IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic...

Z. Phys. Chem.219 (2005) 125–168 by Oldenbourg Wissenschaftsverlag, München

Review Paper

IR Spectroscopy of Microsolvated AromaticCluster Ions: Ionization-Induced Switch inAromatic Molecule–Solvent Recognition

By Otto Dopfer∗Institut für Physikalische Chemie, Universität Würzburg, Am Hubland,97074 Würzburg, Germany

(Received October 14, 2004; accepted in revised form November 24, 2004)

Ion–Ligand Interaction / IR Spectroscopy / Cluster Ions / Ion Solvation /Aromatic Molecules

IR spectroscopy, mass spectrometry, and quantum chemical calculations are employedto characterize the intermolecular interaction of a variety of aromatic cations (A+) withseveral types of solvents. For this purpose, isolated ionic complexes of the type A+–Ln ,in which A+ is microsolvated by a controlled number (n) of ligands (L), are preparedin a supersonic plasma expansion, and their spectra are obtained by IR photodissociation(IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations ina nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarboncations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimersprovides detailed information about the intermolecular interaction between the aromaticion and the neutral solvent, such as ion–ligand binding energies, the competition betweendifferent intermolecular binding motifs (H-bonds,π-bonds, charge–dipole bonds), and itsdependence on chemical properties of both the A+ cation and the solvent type L. IRPDspectra of larger A+–Ln clusters yield detailed insight into the cluster growth process,including the formation of structural isomers, the competition between ion–solvent andsolvent–solvent interactions, and the degree of (non)cooperativity of the intermolecularinteractions as a function of solvent type and degree of solvation. The systematic A+–Ln

cluster studies are shown to reveal valuable new information about fundamental chemicalproperties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Becauseof the additional attraction arising from the excess charge, the interaction in the A+–Ln

cation clusters differs largely from that in the corresponding neutral A–Ln clusters withrespect to both the interaction strength and the most stable structure, implying in mostcases an ionization-induced switch in the preferred aromatic molecule–solvent recognition

* Corresponding author. E-mail: [email protected]

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motif. This process causes severe limitations for the spectroscopic characterization ofion–ligand complexes using popular photoionization techniques, due to the restrictionsimposed by the Franck–Condon principle. The present study circumvents these limitationsby employing an electron impact cluster ion source for A+–Ln generation, which generatespredominantly the most stable isomer of a given cluster ion independent of its geometry.

1. Introduction

Many biophysical and chemical phenomena, including biomolecular recogni-tion, protein folding, biological activity, and chemical reaction mechanismsstrongly depend on the microsolvation environment [1–12]. Such solvationeffects are particularly important for charged species, because of the largerstrength and longer range of ion–solvent interactions compared to corres-ponding neutral–neutral interactions [13–24]. Biological molecules are often(locally) charged due to either (de-)protonation or charge separation [1–5].Moreover, many fundamental chemical reaction mechanisms are ion–moleculereactions, and their properties strongly depend on solvation due to large forcesbetween the ionic species and the solvent molecules [6–8]. The detailed un-derstanding of these important solvation phenomena at the molecular levelrequires the accurate knowledge of the intermolecular interaction potential en-ergy surface. Isolated clusters of the type X±q–Ln are simple and attractivemodel systems to investigate the effects of stepwise solvation of an ion withcharge±q (X±q) by a well-defined number (n) of ligands (L). It is well es-tablished that mass-spectrometric and spectroscopic techniques on the experi-mental side, combined with quantum chemical calculations on the theoreticalside, provide the most direct access and most detailed information on ion–solvent and solvent–solvent interactions in X±q–Ln clusters, and thus on theion–solvent and solvent–solvent interaction potentials as a function of boththe type and the degree of microsolvation [13–24]. This fruitful combinationof experimental and theoretical techniques is applied in the present work toinvestigate ion–ligand complexes involving aromatic ions. Comprehensive re-views describing both the spectroscopy of charged complexes, and discussingthe formation, energetics, and reactivity of ionic complexes are given in recentcompilations [19–22].

The scope of this review focuses mainly on recent results of our group,using IR spectroscopy, mass spectrometry, and quantum chemical calculationsto characterize both the physical and chemical properties of ion–ligand in-teractions involving simple aromatic radical cations (A+) and small solventmolecules [25–35]. The importance of aromatic rings for both biological andchemical recognition has recently been reviewed [10]. The following two pro-totype A+–Ln cluster types shall be discussed.

(1) The first class of A+–Ln clusters (considered in Sect. 3.1) describes theinteraction of acidic aromatic ions A+ with a nonpolar hydrophobic solvent L.The aromatic ions contain acidic functional groups (YHk = OH, NH, NH2),

IR Spectroscopy of Aromatic Cation–Solvent Recognition 127

and examples include phenol (Ph+ [25–27]), para-halogenated phenols (para-fluorophenol and para-chlorophenol, pFPh+ and pClPh+ [28]), 1-naphthol(1-Np+ [32]), aniline (An+ [29, 30]), and indole (In+ [31]). The ligands Lare inert solvents, including the rare gas (Rg) atoms He, Ne, Ar, as well asN2 and CH4. The major issue for these clusters is the competition between(i) the formation of hydrogen bonds (H-bonds) between L and the protonsof the acidic functional YHk group of A+ and (ii) the formation ofπ-bondsbetween L and the aromaticπ-electron system of A+. Moreover, the A+–Ln

spectra reveal the effects of both the solvent type and the degree of asymmet-ric solvation within the first solvation shell on the acidity of the YHk group.The Ph and In complexes are interesting from the biochemical viewpoint, be-cause they represent the chromophores of the aromatic amino acids tyrosinand tryptophan, respectively. Hence, Ph(+)–Ln and In(+)–Ln clusters serve asmodel systems to investigate the interaction between (charged) tryptophan- andtyrosin-containing proteins and surrounding hydrophobic solvent moleculesunder controlled microsolvation conditions [1].

(2) The second class of A+–Ln clusters (discussed in Sect. 3.2) representsthe interaction of bare aromatic hydrocarbon cations A+ with a polar hydro-philic solvent L. The model systems considered here are clusters composed ofthe benzene cation (Bz+) and several water (H2O= W) or methanol ligands(CH3OH= M) [34, 35]. These clusters mimic microhydration of unsubstitutedaromatic hydrocarbon cations. Ion–ligand interactions under controlled micro-hydration conditions are relevant for many biochemcial processes, in whichbiomolecules (such as proteins) are solvated by a small and well-defined num-ber of water molecules [1, 5]. The main interest in the A+–Ln clusters discussedhere arises from the strong competition between (i) hydration of an interiorA+ ion and (ii) the formation of a strongly H-bonded solvent network. Signifi-cantly, the structure and bond energy of the prototypical charge–dipole bond inthe Bz+–W cation has been spectroscopically characterized for the first time.Moreover, stepwise hydration of an interior A+ ion is observed to be a stronglynoncooperative process, whereas the formation of the H-bonded solvent net-work is controlled by large cooperativethree-body forces. Another interestingaspect of these clusters is the onset for chemical reactivity (e.g., proton trans-fer from the hydrocarbon cation to the H-bonded solvent cluster), which isobserved for cluster sizes larger than a certain critical threshold.

In general, the interaction in A+–Ln cation clusters is rather different fromthat in the corresponding neutral A–Ln complexes, because of the substan-tial additional electrostatic, inductive, and charge-transfer attraction arisingfrom the positive charge distribution in A+. Hence, A+–Ln and A–Ln com-plexes often have rather different equilibrium structures and binding ener-gies, corresponding to an ionization-induced switch in the preferred aromaticmolecule–solvent recognition motif. The substantial difference in the top-ology of the interaction potentials in neutral A–Ln and ionic A+–Ln clusters,with respect to both their equilibrium structure and binding energy, leads to

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important consequences for spectroscopic studies of the cation clusters pre-pared by photoionization of the corresponding neutral precursor. Frequently,A+–Ln complexes are generated by the formation of neutral A–Ln complexesin a supersonic molecular beam expansion, followed by resonance-enhancedmultiphoton ionization (REMPI) schemes. REMPI is a popular and convenienttechnique for aromatic cluster spectroscopy, because aromatic chromophoreshave low ionization potentials, and REMPI combines high sensitivity withsize-, isomer-, and state-selectivity [17, 36–41]. For the spectroscopic charac-terization of A+–Ln, the REMPI process is usually combined with photoion-ization efficiency measurements (PIE) [42, 43], mass analysed threshold ion-ization (MATI) [17, 38, 39, 43–45], zero kinetic energy photoelectron (ZEKE)spectroscopy [17, 39, 46], or photodissociation (PD) spectroscopy [47, 48].However, all these ionization techniques suffer from the severe restrictionsimposed by the Franck–Condon (FC) principle, which prevents the efficientpopulation of the most stable A+–Ln isomer in cases where the global min-imum structures of the neutral and the cation complex are rather different.This scenario is quite common for complexes composed of (substituted) aro-matic molecules and both polar and nonpolar molecules [36]. To overcome thelimitations of the REMPI technique for cluster ion generation (“REMPI ionsource”), the present work employs electron impact (EI) ionization of a su-personic expansion to generate cold A+–Ln complexes [22]. This EI clusterion source produces predominantly the most stable isomer of a given A+–Ln

cluster ion, because the reaction sequence begins with EI ionization of A (toyield A+), which is followed by three-body cluster aggregation reactions [25–35]. Hence, the EI source forms predominantly the most stable A+–Ln isomer,independent of the most stable geometry of the corresponding neutral A–Ln

species. This observation is in contrast to the REMPI source, which oftengenerates only local minima of A+–Ln, and as a consequence has frequentlyled to wrong conclusions about the determination of the most stable clusterion structure and the corresponding adiabatic ionization potential. The EI-IRspectra of most A+–Ln clusters discussed in this review have been interpretedwith global minimum structures, which have completely escaped detection inthe corresponding REMPI-IR spectra, as well as other photoionization spectra(PIE, ZEKE, MATI).

2. Experimental approach

The spectroscopic characterization of ion–ligand complexes in the gas phaseis a challenging task, because the inherently low concentrations of chargedclusters produced in available ion sources usually prevent the application ofconventional spectroscopic techniques [19]. As a consequence, most modernspectroscopic methods utilize mass spectrometry to enhance both sensitivityand selectivity. In recent years, resonance-enhanced PD spectroscopy of mass-


Fig. 1. Principle of IRPD spectroscopy of cluster ions in a tandem mass spectrometer il-lustrated for excitation of A+–L dimers. A+–L complexes are generated in a cluster ionsource and mass selected by an initial mass spectrometer. Absorption of an IR photon (ν)leads to resonant vibrational excitation of the intramolecular fundamental (v = 1) from theground vibrational state (v = 0). As thev = 1 level of A+–L lies above the lowest dis-sociation limit, predissociation cleaves the weak intermolecular bond. The produced A+

fragment ions are mass selected by a second mass spectrometer and monitored as a func-tion of ν to obtain the IR action spectrum of A+–L. The excitation may occur on the A+

moiety (as shown in the figure) but it may also occur on the ligand L or involve combina-tion bands with intermolecular vibrations.

selected complexes in the MW, IR, and UV-VIS spectral ranges has developedinto the probably most powerful tool for the spectroscopic investigation ofweakly bound charged aggregates [13, 18–22, 24, 47–62]. In particular, IRspectroscopy has proven to be a very sensitive tool for probing both structureand bonding in ion complexes.

Fig. 1 details the principle of IRPD spectroscopy utilizing a tandemmass spectrometer, with application to a weakly bound A+–L dimer. ColdA+–L complexes are produced in a cluster ion source, which combines low-temperature plasma techniques for ionization (e.g., discharge, electron impact,or laser) with the advantages of a molecular beam expansion [63]. The loweffective temperatures achieved in such supersonic plasma expansions enablethe efficient production of weakly bound isolated ionic aggregates under con-trolled condensation conditions [13, 19–22, 64]. The A+–L dimers generatedare selected by a first mass filter and vibrationally excited by a tunable IRlaser pulse, for example from the ground vibrational state (v = 0) to a high-frequency intramolecular fundamental (v = 1) of either A+ or L. The absorbedvibrational energy is then transferred into the intermolecular degrees of free-dom, leading eventually to the rupture of the weak intermolecular bond. For

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Fig. 2. Experimental setup for the realization of IRPD spectroscopy of cluster cationsin a tandem mass spectrometer illustrated for A+–Ln. Ionic complexes are producedin a cluster ion source, which combines electron impact ionization with a pulsed andskimmed supersonic expansion. The desired A+–Ln parent clusters are then selected bythe first quadrupole mass spectrometer (QMS 1), deflected by 90◦ using a quadrupole ben-der, and injected in an octopole ion guide, where the IRPD process occurs. The A+–Lm

fragment ions generated are filtered by the second quadrupole mass spectrometer (QMS 2)and monitored with a Daly ion detector as a function ofν to obtain the IR action spec-trum of A+–Ln. IR radiation is created by an optical parametric oscillator laser systempumped by a Nd:YAG laser. The IR laser intensity measured with an InSb detector is usedfor normalizing the A+–Lm current for laser intensity variations.

larger aggregates, this direct vibrational predissociation process may be pre-ceded by intracomplex vibrational energy redistribution. The A+ fragment ionscreated are selected by a second mass filter and monitored as a function ofthe IR photon frequency (ν) to obtain the IRPD spectrum of A+–L. Thus,the A+–L absorption is signalled by the appearance of A+. This type of ac-tion spectroscopy combines the high selectivity of mass spectrometry withthe high sensitivity of (nearly) background free ion detection, which can rou-tinely be performed with nearly 100% efficiency at the single particle level.Thus, this approach reaches detection limits below 10 ions/cm3 [19, 22], whichare several orders of magnitude below those required for the most sensitivedirect absorption techniques available to date, such as cavity ring-down spec-troscopy [65].

Fig. 2 illustrates our realization of the IRPD experiment using a tandemquadrupole mass spectrometer (QMS 1/2) coupled to an ion source and an oc-topole ion trap. Details of the setup have been described recently [22]. For the


generation of A+–Ln clusters, a suitable gas mixture, typically composed ofA, L, and a rare gas (Rg), is expanded from high pressure (2–10 bar) througha pulsed valve into a vacuum chamber. Electron impact or Penning ionizationfollowed by three-body association reactions in the high-pressure region of theexpansion produce cold, weakly bound A+–Ln cluster ions [22, 27]:

A +e− → A+ +2e− (1)

A+–Ln−1 +L +B → A+–Ln +B (B = A, L, Rg) . (2)

In case of strong interaction between the ligands, they may form Lp clustersprior to attachment to the (cluster) cation core, which can also result in theproduction of A+–Ln complexes via evaporative or collisional cooling [35]:

A+–Ln+m−p +L p → A+–Ln +mL (3a)

A+–Ln−p +L p +B → A+–Ln +B . (3b)

Significantly, the A+–Ln clusters are not formed in noticeable concentrationsby ionization of the corresponding neutral A–Ln complexes [27, 35], like inREMPI ion sources.

The central part of the plasma expansion is extracted through a skim-mer into QMS 1, which is tuned to the mass of the parent cluster ion underinvestigation, A+–Ln. The mass selected A+–Ln beam is then injected intoan octopole ion trap, where it is overlapped with a tunable IR laser pulse.Single photon absorption of mid-IR radiation leads to resonant excitation ofvibrational levels of A+–Ln lying above the lowest dissociation threshold andinduces the evaporation of one or more ligands:

A+–Ln +hν → A+–Lm + (n −m)L . (4)

Only the rupture of weak intermolecular bonds is observed at the laser inten-sities (I < 200 kW/cm2) and wavelengths employed [34, 66]. The producedA+–Lm fragment ions are selected by QMS 2 and monitored by a Daly iondetector as a function of the laser frequency to obtain the IRPD spectrum ofA+–Ln. For larger clusters (n ≥ 2) several fragment channels (m) may be open.In these cases, action spectra are recorded simultaneously in the dominant frag-ment channels. They are usually similar in appearance for the A+–Ln clustersdiscussed in this review. In general, the range of fragment channels (m) ob-served for a given parent cluster (n) is rather narrow and this information can beused to estimate incremental ligand binding energies (see Sect. 3.1.2). To sep-arate the contribution of fragment ions generated by laser-induced dissociation(LID) from those produced by metastable decay (MD) of hot parent clusters orcollision-induced dissociation (CID) with residual gas in the octopole, the ionsource is triggered at twice the laser frequency and the signals from alternat-ing triggers are subtracted. Pulsed and tunable IR laser radiation is created by

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Fig. 3. (a) Mass spectrum of the EI cluster ion source for an expansion of heated Ph vapor(T ≈ 330 K) seeded in 6 bar N2 [27]. The spectrum is dominated by Nn

+ and Ph+–(N2)n

clusters. (b,c) Mass spectra obtained by mass-selecting Ph+–(N2)3 clusters with QMS 1and scanning QMS 2 [27]. For spectrum (b), the laser is off and the observed Ph+–(N2)m

fragment ions (m = 1, 2) arise from metastable decay and/or collision-induced dissocia-tion. For spectrum (c), the laser is tuned to a resonance of Ph+–(N2)3, ν1 = 3381 cm−1,leading to additional fragmentation into them = 0 and 1 daughter channels (laser-induceddissociation).

a single-mode optical parametric oscillator laser system, which is pumped bya seeded Nd:YAG laser. Relevant attributes of the laser radiation include thetuning range (2500–6800 cm−1), the pulse width (∼ 5 ns), the repetition rate(25 Hz), and the energy per pulse (∼ 1 mJ).

As an example, Fig. 3(a) shows a mass spectrum of the ion source ob-tained for the production of Ph+–(N2)n clusters by expanding heated Ph vapor(T ≈ 330 K) seeded in 6 bar N2 [27]. The dominant ions observed are Nn

+

and Ph+. The inset shows the production of weakly bound Ph+–(N2)n clusters.Fig. 3(b,c) reproduces the mass spectra obtained by mass-selecting Ph+–(N2)3

with QMS 1 and scanning QMS 2 without (b) and with (c) resonant IR excita-tion. The spectrum in Fig. 3(b) shows a strong parent peak (n = 3) and weaksignals in them = 1 andm = 2 fragment channels (< 1%) arising from MDand/or CID. The spectrum in Fig. 3(c) reveals additional fragmentation of then = 3 cluster owing to resonant LID into them = 0 (90%) andm = 1 (10%)fragment channels. In total,∼ 30% of Ph+–(N2)3 is depleted by the LID pro-cess.

For strongly bound ion–ligand complexes, the dissociation energy for lig-and evaporation (D0) often exceeds the vibrational frequency of any of itsfundamental vibrations. In such a case, the complex will not dissociate uponexcitation of a fundamental from the ground vibrational state, because the


Fig. 4. IRPD spectra of Ph+–W (a [75, 76]) and Ph+–W–Ar (b [75, 76]) produced in an EIsource compared to the corresponding spectrum of Ph+–W generated by REMPI (c [48]).The arrows indicate the positions of theν1 andν3 frequencies of bare W.

IRPD process described in Eq. (4) is endothermic. Nevertheless, there are sev-eral possibilities to obtain an IRPD spectrum for a strongly bound complex.First, single-photon IRPD may be observed for initially hot parent clusters con-taining already a significant amount of internal (ro)vibrational energy prior tophotoexcitation [34, 35, 66, 67]. Second, the excitation of overtone or combina-tion bands can result in an exothermicIRPD process, which may be endother-mic for the fundamental vibration [22, 68]. Third, in strong IR laser fields, suchas those of modern free electron lasers, the complex may absorb multiple IRphotons to overcome the dissociation limit (IRMPD) [60, 69]. Fourth, the ef-fective dissociation energy of a strongly bound complex can significantly bereduced by attaching a weakly bound ligand (messenger approach [33, 34, 70–73]). The messenger ligand is nearly not perturbing the properties of the com-plex but can easily be evaporated upon IR excitation. Fig. 4 shows IRPD spec-tra of the strongly H-bound Ph+–W complex [40, 41, 74] in the O–H stretchrange obtained by three different techniques: (a) EI-IR of Ph+–W [75, 76],(b) EI-IR of Ph+–W–Ar [75, 76], and (c) REMPI-IR of Ph+–W [48]. The dis-sociation energy of Ph+–W, D0 = 6520±50 cm−1 [45], is much larger thanboth O–H stretch fundamentals of W (ν1 = 3657 cm−1, ν3 = 3756 cm−1 [77]).Hence, the single-photon EI-IR spectrum of Ph+–W corresponds to excitationof sequence hot bands of the typeν1,3 +νx ← νx , whereνx are vibrations withfrequencies in excess of∼ 2750 cm−1. Consequently, the spectrum arises fromrelatively hot species, resulting in large bandwidths of∼ 25 cm−1 arising fromcross anharmonicities. The smaller linewidths of∼ 15 cm−1 observed in theREMPI-IR spectrum of Ph+–W correspond to somewhat colder species, indi-cating that the production of Ph+–W by REMPI deposits less internal energyin the complex than EI. The coldest spectrum is obtained by the messenger

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approach using Ar tagging. The linewidths of∼ 6 cm−1 in the Ph+–W–Arspectrum indicate low internal energy, and the observed absorptions corres-pond to theν1,3 fundamentals, because the binding energy of Ar,D0 = 518±6 cm−1 [43], is much lower thanν1,3. Thus, the Ar messenger reduces both theeffective dissociation energy and the internal energy of the Ph+–W complex.

3. Results and discussion

3.1 Complexes of acidic aromatic ions with nonpolar ligands

3.1.1 A+–L dimers

Aromatic molecules (A) with acidic functional YHk groups (e.g., Y = N, O, F)offer two major recognition sites for inert ligands L. L can either bind to thearomaticπ-electron system (π-bond) or to one of thek acidic protons of YHk

(H-bond). Alternative binding sites, such as H-bonds to aliphatic or aromaticCH protons, are usually less stable. Thepreferred binding site in A–L dimersdepends on several factors, including the degree of electronic excitation andcharge state of A, the acidity of the protons of the YHk group, and the lig-and type (polar or nonpolar). For example, complexes of neutral A with Rgatoms or other “spherical” nonpolar ligands (such as CH4) haveπ-bound equi-librium structures in the singlet electronic ground state (S0). Examples includePh–Ar and 1-Np–Ar (YHk = OH) [78–81], An–Ar (YHk = NH2) [82], In–Ar(YH k = NH) [83], as well as Bz–Ar [38]. The major contributions to the A–Arattraction arise from dispersion forces between Ar and theπ-electron systemof A, which favorπ-bonding. In fact, H-bound isomers have not been detectedfor any aromatic A–Rg/CH4 complex inS0. The situation changes for A–N2

dimers, because the quadrupole moment of N2 leads to additional electrostaticinteractions with the polar YHk group of A, so that the H-bond may energeti-cally compete with theπ-bond. For example, N2 favors H-bonds to Ph [78, 84]butπ-bonds to less acidic 1-Np [81], An [85] as well as Bz [86, 87].

As already outlined in the Introduction, the topology of the interactionpotential of an A+–L radical cation dimers differs qualitatively from that ofneutral A–L, because of the significant additional electrostatic and inductiveattraction arising from the excess positive charge [17, 19, 36, 45, 47]. As a con-sequence, neutral and ionic A(+)–L dimers often possess rather different equi-librium structures and interaction energies. For example, Ph+–Rg/CH4 [25–28], An+–Ar [30], In+–Ar [31], and 1-Np+–Ar [32] feature H-bound globalminima in the ground electronic state of the cation (D0). The π-bound iso-mers, which are global minima inS0, are only local minima inD0. Similarly,Ph+–N2 [25, 27, 78, 88], An+–N2 [29], In+–N2 [31], and 1-Np+–N2 [32] pre-fer H-bonds overπ-bonds inD0. Similar to A+–Rg/N2, the Rg and N2 dimersof protonated aromatic molecules (AH+) with acidic YHk groups, such asPhH+–Rg/N2 [89–91], also favor H-bonds overπ-bonds. In contrast, the most


stable A(H)+–Rg/CH4/N2 dimers of A(H)+ ions without YHk groups, such asBz(H)+–Rg/CH4/N2 [33, 38, 71, 92], featureπ-bonds inD0, because the aro-matic and aliphatic CH protons are only little acidic even in the cation groundstate [92].

The ionization-induced switch in the preferred ligand binding site inA (+)–L dimers imposes important consequences for spectroscopic studies ofA+–L prepared by photoionization of A–L. Frequently, A+–L is generated byREMPI of A–L formed in a supersonic expansion. Spectroscopic informationof A+–L may then be extracted, for example, from PIE [42, 43], MATI [38,39, 43–45], ZEKE [17, 39, 46], or IRPD spectra [47, 48]. However, all theseionization techniques suffer from the FC principle, which prevents significantpopulation of the most stable A+–L isomer for dimers with largely differentneutral and ionic global minimum structures. This situation is, however, quitecommon for A(+)–L dimers with both polar and nonpolar L [25, 34, 36, 93].To avoid the limitations of REMPI cluster ion generation, in the present workcold A+–Ln complexes are produced in a supersonic plasma expansion createdby EI of a molecular beam [22]. This EI ion source generates A+–Ln com-plexes via EI of A followed by cluster aggregation (Eqs. 1–3). Consequently,the EI source produces predominantly the most stable isomer of a given A+–Ln

complex, independent of the most stable structure of neutral A–Ln. For ex-ample, EI-IR spectra of Ph+–Arn [25–28], An+–Arn [30], In+–Arn [31], and1-Np+–Arn [32] were interpreted with global minimum structures, which com-pletely escaped detection in the corresponding REMPI-IR and other photoion-ization spectra (PIE, ZEKE, MATI).

Fig. 5 compares the REMPI and EI approaches by considering the poten-tial curves in the various electronic and vibrational states of Ph–Ar. Accordingto ab initio calculations [94], Ph–Ar has aπ-bound equilibrium structure inS0,whereas the H-bound geometry is predicted to be a less stable local minimum,which has not yet been identified experimentally. Consequently, REMPI ofneutral Ph–Ar, generated in a molecular beam, via theS1 state leads mainlyto the production ofπ-bound Ph+–Ar cations inD0, because the FC principlestrongly favors vertical transitions (dashed arrows in Fig. 5). Transitions fromtheπ-bound to the H-bound minima (and vice versa) have nearly vanishing FCfactors, because they imply large geometry changes. Therefore, all transitionsobserved in REMPI [42, 94–96], PIE [42], ZEKE [39, 46, 79, 94], MATI [79],and REMPI-IR spectra [78, 97] of Ph(+)–Ar were attributed to theπ-boundisomer. No signature of the H-bound Ph(+)–Ar isomer was observed at all inthese spectra, although the H-bound structure is clearly the global minimum inD0 [25–28]. These limitations of the REMPI ion source are overcome by gen-erating Ph+–Ar in the EI cluster ion source, because there Ph is ionized firstby EI, and Ph+–Ar dimers are subsequentlyproduced by three-body associa-tion reactions (dotted arrows in Fig. 5). As H-bound Ph+–Ar is more stable thanπ-bound Ph+–Ar, the abundance of the former isomer in the EI source is larger.Indeed, both isomers were unambiguously identified by their IRPD spectra,

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Fig. 5. Sketch of potential energy diagrams of Ph–Ar in various electronic (S0, S1, D0)and vibrational states (v1 = 0, 1). The dashed arrows indicate the preparation of Ph+–Arcations in theD0 state by resonance-enhanced multiphoton ionization (REMPI): the neu-tral precursor is formed in a molecular beam in theS0 state and resonantly ionized withtwo photons (νa,b) via the S1 state. The dotted arrows indicate the preparation of Ph+–Arcations in theD0 state by electron impact (EI): Ph is ionized by EI and Ph+–Ar complexesare subsequently formed in the molecular beam.

based upon their characteristic O–H stretch frequency (ν1). Fig. 6 compares theIRPD spectra of Ph+–Ar in the O–H stretch range, where the cation clusters areproduced either by REMPI (a [97]) or by EI (b,c [26, 27]). The EI-IR spectrum(b) shows theν1 bands of both isomers, whereas the REMPI-IR spectrum dis-plays only theν1 absorption of the less stableπ-bound isomer. Theν1 band ofthe H-bound global minimum of Ph+–Ar is completely missing in the REMPI-IR spectrum, clearly demonstrating the limitations of the REMPI ion sourcefor the spectroscopic study of cation complexes. The EI-IR spectra recordedfor decreasing effective temperatureT3 > T2 > T1 in Fig. 6(c) demonstrate thatthe relative abundance of the less stableπ-bound Ph+–Ar isomer decreases fordecreasingT , confirming that this isomer is less stable than H-bound Ph+–Ar.Fig. 7 reproduces the calculated minimum energy path for the isomerizationfrom the π-bound local minimum toward the H-bound global minimum ofPh+–Ar, which involves a barrier of the order ofVb ∼ 150 cm−1 [26]. All avail-able spectroscopic data are consistent with such a potential, and the dynamicsof this π → H isomerization process is currently subject of state-of-the-artpicosecond three-color UV-IR pump-probe spectroscopy [98]. The binding en-ergies ofDe = 415 and 685 cm−1 for π-bound and H-bound Ph+–Ar calculated


Fig. 6. Comparison of the IR photodissociation spectra of Ph+–Ar obtained when thecation cluster is produced by REMPI (a [97]) or in the EI source (b [26, 27]). Both spec-tra show theν1 bands of the less stableπ-bound isomer, whereas theν1 band of theH-bound global minimum is only visible in spectrum (b). (c) EI-IR spectra of Ph+–Arrecorded under different experimental conditions [26]. The relative abundance ofπ-boundPh+–Ar decreases in the orderT3 > T2 > T1, confirming that this isomer is less stable thanH-bound Ph+–Ar. Only under warm conditions (T2,3) the population ofπ-bound Ph+–Aris significant, whereas for cold conditions (T1) its ν1 absorption is below the noise level.

Fig. 7. Minimum energies (Dmin) and intermolecular separations (Rmin in Å) of one-dimensional radial cuts through the three-dimensional intermolecular potential energy sur-face of Ph+–Ar calculated at the UMP2/6-311G(2df,2pd) level [26]. The calculated datapoints are interpolated by a polynomial fit to estimate the potential barrier,Vb, along theisomerization coordinate.

138 O. Dopfer

Fig. 8. (a) IRPD spectra of Ph+–L dimers (L= He, Ne, Ar, CH4, N2) recorded in the Ph+

fragment channel [27]. The strongest band in each spectrum is attributed to theν1 fun-damental of the H-bound structure (ν1). In the case of Ph+–Ar, the ν1 transition of theless stableπ-bound isomer is also observed (νπ

1 ). The arrow indicates theν1 frequency ofbare Ph+ (ν1 = 3534 cm−1 [78]). (b) Plot of theν1 frequency of H-bound Ph+–L versusthe proton affinity of L [27]. The line corresponds to a least squares fit of the data pointsto a linear polynomial. (c) Plot of theν1 frequency of H-bound Ph+–L versus the polar-izability of L [27]. Also included is the data point for Ph+ (open circle [78]). The linecorresponds to a least squares fit of the data points of He, Ne, Ar, CH4 (filled circles) toa second order polynomial.

at the MP2/6-311G(2df,2pd) level [28] are in good agreement with the corres-ponding experimental values ofD0 = 535±3 and 670±140 cm−1 determinedfrom MATI [79] and IRPD [27] spectroscopy, respectively.

Fig. 8(a) compares the IRPD spectra of a variety of Ph+–L dimers withL =He, Ne, Ar, CH4, and N2 in the range of theν1 vibration of bare Ph+ (in-dicated by the arrow) [27]. All spectra are dominated by theν1 fundamental ofthe H-bound Ph+–L dimers, which correspond to the global minima on theirpotentials. Only for Ph+–Ar, the ν1 band of the less stableπ-bound isomercould be identified. Theν1 transitions of the H-bound Ph+–L dimers displaya complexation-induced red shift,∆ν1, which is characteristic for H-bonding.H-bonding of L to the acidic OH group induces a partial proton shift towardL and weakens the O–H bond. As a result, theν1 frequency is reduced andthe magnitude of the reduction is a measure of the H-bond strength:e.g.,∆ν1 = −70 and−169 cm−1 for Ar and N2, respectively. Fig. 8(b) demonstratesthat the frequency shift is roughly linearly correlated with the proton affinityof L, PAL [27]. For nonpolar L, the attractive part of the potential in Ph+–Lis dominated by charge–induced dipole interaction. Hence, theν1 frequencyof Ph+–L with nonpolar L correlates also with the polarisability of L,αL, asshown in Fig. 8(c) [27]. Ph+–N2 features additional electrostatic attraction aris-ing from charge–quadrupole interaction. Consequently, it exhibits a largerν1

shift compared to that predicted from its polarisability alone. In contrast to theH-bound Ph+–L dimers, theπ-bound Ph+–Ar isomer displays only an insignif-icant change inν1 (< 2 cm−1), asπ-bound ligands do not affect the propertiesof the OH group.


Fig. 9. IRPD spectra of a variety of A+–L dimers recorded in the A+ fragment chan-nel (A= pClPh, pFPh, Ph, and 1-Np; L= Ar and N2 [27, 28, 32]). A represents a var-iety of hydroxyarenes, namely parahalogenated phenols (pXPh with X= H, F, Cl) and1-naphthol. The spectra are recorded in the O–H stretch range (ν1). The strongest bandin each spectrum is attributed to theν1 fundamental of the H-bound structure (H). Thearrows and the numbers indicate the complexation-induced red shifts,∆ν1, from the cor-responding monomer transitions. The A+–Ar spectra also show weakν1 bands of the lessstableπ-bound isomers (π), which occur close to the monomerν1 frequencies.

Fig. 9 summarizes all EI-IR spectra obtained so far for A+–Ar/N2 dimers,in which A+ is a hydroxyarene radical cation, namely (monohalogenated)phenol (Ph+, pFPh+, pClPh+) and 1-Np+ [25–28, 32]. Similarly, Fig. 10 re-produces the corresponding EI-IR spectra recorded for In+–Ar/N2 [31] andAn+–Ar/N2 [29, 30], respectively. All these IR spectra are clearly dominatedby the O/N–H stretch fundamentals of their H-bound isomers, indicating thatH-bonds are favored overπ-bonds in both A+–Ar and A+–N2. All A +–Arspectra in Figs. 9 and 10 also show weak transitions assigned to the less sta-ble π-bound isomers, which display only negligible frequency shifts uponcomplexation. The A+–N2 spectra typically lack transitions of theπ-boundisomers, with the notable exception of In+–N2.

Significantly, the EI-IR spectra in Figs. 8–10 have demonstrated for thefirst time the general phenomenon of anionization-induced switch in the pre-ferred intermolecular binding or recognition motif fromπ → H for acidic

140 O. Dopfer

Fig. 10. IRPD spectra of a variety of A+–L dimers recorded in the A+ fragment channel(A = An and In; L= Ar and N2 [29–31]). The spectra are recorded in the N–H stretchrange. The strongest bands in each spectrum are attributed to the N–H stretch fundamen-tals of the H-bound structure (H). The arrows and the numbers indicate the complexation-induced red shifts from the corresponding monomer transitions. The A+–Ar and In+–N2

spectra also show weak N–H stretch bands of the less stableπ-bound isomers (π), whichoccur close to the monomer frequencies.

aromatic molecules interacting with anonpolar ligand. All considered neutralA–Rg/CH4 dimers (A= pXPh, An, In, and 1-Np), as well as A–N2 complexeswith A = An, In, and 1-Np preferπ-bonding inS0, because dispersion dom-inates the attraction. On the other hand, the corresponding A+–Rg/CH4/N2

cation dimers favor H-bonding inD0, because electrostatic and inductive forcesbetween the acidic protons and L provide the major contribution to the at-traction. As a consequence of this switch in the preferred binding motif, allprevious spectroscopic experiments [39, 42, 45, 46, 58, 78–80, 94, 97, 99–112]missed the H-bound global minima of A+–L, because they exclusively usedREMPI for cluster ion preparation. Consequently, the ionization potentials ob-served in these studies are not adiabatic ones (as claimed by the authors) butonly vertical ones. It was only the application of the EI ion source, which en-abled the spectroscopic detection and characterization of the H-bound isomers,


Fig. 11. Relative complexation-induced frequency shifts of the proton donor stretch vibra-tion in H-bound X–H+–L dimers,|∆νX–H|/νX–H, as a function of PA(X) for XH+ = SiOH+,Ph+, t-1-Np+, In+, and An+ and L= Ar and N2. Interpolation of the data for XH+ =t-1-Np+ (dotted lines) and XH+ = In+ (dashed lines) yields PA= 908± 5 and 923±3 kJ/mol for the t-1-naphthoxy and indolyl radicals, respectively [31, 32].

and thus this prototype interaction for acidic aromatic cations with nonpo-lar solvents, for the first time. It is noted that there is no difference betweenREMPI and EI ion sources for A+–L complexes composed of acidic aromaticmolecules and strongly quadrupolar and dipolar ligands, because H-bondingis preferred in both the neutral and cationic ground states of these dimers.Examples for such complexes include Ph+–L with L = N2 [25, 27, 78, 88, 99],CO(2) [99, 113, 114], W (see also Fig. 4) [17, 40, 41, 48, 74–76], M [17, 115,116], dimethyl ether [117], ethanol [118], or Ph [116], In+–W [45, 119], and1-Np–W [45, 120]. However, difficulties have been encountered in preparingcold complexes of unsubstituted or halogenated aromatic hydrocarbon ionswith polar ligands by REMPI due to aπ → H structural switch upon ionization(see also Sect. 3.2) [36].

Previous spectroscopic experiments revealed that the magnitude of thecomplexation-induced red shift of the proton donor stretch vibration in proton-bound dimers of the type X–H+–L, ∆νX–H, is correlated with the differencein the proton affinities (PA) of the two bases X and L [19, 27, 121–123]. Thesmaller PA(X)-PA(L), the stronger theintermolecular H–L bond and the larger∆νX–H. This relation may be used to estimate unknown PA values of basesX from IR spectra of their X–H+–L dimers. Recently, this procedure wasapplied to XH+ = In+ and t-1-Np+ to obtain the first experimental determin-ation of the PA of the indolyl and trans-1-naphthoxy radicals [31, 32]. Fig. 11plots the relative red shifts|∆νX–H|/νX–H as a function of PA(X) for a series ofH-bound XH+–N2 and XH+–Ar dimers, including XH+ = SiOH+, Ph+, In+,An+, and t-1-Np+. The shifts are larger for L=N2 compared to L= Ar,

142 O. Dopfer

Table 1. Calculated structural, energetic, and vibrational parameters of pXPh+ (X = H, F,Cl) and theirπ-bound and H-bound pXPh+–Ar dimers (Fig. 12),(a) along with availableexperimental frequencies (Fig. 9).

Species re Re φe De ω1 ωs ν1

[Å] [Å] [cm −1] [cm−1] [cm−1] [cm−1]

Ph+ 0.9693 3654 3534(b)

Ph+–Ar(π) 0.9691 3.415 83.3◦ 415 3536Ph+–Ar(H) 0.9721 2.3229 174.5◦ 685 3599 71 3464pFPh+ 0.9674 3664 3546(b)

pFPh+–Ar(π) 0.9671 3.502 67.2◦ 424 3546pFPh+–Ar(H) 0.9696 2.3534 176.7◦ 644 3609 68 3477pClPh+ 0.9667 3669pClPh+–Ar(π) 0.9665 3.419 78.3◦ 482 3552pClPh+–Ar(H) 0.9687 2.3748 176.4◦ 605 3620 66 3488

(a) Structural and energetic data are obtained at the MP2(fc)/6-311G(2df,2pd) level,whereas vibrational data are derived at the B3LYP/6-31G* level [28].(b) From Ref. [198].

because PA(N2) > PA(Ar) (494> 369 kJ/mol [124]). For a given L, theshifts decrease in the order XH+ = SiOH+ > Ph+ > t-1-Np+ > In+ > An+ be-cause of increasing PA(X). Linear extrapolation from the known PA valuesof the anilino and phenoxy radicals (950 and 873 kJ/mol [124, 125]) yieldsPA≈ 908±5 and 923±3 kJ/mol for t-1-naphthoxy and indolyl (Fig. 11). Asthe uncertainty of this method for determining PA values is not well doc-umented, the errors have been enlarged to±30 kJ/mol. The spectrum ofc-1-naphthoxy indicates that the PA of the cis isomer of is about 10 kJ/mollarger than that of the trans isomer. This example demonstrates that IRspectroscopy of cluster ions can be used to selectively probe thermo-chemical properties of specific conformers of transient radicals, such astheir PA [32].

The IRPD spectra of the Ar dimers of the para-halogenated phenol cations(Ph+–Ar, pFPh+–Ar, pClPh+–Ar) in Fig. 9 nicely illustrate that IR spec-troscopy of cluster ions can also be used to probe subtle effects of substitutionof functional groups on the acidity of radical cations, in particular in connec-tion with quantum chemical calculations. Table 1 and Fig. 12 summarize thecalculated properties of both the H-bound global minima and theπ-boundlocal minima of pXPh+–Ar (X = H, F, Cl) [28]. The O–H bond parametersof pXPh+ indicate that the strength of this intramolecular bond increases inthe order X= H <F< Cl, i.e., the acidity decreases in the same order. TheO–H bond length,re, decreases from 0.9693 to 0.9667 Å, and the harmonicO–H stretch frequency,ω1, increases from 3654 to 3669 cm−1. The planar andslightly trans-linear H-bound pXPh+–Ar geometries are the global minima onthe dimer potentials. The intermolecular interaction strength,De, decreases inthe order H>F> Cl from 685 to 605 cm−1. Consequently, the intermolecular


Fig. 12. (a) Sketch of the calculated minimum structures of the H-bound global minimaand theπ-bound local minima of pXPh+–Ar (Table 1; X= H, F, Cl). (b) ExperimentalO–H stretch frequencies ofπ-bound pXPh+–Ar. (c) Difference of the O–H stretch fre-quencies ofπ-bound and H-bound isomers of pXPh+–Ar. The indexN corresponds to therow of X in the periodic table [28].

H–Ar bond length,Re, increases from 2.32 to 2.37 Å, and the intermolecularstretch frequency,ωs, decreases from 71 to 66 cm−1. Ar complexation causesmodifications of the O–H bond properties, which are typical for H-bonding:re increases (∆re = 0.002–0.003 Å) andω1 decreases (|∆ω1| = 49–55 cm−1).In general, the weaker the O–H bond in pXPh+, the stronger the inter-molecular H–Ar bond and its impact on the intramolecular O–H bond inH-bound pXPh+–Ar (H > F> Cl). The π-bound pXPh+–Ar geometries arelocal minima, withDe = 415, 424, and 482 cm−1 for X = H, F, and Cl, respec-tively. Apparently, the larger the substituent X, the stronger the intermolecularπ-bond. This effect is attributed to increased dispersion forces between Arand X. Moreover, halogen substitution tends to move the Ar ligand furtheraway from the centre of the aromatic ring in direction of X (φe = 83◦, 67◦,78◦ for X = H, F, Cl). In contrast to H-bonding,π-bonding has essentiallyno influence on the O–H bond properties (re, ω1). The O–H bond contractsonly slightly (|∆re| ≤ 0.0003 Å), causing a minor blue shift inω1 of the orderof 1–2 cm−1 [25]. The IRPD spectra of pXPh+–Ar in Fig. 9 nicely repro-duce the trends predicted theoretically. Theνπ

1 bands ofπ-bound pXPh+–Arare close toν1 of the bare pXPh+ (νπ

1 ≈ ν1). Consequently,νπ1 of pXPh+–Ar

can directly be used to reliably approximate the unknown pClPh+ transi-tion as 3552±2 cm−1. Moreover,νπ

1 of pXPh+–Ar, and thusν1 of pXPh+,increases in the order H< F< Cl, confirming that the O–H bond becomesindeed stronger along the series Ph+ < pFPh+ <pClPh+ (Fig. 12(b)). Simi-lar to νπ

1 , the red shift∆νH1 of H-bound pXPh+–Ar (≈ νH

1 −νπ1 , Fig. 12(c))

is a sensitive spectroscopic indicator of the PA of the corresponding phen-oxy radical. In line with the trend of the O–H bond strengths derivedfrom νπ

1 , the decreasing∆νH1 values (−72, −69, and−64 cm−1 for X =

H, F, and Cl) suggest that the PA of the para-substituted phenoxy radicalsincreases slightly in the order H< F< Cl, leading to weaker intermolecularH-bonds to Ar.

144 O. Dopfer

Table 2. Dissociation energies (De) of selected H-bound A(H)+–Ar and A(H)+–N2 dimersinvolving aromatic A(H)+ ions, along with the positive partial charge (AIM) on the inter-mediate proton (qH), calculated at the MP2/6-311G(2df,2pd) level.

A(H)+ qH [e] De (Ar) [cm−1] De (N2) [cm−1]

BzH+ (CH) 0.12 220 728BzH+ (CH2) 0.16 293 932c-C3H3

+ (CH)(a) 0.27 365 1227An+ (NH2)(b) 0.48 513 1431PhH+ (OH)(c) 0.67 633 1808Ph+ (OH)(d) 0.66 656 1910

(a) Refs. [138, 139].(b) Refs. [29, 30].(c) [76] (assuming para protona-tion). (d) Refs. [25, 76].

Fig. 13. Dissociation energies (De) of selected H-bound A(H)+–Ar/N2 dimers involvingaromatic A(H)+ ions as a function of the positive partial charge (atoms-in-molecules ap-proach) on the intermediate proton (qH) calculated at the MP2/6-311G(2df,2pd) level(Table 2) [92].

In general, the long-range attraction in weakly H-bound A(H)+–L com-plexes is dominated by the electrostatic and inductive forces between thecharge distribution of A(H)+ and the permanent multipole moments and po-larisabilities of L [19, 121, 126, 127]. A key factor for the interaction strengthis thus the positive partial charge localized on the intermediate proton (qH).Table 2 and Fig. 13 summarize the calculated binding energies (De) for a var-iety of H-bound A(H)+–L dimers (L= Ar, N2) involving aromatic radicalcations or protonated aromatic molecules, namely BzH+, the cyclopropenylcation (c-C3H3

+), An+, Ph+, and PhH+ [92]. As a general trend,De increaseswith qH. In particular, the CH1/2–L bonds are much weaker than the OH–L andNH1/2–L bonds.


3.1.2 Larger A+–Ln clusters

The IRPD spectra of larger A+–Ln clusters (n > 1) provide detailed insightinto the cluster growth process, including the determination of incrementalligand binding energies, the formation of structural isomers, the competi-tion between ion–solvent and solvent–solvent interactions, and the degree of(non)cooperativity of the intermolecular interactions as a function of solventtype and degree of solvation.

As an example, Fig. 14(a) shows the IRPD spectra of Ph+–Ln with L = Arand N2 recorded in the dominant Ph+–Lm fragment channel (indicated asn → m) [27]. All spectra are dominated by the strongν1 (O–H stretch) transi-tion of the most stable isomer of each cluster ion. The positions of these bandsare plotted in Fig. 14(b,c) as a function of the cluster sizen for L = Ar and N2.As discussed in Sect. 3.1.1, theν1 bands of the H-bound Ph+–L dimers dis-play large red shifts with respect to the Ph+ transition (indicated by an arrow inFig. 14(a)), because the formation of the intermolecular proton bond weakensthe O–H bond. Further complexation of the H-bound Ph+–L dimer core occursat the aromatic ring by the formation ofπ-bonds (Fig. 14(d)). Other bindingsites, such as binding to the O atom or to H atoms of the ring are calculated tobe less stable [26]. Theπ-bound ligands cause only modest incrementalν1 blueshifts of a few cm−1, becauseπ-bonding has only a minor influence on the O–Hbond strength. The small blue shifts indicate thatπ-bonding leads to a slightincrease of the O–H bond strength, which is accompanied by a weakening ofthe intermolecular proton bond to the first ligand. Such noncooperative three-body effects are typical for solvation of an ion by nonpolar ligands (vide infra).In general, the shifts are larger for L= N2 than for L= Ar, due to the strongerinteraction in the former complexes. Moreover, the incremental shifts tend tobecome smaller for increasingn, although they are not converged to zero at thelargest cluster sizes investigated (n = 7). Unfortunately, there appear to be nomatrix isolation studies of Ph+ in argon or nitrogen, making it impossible atthis stage to compare the cluster data with the bulk limit. Fig. 14(b,c) comparesthe ν1 frequencies of Ph+–Ln with those of the corresponding t/c-1-Np+–Ln

clusters [32]. The acidity of the OH group increases in the order c-1-Np+ <

t-1-Np+ < Ph+, as demonstrated by the decreasingν1 frequencies of themonomer ions (n = 0). This trend is drastically amplified for theν1 frequen-cies of the H-bound dimers, because the strengths of the H-bonds are largelydepending on the acidity of the OH group. This enhancement demonstrates thatcluster ion spectroscopy can probe the acidity of bare ions with very high sensi-tivity and resolution. The dramatic dependence of theν1 frequencies of Ph+–Ln

on both the ligand L and the cluster sizen illustrates that the strength of theO–H bond, and thus the acidity of the OH group, largely depends on the degreeof asymmetric solvation (n) and the solvent type (L).

Similar to the A(+)–L dimers discussed in Sect. 3.1.1, the larger A(+)–Ln

clusters exhibit an ionisation-induced change in the preferred cluster structure,

146 O. Dopfer

Fig. 14. (a) IRPD spectra of Ph+–Ln (L = Ar and N2) recorded in the dominant Ph+–Lm

fragment channel (indicated asn → m) [27]. The strongest band in each spectrum is at-tributed to theν1 fundamental of the most stable isomer. For Ph+–Ar, theν1 transition ofthe less stableπ-bound isomer is indicated asνπ

1 . The arrow indicates theν1 frequencyof bare Ph+ (ν1 = 3534 cm−1). (b,c) Plots of theν1 maxima of the most stable isomers ofA+–Arn and A+–(N2)n for A+ = t-1-Np+, c-1-Np+, and Ph+ as a function of the clustersize (n) [27, 32]. The calculated value is used forν1 of bare c-1-Np+ [80]. (d) Sequenceof the cluster growth for the most stable Ph+–Ln complexes (n = 1–3).

because the excess charge changes completely the topology of the intermo-lecular potential. For example, the most stable neutral Ph–Ar2 trimer has a Cssymmetric (1|1) structure, with twoπ-bound ligands located on opposite sidesof the aromatic ring [42]. The IRPD spectra of Ph+–Ar2, however, indicate


that the most stable trimer cation features one H-bound and oneπ-bound lig-and [27]. The fact that the weakνπ

1 band of Ph+–Ar disappears in the IRPDspectra of larger Ph+–Arn clusters shown in Fig. 14(a) confirms that the H-bondin the cation is more stable than theπ-bond. On the other hand, all photoioniza-tion experiments on Ph+–Ar2 detected only the less stable (1|1) structure [42],again demonstrating the severe limitations of REMPI ion sources for the spec-troscopy of cluster ions arising from the FC principle. These limitations arecircumvented here in the case of Ph+–Arn and also other cluster systems byusing the EI ion source, which produces the most stable isomer of a givencluster ion.

IRPD spectra similar to those of Ph+–Ln [27] were obtained for otherrelated A(H)+–Ln clusters of acidic aromatic A(H)+ ions with L= Ar, N2,and CH4, including A(H)+ = In+ [31], An+ [29, 30], t/c-1-Np+–Ln [32], andPhH+ [91]. In all these clusters, the preferred cluster growth begins with thesolvation of all available protons of the acidic YHk group with nonpolar lig-ands. Subsequently, less stable bindingsites in the first solvation shell areoccupied. As the ligand–ligand interaction is much weaker than the ion–ligandinteraction, the A(H)+–Ln clusters display solvation of ligands L around aninterior A(H)+ ion core rather than the formation of a Ln subcluster attachedto a surface A(H)+ ion. Significantly, also the solvation of small acidic AHk

+

ions by nonpolar hydrophobic solvents show the same cluster growth sce-nario, including OCH+–Ln [128, 129], N2H+–Ln [130, 131], SiOH+–Ln [132]H2O+–Ln [127, 133], H3O+–Ln [22], NH3

+–Ln [134, 135], NH4+–Ln [136],

and c/l–C3H3+ [137–139]. In all these clusters, the ligands form first intermo-

lecular H-bonds to thek equivalent acidic protons of AHk +, before less stablebinding sites around an interior ion core are occupied. On the other hand, thereare several examples of both small and also aromatic ions, where the protonsare only little acidic so that other binding sites, such asπ-electrons or elec-trophilic p orbitals, are more attractive than H-bonding sites. Examples for thelatter scenario include CH3+–Rgn [22, 68, 140–143], C2H2

+–Arn [144, 145],Bz+–Ln [33, 38, 44, 146] and BzH+–Ln [71, 92], in which first the availablep/π-binding sites are occupied before H-bonding sets in. In general, the solva-tion of an ion by nonpolar ligands exhibits strongly noncooperative three-bodyeffects, which mainly arise from the nonadditivity of the induction forces [19,22, 147, 148]. For example, in A+–Ln complexes, the dipole moments on theligands L induced by the central positive charge of A+ are oriented in unfa-vorable configurations, resulting in repulsive three-body contributions to theintermolecular potential.

The analysis of photofragmentation branching ratios observed in the IRPDprocess of larger A+–Ln clusters described in Eq. (4) enables the estimationof incremental ligand binding energies. For all cluster systems investigated,the range of fragment channels (m) observed for a given parent cluster (n)is rather narrow [22, 27, 29, 31–33, 35, 68, 72, 91, 92, 127, 128, 130–137] – seeFig. 3(c) for the case of Ph+–(N2)3 – and this information can be used to ex-

148 O. Dopfer

Table 3. Experimental dissociation energies (D0 in cm−1) of π-bound and H-bound com-plexes of Ar and N2 with various aromatic A(H)+ cations.

A(H)+ Ar(π) Ar(H) N2(π) N2(H)

Bz+ 512±3 [199]BzH+ ∼ 800 [92]c-C3H3

+ 860±170 [137] 900±130 [137]An+ 414±28 [58] 700±200 [29] 1100±300 [29]In+ 537±10 [45] 670±140 [31] 600±100 [31] 1475±825 [31]1-Np+ 650±150 [32] 650±150 [32] 1250±750 [32]p/o-PhH+ 600±100 [91] 650±150 [91] 750±150 [91] 1300±350 [91]Ph+ 535±3 [79] 670±140 [27] 750±150 [27] 1640±10 [113]O-PhH+ 650±200 [91] 1150±300 [91] 950±150 [91] 3400±150 [91]

tract ligand binding energies within the framework of a simple model. Thismodel relies on the following approximations. (i) The difference in the in-ternal energies of the parent cluster and the fragmentation products as wellas the kinetic energy release are neglected. (ii) Only single ligands and nolarger Lk oligomers are evaporated. (iii) Ligands with smaller binding en-ergies are evaporated first. (iv) Three-body forces are assumed to be small.(v) The whole absorbed photon energy (single photon absorption) is availablefor ligand evaporation. (vi) Ligands at similar binding sites are assumed tohave the same binding energy. Table 3 summarizes the dissociation energiesD0(H) and D0(π) of a variety of A(H)+–Ar/N2 complexes derived from theIR photofragmentation data and otherphotoionization techniques. In general,these experimentalD0 values are in good agreement with binding energies ofthe corresponding dimer potentials (De) derived from quantum chemical cal-culations (Table 2).

The following important trends can be extracted from Table 3. First, theinteraction ofπ-bound ligands with A(H)+ appears to be relatively insen-sitive to the details of A(H)+, such as substitution of functional groups orprotonation. This observation suggests that dispersion interactions betweenL and theπ-electron system of the aromatic ring provide still a major con-tribution to the attraction. Theπ-bonds to Ar have dissociation energiesof the order of 500 cm−1. The π-bonds of A(H)+ to N2 are stronger andof the order of 800 cm−1, because of additional electrostatic forces aris-ing from the quadrupole moment of N2. For all acidic A(H)+ ions, theH-bonds to both Ar and N2 are stronger than the correspondingπ-bonds.Moreover, in contrast toπ-bonds, the strengths of these H-bonds stronglydepend on the acidity of the A(H)+ ion, which increases in the orderBz+ < BzH+ < c-C3H3

+ < An+ < In+ < 1-Np+ < p/o-PhH+ < Ph+ < O-PhH+

(see also Figs. 11, 13, 14, and Table 2). Apparently, the acidity of X–H bondsin these A(H)+ ions increases in the order X= C< N < O.


Fig. 15. IRPD spectra of Bz+–Ar, Bz+–N2, and Bz+–(CH4)1–4 in the C–H stretch range.The Bz+–(CH4)4 spectrum is recorded in the Bz+–CH4 fragment channel, whereas allother spectra are monitored in the Bz+ channel [33].

Sect. 3.1 has been dealing with the interaction between acidic aromaticions and nonpolar ligands. For completeness, these results shall briefly becontrasted with the interaction of bare aromatic hydrocarbon ions and nonpo-lar ligands. IRPD spectra of Bz+–Ln with L = Ar, N2, and CH4 in the C–Hstretch range are reproduced in Fig. 15 [33]. They demonstrate that the C–Hstretch spectrum of the Bz+ core is independent of the type and number ofligands, suggesting that these ligands preferπ-bonds to Bz+ over H-bonds.These conclusions are in line with available photoionization and quantumchemical data [38, 73, 146, 149]. Similarly, the most stable complexes of BzH+

with nonpolar ligands featureπ-bonds [71, 92]. These results illustrate thatthe C–H bonds in unsubstituted (protonated) aromatic hydrocarbon cations areonly weakly acidic, so that dispersion forces (favoringπ-bonding) overridethe induction forces (favoring H-bonding) even in the cationic ground state.Hence, ionization of Bz–Ln clusters with nonpolar ligands does not signifi-cantly change the preferred binding pattern between the aromatic molecule andthe hydrophobic environment. This is in contrast to the interaction with a polarsolvent, which is the topic of the next section.

3.2 Complexes of bare aromatic hydrocarbon cations with polarligands

This section describes IRPD spectra and quantum chemical calculations ofclusters of the benzene cation (Bz+) solvated by several water (W) or methanol

150 O. Dopfer

(M) ligands, Bz+–Wn and Bz+–Mn [34, 35, 93, 150–152]. These clusters rep-resent benchmark systems for the interaction between an aromatic hydrocar-bon cation and polar ligands. Similar to the A+–Ln cluster systems charac-terized in Sect. 3.1, the Bz(+)–Ln complexes exhibit a large change in thepreferred solvation structure upon ionization. Such interactions are relevantfor biophysical phenomena where, for example, positively charged aromaticbiomolecular building blocks are surrounded by a well-defined number of po-lar solvent molecules [1, 5]. Aromatic molecules occur in nearly every facetof biochemistry and water is the major solvent in biological systems. Further-more, aromatic cations are highly reactive in aqueous solution (e.g., protonand/or electron transfer from A+ to the solvent) [153–157], whereas they formnonreactive stable complexes with only one water ligand. Hence, their reactiv-ity toward the solvent depends on the degree of hydration, that is intraclusterchemical reactions require a certain critical amount of hydration. Spectro-scopic, mass spectrometric, and quantum chemical studies of isolated Bz+–Ln

clusters are ideal tools to characterize and understand these phenomena at themolecular level.

3.2.1 Bz+–L dimers

Neutral Bz–Ln (L = W, M) complexes have attracted much interest in re-cent years, because they represent prototypes for neutral aromatic–hydrophilicinteractions [36, 158–161]. Spectroscopic [36, 158–163] and theoretical [161,164–168] studies demonstrated that the equilibrium structure of Bz–W fea-tures an archetypalπ H-bond, in which one proton of W binds to the aromaticπ electron system of Bz (Fig. 16(a)). However, the complex is highly nonrigid,and low barriers for various internal motions lead to a vibrationally averagedstructure, in which both protons of W are equivalent (Fig. 16(b)). The most ac-curate measured dissociation energy,D0 = 2.44±0.09 kcal/mol [169], agreeswithin chemical accuracy with the value obtained at sophisticatedab initiolevels,D0 = 2.9±0.2 kcal/mol [165].

In contrast to neutral Bz–W and Bz–M, theoretical and spectroscopic in-formation about the structure and interaction strength in the correspondingcation clusters has been rather sparseprior to the recent IRPD studies [34,35, 93, 150–152]. Theoretical studies of Bz+–W [34, 93, 169–171] consideredthe three minima on the intermolecular potential shown in Fig. 16(c–e). TheH-bound structure in Fig. 16(c), in which W approaches Bz+ in the aromaticplane to form two H-bonds between the lone pairs of oxygen and adjacent pro-tons of Bz+, is calculated as the global minimum, withDe values ranging from9 to 14 kcal/mol. The C-bound andπ-bound structures in Fig. 16(d,e) are pre-dicted to be less stable, with binding energies of≈ 9 and≈ 7±1 kcal/mol,respectively. Significantly, all three structures are mainly stabilized by thecharge–dipole attraction between the positive charge distribution in Bz+ andthe dipole moment of W. As a consequence, they are characterized by charge–


Fig. 16. Sketch of various structures of Bz–W (a,b), Bz+–W (c–e), and Bz+–W2 (f,g) inthe ground electronic state. Neutral Bz–W prefers the formation of aπ H-bond, whereasthe favorable structures of the Bz+–W cation are charge-dipole geometries with the Oatom of W pointing toward the positive charge of Bz+. The H-bound structure (c) is cal-culated to be the global minimum of Bz+–W, whereas the C-bound (d) andπ-bound (e)geometries are less stable configurations. The two Bz+–W2 structures visualize the com-petition of hydration of an interior Bz+ (f, isomer class I) and the formation of a H-bondedwater network (g, isomer class II).

dipole configurations, in which the oxygen of W points toward the Bz+ charge.Hence, ionization of the aromatic solute has drastic effects on the topology ofthe intermolecular potential, including both the structure and interaction energyof the global minimum, that is, one observes an ionization-induced switch inthe preferred binding motif for this type of interaction. The largely differentbinding energies and equilibrium structures of Bz–W and Bz+–W lead to in-teresting photoionization and subsequent fragmentation dynamics. Accordingto the FC principle, vertical ionization of cold Bz–W mainly accesses repul-sive parts of the Bz+–W potential, leading to extensive fragmentation [169,170, 172]. The large geometry change upon ionization prevents the prepar-ation of significant concentrations of cold Bz+–W cations by REMPI [36].Such photoionization schemes have frequently been used for the spectroscopyof a variety of H-bound acidic aromatic molecule–water cations, includingPh+–W [17, 40, 41, 45, 47, 48, 116, 173] and In+–W [45, 119], which experi-ence only small structural changes upon ionization. However, this strategy hasfailed to prepare cold water clusters of (halogenated) Bz+ due to the largeionization-induced geometry changes [36]. As an alternative to photoioniza-tion of the neutral precursor, the EIcluster ion source is able to producelarge quantities of cold Bz+–W dimers. With this strategy, the fundamentalBz+–W interaction could recently be characterized for the first time by IRPDspectroscopy [34, 35], yielding direct information of both the structure anddissociation energy of this prototype interaction. Using a similar pick-up ionsource, which employs REMPI rather than EI to produce cold Bz+ monomer

152 O. Dopfer

Fig. 17. IRPD spectra of Bz+–W= Bz+–H2O (a), Bz+–HDO (b), and Bz+–W–N2 (c) [34].Spectra (a) and (b) are recorded in the Bz+ fragment channel, whereas spectrum (c) isobtained in the Bz+–W channel. The two transitions of Bz+–W and Bz+–W–N2 are as-signed to the symmetric and antisymmetric O–H stretch vibrations of W by compari-son with the O–H stretch fundamentals of bare W (indicated by arrows,ν1 = 3657 cm−1,ν3 = 3756 cm−1). The single band in the Bz+–HDO spectrum is assigned to the O–Hstretch mode.

ions in the high-pressure region of the expansion (for subsequent complexa-tion), IRPD spectra of cold Bz+–W and related complexes could recently berecorded as well [93, 150–152].

Fig. 17(a) shows the IRPD spectrum of Bz+–W in the vicinity of the O–Hstretch fundamentals of bare water (ν1,3). It displays two transitions, which are,by comparison withab initio calculations, assigned to theν1,3 bands of the moststable H-bound Bz+–W isomer shown in Fig. 16(c) [34]. The small red shiftsupon complexation demonstrate that both OH groups of W are free and notengaged in H-bonding, consistent witha charge–dipole geometry. They alsoimply that the O–H bonds of W become slightly weaker and more acidic uponcomplexation with Bz+. The fact that the Bz+–HDO spectrum in Fig. 17(b)displays only a single transition in the O–H stretch range confirms that onlyone isomer contributes to both the Bz+–W and the Bz+–HDO spectrum [34].The Bz+–W–N2 spectrum in Fig. 17(c) is similar in appearance to the Bz+–Wspectrum, except that theν1,3 bands are much narrower. As outlined in detailin Sect. 2 and Fig. 4 for the case of Ph+–W(–Ar), the attachment of a weaklybound messenger ligand reduces both the internal temperature and the effectivedissociation energy leading to sharper transitions. Consequently, complexationof Bz+–W with N2 ensures that all isomers of Bz+–W (even strongly boundones) are detected in the Bz+–W–N2 spectrum because the N2 dissociation en-ergy lies well belowν1,3 vibrational energies for all of them [33, 34]. As theBz+–W–N2 spectrum merely displays two narrow bands, only one Bz+–W iso-mer is present in the supersonic plasma expansion, namely the most stableH-bound one. The absence of the less stable Bz+–W isomers in the expansion


Table 4. Frequencies of the O–H stretch vibrations under warm (w) and cold (c) condi-tions and binding energies of H-bound aromatic ion–H2O dimers. Frequency shifts com-pared to free H2O (ν1 = 3657 cm−1, ν3 = 3756 cm−1) are listed in parentheses.

A+–H2O ν1(∆ν1) [cm−1] ν3(∆ν3) [cm−1] D0 [cm−1] References

Bz+–H2O 3637 (−20, w) 3718 (−38, w) 3290±120 [34, 151]3639 (−18, c)(a) 3722 (−34, c)(a)

In+–H2O 3636 (−21, w) 3720 (−36, w) 4790±10 [45, 119]3641 (−16, c) 3725 (−31, c)

Ph+–H2O 3619 (−38, w) 3702 (−54, w) 6520±50 [45, 75, 76]3624 (−33, c)(b) 3707 (−49, c)(b)

(a) Approximated by the Bz+–H2O–N2 transitions.(b) Approximated by the Ph+–H2O–Artransitions (Fig. 4).

can be attributed to their significantly lower stabilization energies and/or lowisomerization barriers toward the most stable H-bound minimum [34].

The complexation-induced red shifts in aromatic ion–W dimers,∆ν1,3, area sensitive probe of the intermolecular bond strength. Table 4 compares the∆ν1,3 shifts of Bz+–W with those of Ph+–W and In+–W, for which precise ex-perimental dissociation energies are available from MATI spectroscopy [45].The Bz+–W and In+–W shifts are comparable suggesting that both complexeshave similar binding energies, leading to an estimate ofD0 = 14±3 kcal/mol(4900±1050 cm−1) for Bz+–W [34]. The analysis of very recent IRPD spectraof Bz+–W–Ar yields, however, a somewhat smaller but more accurate value,D0 = 9.4±0.34 kcal/mol (3290±120 cm−1) [151], indicating that the∆ν1,3

shifts slightly overestimate theD0 value for Bz+–W. This discrepancy mayarise from the special nature of the doubly H-bonded Bz+–W cation–dipoleconfiguration. Ph+–W features a stronger H-bond and correspondingly larger∆ν1,3 shifts than both Bz+–W and In+–W. In general, the A+–W binding ener-gies are correlated with the acidity of the A+ ion, which rises in the order Bz+

< In+ < Ph+ (see also Table 2 and Figs. 11 and 13).The Bz+–M spectrum shown in Fig. 18 displays a single broad band in

the O–H stretch range, which is assigned to the free O–H stretch of a H-bound dimer with a structure similar to that of Bz+–W in Fig. 16(c) [35]. Therelative complexation-induced red shift is slightly larger for Bz+–M than forBz+–W, consistent with a stronger intermolecular bond arising from a largerPA of the ligand (PA= 754 and 691 kJ/mol for M and W [124]). A simi-lar trend is observed for the related H-bonded dimers involving the phe-nol cation [17, 40, 115, 174]:D0 = 18.54±0.11 and 21.40±0.18 kcal/molfor Ph+–W and Ph+–M, respectively [174]. The IRPD spectra of Bz+–W,Bz+–W–N2, and Bz+–M recorded in the C–H stretch range are also shownin Fig. 18 [35]. Although less informative, they confirm the conclusions de-rived from the O–H stretch spectra. The Bz+–W(–N2) spectra display a single

154 O. Dopfer

Fig. 18. IRPD spectra of Bz+–W–N2, Bz+–Wn (n = 1–4), and Bz+–M in the C–H (left)and O–H stretch (right) ranges. The Bz+–W–N2 spectrum is recorded in the Bz+–W frag-ment channel, whereas those of Bz+–Wn and Bz+–M are measured in the Bz+–Wn−1 andBz+ fragment channels, respectively. Corresponding transitions are connected by dottedlines. The arrows indicate the symmetric and antisymmetric O–H stretch vibrations ofbare W (νW

1 = 3657 cm−1, νW3 = 3756 cm−1 [77]), the O–H stretch fundamental of bare

M (νM1 = 3681 cm−1 [196]), the intense C–H stretch band of bare Bz+ estimated from

Bz+–Rg spectra (νBz+20 and/or νBz+

13 at ≈ 3195 cm−1 [33, 146, 197]), and the C–H stretchfundamentals of bare M (νM

2 = 3000 cm−1, νM3 = 2844 cm−1, νM

9 = 2960 cm−1 [196]). Thebands marked by filled and open circles are assigned to class I isomers (hydration of in-terior Bz+) and class II isomers (formation of a H-bonded water network) of Bz+–Wn,respectively. The signals marked by an asterisk are tentatively attributed to [Bz–W4]+

isomers, in which the proton is transferred from Bz+ to W4, i.e. C6H5–W4H+ (class IIIisomers).

band in the C–H stretch range, which is assigned toν13/20 of Bz+ (νBz+CH ). The

corresponding frequency in isolated Bz+ can be estimated from the IR spec-tra of the weaklyπ-bound Bz+–L dimers shown in Fig. 15. The influence offormation of the two nonlinear C–H· · ·O H-bonds to W onνBz+

CH is surpris-ingly small (< 10 cm−1). However, its IR intensity is considerably enhancedupon hydration of Bz+, and this effect has been taken as strong evidencefor the formation of the C–H· · ·O H-bonds [93]. The Bz+–M spectrum inFig. 18 shows additional absorptions in the C–H stretch range arising fromthe CH3 group of M. They are barely shifted from those of free methanol,confirming that the CH3 group is not involved in the intermolecular bond ofBz+–M [35].

3.2.2 Larger Bz+–Ln clusters

Comparison of larger neutral and ionic Bz(+)–Ln clusters will reveal the drasticeffect of ionization on polar solvation of bare aromatic hydrocarbon molecules.


One fundamental aspect is the competition between interior solvation of thearomatic solute and the formation of a H-bonded ligand network (surface sol-vation). This competition will strongly be influenced by the relative magnitudeof the Bz(+)–L and L–L interactions, as well as the nonadditive three-body con-tributions to the interaction. All these factors largely depend on the charge stateof Bz(+) and the solvent.

IR spectra of neutral Bz–W2, Bz–W3–5, and Bz–Wn≥6 complexes were in-terpreted by cluster structures, in which a W2 dimer, a cyclic W3–5 cluster, anda larger three-dimensional H-bonded Wn≥6 network are weaklyπ H-bonded toBz, respectively [36, 158–160]. IR spectra of all Bz–Wn clusters investigatedto date (n ≤ 9 [175]) were explained with geometries, in which a more or lessunperturbed Wn cluster is attached to one side of the aromatic Bz plane, that isBz is surface solvated to Wn [160, 176]. Such Bz–Wn structures are in line withthe hydrophobic character of Bz and can be rationalized by the fact that theW–W interaction (De ≈ 5 kcal/mol [177]) is slightly stronger than the Bz–Winteraction (De ≈ 4 kcal/mol [165]). In general, the H-bonds in Bz–Wn tend tobecome stronger as the cluster size increases, reflecting the cooperative natureof the intermolecular forces inthis cluster type [159, 160]. Bz–Mn complexesdiffer in various aspects from the corresponding Bz–Wn clusters, because theCH3 group of M cannot participate in H-bonding [158, 178, 179]. For example,although Bz–M has also aπ H-bonded equilibrium structure, the complex ismore rigid than Bz–W, because reduction of symmetry and additional sterichindrance quenches the low-barrier internal motions present in Bz–W. More-over, Bz–Mn with n = 2 and 3 feature chain-like Mn clusters which areπH-bonded to Bz, whereas only forn > 3 cyclic Mn subclusters are observed.These studies on neutral Bz–Ln clusters demonstrated that IR spectroscopy inthe O–H stretch range provides a sensitive probe of the details of the structureand interaction of the H-bonded ligand network. A similar strategy has beenfollowed recently to characterize the corresponding cationic Bz+–Ln speciesfor the first time [35, 150, 152, 180].

As the Bz+–L interaction is significantly stronger than the L–L interac-tion, the most stable Bz+–Ln clusters were expected to have structures in whichthe ligands L are solvated around an interior Bz+ cation. This is in strikingcontrast to neutral Bz–Ln clusters, which prefer geometries in which Bz is at-tached to the surface of a Ln network. Moreover, induction forces are highlynonadditive and provide a significant contribution to the interaction in chargedcomplexes [19, 22, 147, 148]. Thus, the Bz+–Ln spectra probe also the degreeof (non)cooperativity of the three-body interactions in this type of clusters.

The IR spectrum and quantum chemical calculations clearly demonstratethat the H-bound isomer in Fig. 16(c) is the most stable Bz+–W structure,with a dissociation energy ofD0 = 9.4±0.34 kcal/mol [34, 93, 151]. The lat-ter value agrees well with a very recent determination of the binding enthalpyby mass spectrometry (−∆H 0 = 9.0±1.5 kcal/mol) [181]. On the other hand,the W2 dimer has a H-bonded equilibrium structure, in which one H atom

156 O. Dopfer

of the proton donor forms a nearly linear H-bond to the O atom of the pro-ton acceptor [182, 183]. The best experimental estimate for the W2 bindingenergy is probablyD0 = 3.34±0.7 kcal/mol [184, 185]. Thus, the W–W inter-action is roughly three times weaker than the Bz+–W attraction. Consequently,the most stable Bz+–W2 structure is expected to be one in which two singleW molecules form separate H-bonds to Bz+. One example of such a struc-ture, representing interior Bz+ hydration (denoted isomer class I), is shownin Fig. 16(f). As Bz+ offers six such bifurcated H-bonding sites for W lig-ands, the first “planar” hydration subshell around an interior Bz+ is closed atBz+–W6. The sequence of filling this subshell is not obvious and several iso-mers lying close in energy may exist. Simple electrostatic considerations of thedipole–dipole (µ−µ) interactions suggest that the hydration energy of possibleBz+–W2 isomers decreases in the order para> meta> ortho for the bindingsites of the two W ligands [35]. As the individual W ligands in the Bz+–Wn

structures featuring interior Bz+ hydration are (nearly) equivalent and the inter-action between the ligands is much smaller than the interaction with the centralcation, the mid-IR spectra of these class I isomers closely resemble that of theH-bound Bz+–W dimer. Indeed, the Bz+–W2–4 spectra in Fig. 18 show two in-tense bands in the O–H stretch range (filled circles), which are assigned to theν1,3 modes of the equivalent W ligands of the class I isomers [35]. Closer in-spection reveals that their band centers demonstrate small but noticeable blueshifts asn increases, indicating that theintermolecular H-bonds in Bz+–Wn be-come significantly weaker when the number of W ligands is increased in thefirst hydration shell. For example, the incremental blue shifts forn = 2 (∆ν3

and∆ν1 = 6 and 5 cm−1) reduce the absolute red shifts induced by the firstligand in Bz+–W (−38 and−20 cm−1) by 16 and 25%, respectively, empha-sizing the remarkable influence of the second ligand on the interaction withthe first ligand. Several factors may contribute to this large noncooperative ef-fect. First, the permanent and induced dipole moments (µ) of the W ligandsare not aligned in an optimalµ−µ orientation, producing arepulsive contribu-tion of theµ−µ interaction to the total binding energy. In addition, structuralreorganization of Bz+ upon sequential hydration may cause noncooperativeeffects. Complexation of Bz+ by the first W induces significant changes ofits structure. For example, the compressed form of Bz+ (which is Jahn–Tellerdistorted in its2E1g electronic ground state [186]) is calculated to be slightlymore stable than the elongated form, whereas the reverse situation is predictedfor Bz+–W [171]. It is likely that the structural reorganization of Bz+ inducedby the first W is not favorable for the interaction with the second W, caus-ing a decrease in the average ligand binding energy in Bz+–W2. Moreover, thewidths of the Bz+–Wn>1 transitions are smaller than those of the correspondingBz+–W bands, providing further evidence that the strength of the intermolecu-lar bonds in the class I isomers of Bz+–Wn decreases withn. To cleave weakerbonds by IR absorption requires less internal energy prior to photoexcitationand, as a consequence, photodissociation spectra with lower effective rotational


and vibrational temperatures are obtained. The IR spectrum of Bz+–M2 (notshown) reveals also the presence ofclass I isomers [35]. The magnitude ofthe noncooperativity in Bz+–M2 is found to be similar to that observed forBz+–W2.

In addition to theν1,3 transitions assigned to thefree O–H stretch vibra-tions of the equivalent W ligands of the class I isomers (interior Bz+ hydration),the Bz+–W2,3 spectra in Fig. 18 show additional bands in the O–H stretchrange (open circles), which are not present in the Bz+–W spectrum. They areattributed to class II isomers, in which a perturbed H-bonded Wn network clus-ter is attached to Bz+ via bifurcated H-bonding. As an example, Fig. 16(g)shows the structure for the class II isomer of Bz+–W2. The band at 3494 cm−1

in the Bz+–W2 spectrum displays a large red shift of−213 cm−1 from theaverage O–H stretch frequency of isolated W (3707 cm−1) and provides thusclear evidence that one of the W molecules in the considered Bz+–W2 iso-mer acts as a proton donor in a H-bond. Isolated W2 features a nearly linearO–H· · ·O H-bond, that is one W acts as the proton donor and the other oneas the proton acceptor [182, 183]. Three of the four O–H stretch frequenciesof W2 could be determined as 3735 (free donor stretch), 3601 (bound donorstretch), and 3745.5 cm−1 (antisymmetric acceptor stretch), whereas that ofthe only weakly IR active symmetric acceptor stretch was estimated as 3655–3660 cm−1 [182, 187]. Hence, the red shift of the bound O–H stretch of theproton donor of W2 in Bz+–W2 (−213 cm−1) is twice the corresponding shiftof bare W2 (−106 cm−1), implying that the presence of Bz+ drastically en-hances the strength of the H-bond in the W2 moiety. This large cooperativethree-body effect is not surprising because Bz+ increases the acidity of theO–H bonds of the first W ligand, as is deduced from the∆ν1,3 red shifts ofBz+–W. Moreover, the positive charge in Bz+ increases the effective dipolemoment of the first W ligand (by charge–induced dipole interaction), whichin turn provides an additional contribution to the W–W attraction in Bz+–W2.The other isolated spectroscopic signature of the class II isomer of Bz+–W2

near 3690 cm−1 is probable due to the free O–H stretch of the donor. Similarto Bz+–W2, the Bz+–W3 spectrum displays a broad band in the low-frequencyO–H stretch range, which clearly indicates the presence of an isomer in whichat least one W ligand acts as a proton donor. The bands marked with an opencircle are thus again attributed to free and bound O–H stretch modes of vari-ous isomer type II structures, which show at least the onset of the formation ofa water ligand network. Several such structures may be considered. The proba-bly most stable type II isomers are those which have a W2 dimer and a singleW molecule separately attached to two of the six available H-bonding sitesof Bz+. These isomers are expected to have similar O–H stretch spectra ofBz+–W2 (type I plus type II), as the W2 unit is not interacting much with thethird W ligand. A second group of presumably less stable type II isomers ischaracterized by geometries in which a W3 complex is linked to Bz+. The W3

unit can have either a chain-like structure, with the terminal W ligand forming

158 O. Dopfer

a H-bond to Bz+, or a branched configuration, in which two single W ligandsare H-bonded to the two protons of the first W ligand [48]. The branched struc-ture is likely to be more stable than the chained one, because of the shorteraverage separation of the W ligands from the Bz+ charge. In contrast to isolatedcyclic W3 [188], a Bz+–W3 structure with a W3 ring appears to be an unfavor-able configuration because one W ligand needs to act as a double donor in thelatter system. Unfortunately, the quality of the Bz+–W3 spectrum is insufficientto determine which specific type II isomer is actually present in the expan-sion and higher quality spectra and reliable high-level quantum calculations arerequired to determine the geometries of these clusters.

The IR spectra of Bz+–W2,3 clearly reveals the presence of two classes ofisomers, which raises the interesting question of their relative stabilities. On thebasis of the dimer interactions alone (D0 = 9.4±0.34 and 3.34±0.7 kcal/molfor Bz+–W and W–W, respectively), isomers I are predicted to be much morestable than isomer II (by≈ 18.8−12.7 = 6.1 kcal/mol). On the other hand,cooperative and noncooperative (three-body) effects are observed to becomeimportant for both isomer classes as the number of ligands in the cluster isincreased. Apparently, these are extremely cooperative for the W–W bond inisomer II of Bz+–W2, whereas they are significantly noncooperative for theBz+–W bond in isomer I of Bz+–W2. Consequently, the difference in the sta-bility of both types of isomers is possibly much smaller than predicted bythe dimer bond strengths alone. It is difficult to reliable estimate their relativestabilities from the experimental IR spectra, because the observed band inten-sities are influenced by several factors. First, photodissociation spectra do notdirectly reflect the absorption crosssection but correspond to the product ofabsorption and dissociation cross sections. The latter may sensitively dependon the type of isomer and also on the type of vibration because of differentdissociation energies and predissociation dynamics. For example, the signalsof an isomer with lower binding energy and larger dissociation rates will beenhanced compared to those from another isomer with a stronger bond andlower dissociation rates. Moreover, the IR intensities are difficult to analyze ina quantitative fashion because the contributions of sequence hot bands and fun-damental transitions cannot easily be separated. In addition, reliable quantumchemical calculations of IR intensities are challenging for complexes com-posed of Bz+ because of the degeneracy of its2E1g electronic ground stateand the resulting dynamic Jahn–Teller distortion [149, 189]. Such calculationsare, however, required for a quantitative analysis of the O–H stretch intensi-ties, because Bz+ complexation has a large effect on the IR intensities of boththe W and Bz+ vibrations [34, 93, 171]. Finally, even if it were possible toreliably extract the relative abundance of both isomer types from the IR spec-tra, it would not be possible to use this information to evaluate their relativestability because their concentration in the molecular beam is not only con-trolled by thermodynamic considerations. The competing production schemesdescribed in Eqs. (2) and (3) suggest that kinetic factors may also play a sig-


nificant role in the determination of the production ratio of certain isomers.For example, isomers I of Bz+–W2 are predominantly produced by sequen-tial three-body association of two single W ligands to Bz+ (Eq. 2). Similarly,isomer II of Bz+–W2 can be generated via this route, although the probabilityfor the second W to bind to the first W in Bz+–W is much lower than bind-ing directly to Bz+ because of both energetic and entropic arguments. However,significant concentrations of isomer II of Bz+–W2 may also be produced by at-taching a preformed neutral water cluster to Bz+ with subsequent stabilizationby collisional or evaporative cooling (Eq. 3). The relative importance of thedifferent production pathways depends on the expansion conditions and maylead to isomer concentrations which are rather different from those predictedby thermodynamic considerations alone. Recently, the incremental associationenthalpies of Bz+–Wn have been determined by mass spectrometry [181]. Thevalues of−∆H 0 = 9.0±1.5 and 8.0±1.5 kcal/mol were obtained forn = 1and 2. However, this experiment is based on mass spectrometry only and doesnot provide any isomer specific information. Thus, it is not clear whether thevalue forn = 2 corresponds to class I or class II isomers of Bz+–W2.

In contrast to Bz+–W2,3, the Bz+–W4 spectrum lacks the spectroscopic sig-nature of the class II isomers in thelow-frequency O–H stretch range near3500 cm−1. This observation is attributed to intracluster proton transfer in theclass II isomer of Bz+–W4 from Bz+ to the W4 cluster [35, 150]. Consequently,these isomers are better described as C6H5–W4H+ (class III isomers), that iscomplexes of the phenyl radical with a protonated water cluster. The PA ofWn clusters drastically increases with cluster sizen: PA= 167±1, 197±1,210±1, and 218±3 kcal/mol for n = 1–4, respectively [180]. Thus, forn ≥ 4the PA of Wn exceeds the PA of the phenyl radical, implying that intraclusterproton transfer in [Bz–Wn]+ is exothermic for clusters larger than this criticalsize. Both IR and electronic spectroscopy have recently confirmed the occur-rence of intracluster proton transfer forn ≥ 4. The IR spectra of [Bz–Wn≥4]+

reveal the unambiguous spectroscopic signatures of WnH+ [150], whereasthe electronic spectra show fingerprint absorptions of the C6H5 radical [180].Both observations provide convincing evidence for the C6H5–WnH+ type com-position of [Bz–Wn≥4]+ [180]. The unambiguous fingerprint IR absorptionsof WnH+ in C6H5–W4H+ occur in the bound O–H stretch range near 3000and 3200 cm−1 and are outside of the spectral range investigated in Fig. 18.Nonetheless, the absorptions in the free O–H stretch range of the [Bz–W4]+

spectrum marked by asterisks are tentatively attributed to the C6H5–W4H+ iso-mers. The recent mass spectrometric data are also consistent with intraclusterproton transfer for [Bz–Wn≥4]+ clusters [181]. Interestingly, the IRPD spectraof [Bz–Wn ]+ in the size rangen = 4–21 [152] are found to be quite similar tothose of bare Wn+1H+ clusters [61, 62], demonstrating that the phenyl radicalhas essentially no influence on the water network [152]. This observation is ex-pected because, on the basis of “microscopic hydrophobicity” [152], the C6H5

radical is pushed to the surface of the protonated water cluster.

160 O. Dopfer

The large effect of ionization on the Bz–W interaction (energy and geom-etry) causes drastic differences between the cluster growth in Bz+–Wn andBz–Wn, including the dimer geometry as well as the structure of the hydrationshells. For neutral Bz–Wn, the solute–solvent (Bz–W) interaction is weakerthan the solvent–solvent (W–W) interaction, leading to cluster structures inwhich Bz is surface solvated to a Wn network (hydrophobic behaviour of Bz).In contrast to the weakπ H-bond in neutral Bz–W, the charge–dipole bond ofthe Bz+–W cation is much stronger than the W–W interaction. Consequently,the probably most stable Bz+–Wn clusters (isomer type I) prefer interior Bz+

solvation (hydrophilic behaviour of Bz+) at least for small cluster sizes. Inaddition, the Wn network in neutral Bz–Wn differs from those in the type IIisomers of Bz+–Wn. Whereas in neutral Bz–Wn a freely dangling H atom ofa barely perturbed Wn cluster isπ H-bonded to Bz, it is the O atom of thefirst W ligand that forms a H-bond to Bz+ in Bz+–Wn. Consequently, the waternetwork in Bz+–Wn differs largely from that in bare Wn clusters. Similar con-clusions derived for the difference between Bz–Wn and Bz+–Wn apply also toBz–Mn and Bz+–Mn and probably quite generally to bare neutral and cationaromatic hydrocarbon molecules interacting with polar solvents. Similar toionization, also protonation of Bz is expected to have a dramatic effect on itsmicrohydration. Initial spectra have already been obtained for the [Bz–W]H+

dimer and could identify an isomer in which the proton is localized on Bz,e.g.,BzH+–W [92]. Such a configuration is in line with the PA values of Bz andW (750 and 691 kJ/mol [124]). Currently, IRPD and mass spectrometric stud-ies are underway to explore the proton delocalization in larger [Bzm–Wn]H+

clusters as a function of the number of Bz and W molecules in the complex.Sect. 3.2 has been dealing with the interaction between a bare aromatic

hydrocarbon cation (Bz+) with polar ligands. Comparison of Bz+–Ln (L =W, M) with corresponding clusters of the phenol and aniline cations (Ph+–Ln

and An+–Ln [17, 40, 45, 47, 48, 115, 116, 173, 174, 190–192] reveals the effectof substitution of acidic functional groups (OH, NH2) on the solvation of anaromatic hydrocarbon cation by polar ligands. The Ph+–L and An+–L dimersfeature nearly linear H-bonds between the protons of the polar group and theO atom of L= W and M (Fig. 4). These charge–dipole structures are similarto the corresponding H-bound Bz+–L dimers, with the main difference that thepolar group enhances the interaction strength. All spectroscopic data for largerPh+–Ln (n ≤ 8 for L = W, n ≤ 1 for L = M) and An+–Ln (n ≤ 6 for L = W,n ≤ 2 for L = M) clusters have been interpreted with geometries in which theligands solvate the polar group, that is the aromatic ion is surface solvated tothe Ln network. No evidence was presented for isomers in which a ligand isattached to the aromatic ring, such as in Fig. 16(c–g). This is in striking con-trast to the Bz+–Ln clusters. Consequently, one may conclude that substitutionof a H atom by a polar acidic group has a drastic effect on the competitionbetween interior solvation of the aromatic cation and the formation of theH-bonded Ln network. On the other hand, one has to bear in mind that the stud-


ies of the Ph+–Ln and An+–Ln clusters with largern used REMPI [48, 192],which may not necessarily produce the most stable cluster ion structures.

Aromatic cations are known to be highly reactive in aqueous solution [153–156]. The PA of the phenyl radical is much higher than those of W and M(PA= 691, 753, and 844 kJ/mol for W, M, and C6H5 [124]). Moreover, the ion-ization potential of Bz is lower than those of W and M (IP= 9.2, 10.9, and12.6 eV for Bz, M, and W [193]). Consequently, in the most stable Bz+–L(L = W, M) dimers the positive charge is largely localized in Bz+ and pro-ton transfer from Bz+ to L is not observed. The PA of larger Ln clustersincreases withn, whereas the IP decreases. Hence, both proton and chargetransfer from Bz+ to Ln become less endothermic for increasingn and at a cer-tain cluster size intracluster reactions can occur. Indeed, charge and protontransfer was observed in Bz+–Mn after photoionization forn ≥ 3 andn ≥ 4,respectively [157]. Similarly, proton transfer was observed in Bz+–Wn forn ≥ 4 [35, 150, 152, 180]. Due to the lower PA and higher IP, W is a less reac-tive solvent than M. Moreover, in Ph+–Wn proton transfer from Ph+ to the Wn

network was observed already forn ≥ 3 [47, 48, 173, 190], in line with fact thatBz+ is less acidic than Ph+.

4. Concluding remarks and outlook

This review summarizes recent progressachieved in the characterization ofthe microsolvation process of aromatic ions in both polar and nonpolar sol-vents by the fruitful application of spectroscopy, mass spectrometry, and quan-tum chemistry to size-selected A+–Ln clusters. Two prototype interactionsof aromatic ion–solvent recognition have been considered. The first class ofA+–Ln clusters discussed involves microsolvation of acidic aromatic cationsin a nonpolar hydrophobic solvent. The IR spectra provide detailed informa-tion about the competition between various fundamental ion–ligand bindingmotifs (H-bond versusπ-bond), their interaction strengths, the physical na-ture of these interactions (electrostatics, induction, dispersion), the sequenceof the preferred cluster growth within the first solvation shell, the influenceof degree and type of solvation and the substitution of functional groups onthe acidity of the central ion, and thedegree of noncooperativity arising fromnonadditive three-body induction forces. In general, H-bonding to all avail-able acidic protons is preferred overπ-bonding to the aromatic ring, yieldinga solvation sequence in which first all acidic protons are solvated before lessstable binding sites in the first solvation shell are occupied. The second class ofA+–Ln clusters considered describes themicrosolvation of bare aromatic hy-drocarbon cations in a polar hydrophilic solvent, which can form a H-bondedsolvent network. For this purpose, controlled microhydration of the benzenecation, Bz+–Wn, has served as a fundamental model system. The most sta-ble ion–ligand binding motif in Bz+–W and related dimers corresponds to

162 O. Dopfer

a charge–dipole bond supported by bifurcated H-bonding. Bz+–Wn clusterswith n = 2 and 3 exhibit strong competition between interior Bz+ hydra-tion (noncooperative) and the formation of a H-bonded Wn network to whichBz+ is surface solvated (cooperative). [Bz–Wn]+ clusters withn ≥ 4 demon-strate intracluster proton transfer from Bz+ to Wn, leading to the formation ofC6H5–WnH+ structures. The geometries and binding energies of the A+–Ln

clusters differ largely from those of the corresponding neutral A–Ln clusters.Thus ionization induces a switch in the preferred aromatic molecule–solventrecognition motif. An important aspect of the experimental strategy appliedin the present work has been the production of A+–Ln complexes in an EIcluster ion source, which generates predominantly the most stable isomer ofa given A+–Ln cluster ion. In this way, several fundamental binding motifscould be characterized for the first time, including the H-bonded structuresof acidic A+–L dimers with nonpolar L and the Bz+–W charge–dipole bond.These binding motifs have completely escaped previous photoionization stud-ies of A+–Ln, because of the restrictions arising from the Franck–Condonprinciple.

A straightforward extension of the present work is the application ofthe combined spectroscopic and quantum chemical strategy to the solvationof biomolecular building blocks by both polar and nonpolar ligands,e.g.,the microhydration of amino acids, peptides, and proteins, as well as DNAbases, base pairs, and larger DNA oligomers. Particularly interesting ques-tions include the influence of ionization, protonation, and the attachment ofmetal ions on the microhydration environment. Furthermore, the dynamicsof the ionization-induced switch in the preferred binding pattern (π → H)in these A+–L clusters, such asπ → H isomerization, may be followed inreal time using picosecond pump-probe spectroscopy [98, 170, 194]. Otherinteresting targets for the present combined experimental and theoretical ap-proach include the large effects of microhydration on chemical ion–moleculereaction mechanisms [6–8], such as proton transfer, electrophilic aromaticsubstitution, or nucleophilic displacement reactions. For example, protonatedaromatic molecules, AH+, are invoked as reactive intermediates in elec-trophilic aromatic substitution reactions, the probably most important reactionmechanism of aromatic molecules. Systematic studies of AH+–Wn clusterscan unravel the effects of stepwise microhydration on this mechanism. Re-cently, AH+–Ln clusters involving inert ligands have spectroscopically beencharacterized for the first time [66, 67, 71, 72, 89–92], providing direct struc-tural information about competing protonation sites in simple (substituted)aromatic molecules. Similar studies may be performed for cationic nucle-ophilic substitution reactions involving both aromatic [8] and nonaromatic [22]cations. The characterization of the extent of proton delocalization, protontransfer, and proton transport in small H-bonded solvent networks is a fur-ther important topic, which has recently been addressed by cluster ion IRspectroscopy [60–62, 195].


Acknowledgement

This work is part of the projects DO 729/2-1 and DO 729/2-2 funded bythe Deutsche Forschungsgemeinschaft (DFG). Support from the Fonds derchemischen Industrie is gratefully acknowledged. The author is presently sup-ported by the DFG via a Heisenberg Fellowship (DO 729/1-2). Some of theexperimental studies at the University of Basel were supported by the SwissNational Science Foundation. The author would like to thank his former andpresent PhD students, R. V. Olkhov, N. Solcà, and H.-S. Andrei, for their in-valuable contribution to this work, J. P. Maier (University of Basel) for hiscontinuous interest in this work, and E. Rühl and W. Kiefer (both Universityof Würzburg) for their generous hospitality. He also thanks K. Müller-Dethlefs(University of York) for pointing out the possibility for the ionization-inducedswitch in case of Ph–Ar at the International Symposium on Molecular Clustersin Niederpöcking in May 1999, which initiated many of the studies describedin this review.

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