Home > Documents > Infrared Spectroscopy of the Microhydrated Nitrate Ions ... · PDF fileInfrared Spectroscopy...

Infrared Spectroscopy of the Microhydrated Nitrate Ions ... · PDF fileInfrared Spectroscopy...

Date post: 02-Feb-2018
Category:
Author: doankhuong
View: 214 times
Download: 1 times
Share this document with a friend
Embed Size (px)
of 9 /9
Infrared Spectroscopy of the Microhydrated Nitrate Ions NO 3 - (H 2 O) 1-6 Daniel J. Goebbert, ‡,¶ Etienne Garand, § Torsten Wende, Risshu Bergmann, Gerard Meijer, Knut R. Asmis,* ,‡ and Daniel M. Neumark* ,§,Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany, Department of Chemistry, UniVersity of California, Berkeley, California 94720, and Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: February 24, 2009; ReVised Manuscript ReceiVed: April 15, 2009 We present infrared photodissociation spectra of the microhydrated nitrate ions NO 3 - (H 2 O) 1-6 , measured from 600 to 1800 cm -1 . The assignment of the spectra is aided by comparison with calculated B3LYP/aug- cc-pVDZ harmonic frequencies, as well as with higher-level calculations. The IR spectra are dominated by the antisymmetric stretching mode of NO 3 - , which is doubly degenerate in the bare ion but splits into its two components for most microhydrated ions studied here due to asymmetric solvation of the nitrate core. However, for NO 3 - (H 2 O) 3 , the spectrum reveals no lifting of this degeneracy, indicating an ion with a highly symmetric solvation shell. The first three water molecules bind in a bidentate fashion to the terminal oxygen atoms of the nitrate ion, keeping the planar symmetry. The onset of extensive water-water hydrogen bonding is observed starting with four water molecules and persists in the larger clusters. I. Introduction Nitrate ions, NO 3 - , are ubiquitous in aqueous-phase chem- istry. They are of importance in diverse fields like atmospheric chemistry 1 and nuclear waste treatment 2 and as a source of nitrogen for the production of nucleic acids and proteins. 3 NO 3 - is one of the most abundant ionic species in the troposphere and can be formed by rapid acid dissolution of nitric acid in aerosol particles. 1 The heterogeneous chemistry of aerosols containing such strong acids is of importance for many atmospheric cycles and is primarily governed by the nature of the aerosol surface, which determines if the adsorbed acid molecule dissociates and how the conjugate base anion is hydrated. 4 It is difficult to obtain such detailed microscopic information directly from bulk measurements. Spectroscopic experiments on isolated anion-water clusters can aid in characterizing the solvent-solute (and solvent-solvent) interac- tion at the molecular level one water molecule at a time. 5,6 In this work we investigate the microhydration of NO 3 - through the infrared spectroscopy of NO 3 - (H 2 O) n clusters, n ) 1-6. The gas-phase vibrational spectroscopy of NO 3 - · Ar was studied recently by Johnson and co-workers. 7 NO 3 - is planar and has three equivalent N-O bonds (D 3h symmetry). The six vibrational degrees of freedom lead to two bending and two stretching modes, of which two are doubly degenerate. The two bending modes (eand a 2 ′′) are infrared-active but have low intensity. The symmetric stretching vibration ν 1 (a 1 ) is not infrared-active, while the degenerate ν 3 (e) antisymmetric stretching vibration is infrared-active. The vibrational predis- sociation spectrum of NO 3 - · Ar reveals only a single band at 1349 cm -1 , which was assigned to the ν 3 mode, suggesting that the nitrate ion retains its three-fold symmetry in the Ar complex. In contrast, this degeneracy does not persist in extended solid and liquid environments. Solvent as well as counterion interac- tions in aqueous media are sufficiently strong to perturb the electronic structure of NO 3 - , resulting in a distortion from the D 3h symmetry of the bare anion. These effects lift the degeneracy of vibrational transitions, and render all six vibrational modes both Raman- and IR-active. 8-10 For finite systems, either aqueous clusters or systems with extended interfaces, molecular dynamics simulations have predicted that nitrate ions, due to their relatively large molecular polarizability, prefer interfacial rather than bulk solvation. 11 However, vibrational sum-frequency spectroscopy, 12 in combination with more recent calculations, 13,14 suggests that acid dissociation is thermodynamically disfavored at the water/air interface, so that nitrate ions are bulk-solvated and molecular HNO 3 is present at the surface. Several experiments have been performed on microhydrated NO 3 - ions, that is, small NO 3 - (H 2 O) n clusters. Lee et al. 15 measured the sequential hydration energies for NO 3 - (H 2 O) n up to n ) 3, obtaining values of 61 (1 f 0), 60 (2 f 1), and 58 kJ/mol (3 f 2). Wang and co-workers 16 measured the photo- electron spectra for these ions up to n ) 6. Aided by B3LYP/ aug-cc-pVXZ (X ) D or T) calculations, they concluded that the first three water molecules form the first solvation shell, giving rise to a highly symmetric NO 3 - (H 2 O) 3 cluster. The next three water molecules form a second solvation shell without direct contact to NO 3 - . More recently, Pathak et al. 17 used B3LYP/6-311++G** calculations to investigate the energies for a large number of different isomers up to NO 3 - (H 2 O) 8 . Their findings are distinctly different from previous findings; starting with n ) 3, interwater H-bond formation is competitive with nitrate-water binding, leading to the formation of water rings with nitrate preferring a surface position. With the exception of the smallest ions, they predict different structures compared to those obtained by Wang et al. 16 Differences from theory for hydrated NO 3 - , even in the case of small ions, are well- documented. Several publications suggest either a symmetric C 2V isomer for the monohydrated ion with two equivalent H-bonds 17-19 or a slightly asymmetric variant of C s symmetry Part of the “Robert Benny Gerber Festschrift”. * To whom correspondence should be addressed. E-mail: [email protected] fhi-berlin.mpg.de (K.R.A.); [email protected] (D.M.N.). Fritz-Haber-Institut der Max-Planck-Gesellschaft. § University of California. Lawrence Berkeley National Laboratory. Current address: Department of Chemistry, University of Arizona, Tucson, Arizona 85721-0041. J. Phys. Chem. A 2009, 113, 7584–7592 7584 10.1021/jp9017103 CCC: $40.75 2009 American Chemical Society Published on Web 05/15/2009
Transcript
  • Infrared Spectroscopy of the Microhydrated Nitrate Ions NO3-(H2O)1-6

    Daniel J. Goebbert,, Etienne Garand, Torsten Wende, Risshu Bergmann, Gerard Meijer,

    Knut R. Asmis,*, and Daniel M. Neumark*,,

    Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany, Department ofChemistry, UniVersity of California, Berkeley, California 94720, and Chemical Sciences DiVision, LawrenceBerkeley National Laboratory, Berkeley, California 94720

    ReceiVed: February 24, 2009; ReVised Manuscript ReceiVed: April 15, 2009

    We present infrared photodissociation spectra of the microhydrated nitrate ions NO3-(H2O)1-6, measuredfrom 600 to 1800 cm-1. The assignment of the spectra is aided by comparison with calculated B3LYP/aug-cc-pVDZ harmonic frequencies, as well as with higher-level calculations. The IR spectra are dominated bythe antisymmetric stretching mode of NO3-, which is doubly degenerate in the bare ion but splits into its twocomponents for most microhydrated ions studied here due to asymmetric solvation of the nitrate core. However,for NO3-(H2O)3, the spectrum reveals no lifting of this degeneracy, indicating an ion with a highly symmetricsolvation shell. The first three water molecules bind in a bidentate fashion to the terminal oxygen atoms ofthe nitrate ion, keeping the planar symmetry. The onset of extensive water-water hydrogen bonding is observedstarting with four water molecules and persists in the larger clusters.

    I. Introduction

    Nitrate ions, NO3-, are ubiquitous in aqueous-phase chem-istry. They are of importance in diverse fields like atmosphericchemistry1 and nuclear waste treatment2 and as a source ofnitrogen for the production of nucleic acids and proteins.3 NO3-

    is one of the most abundant ionic species in the troposphereand can be formed by rapid acid dissolution of nitric acid inaerosol particles.1 The heterogeneous chemistry of aerosolscontaining such strong acids is of importance for manyatmospheric cycles and is primarily governed by the nature ofthe aerosol surface, which determines if the adsorbed acidmolecule dissociates and how the conjugate base anion ishydrated.4 It is difficult to obtain such detailed microscopicinformation directly from bulk measurements. Spectroscopicexperiments on isolated anion-water clusters can aid incharacterizing the solvent-solute (and solvent-solvent) interac-tion at the molecular level one water molecule at a time.5,6 Inthis work we investigate the microhydration of NO3- throughthe infrared spectroscopy of NO3-(H2O)n clusters, n ) 1-6.

    The gas-phase vibrational spectroscopy of NO3- Ar wasstudied recently by Johnson and co-workers.7 NO3- is planarand has three equivalent N-O bonds (D3h symmetry). The sixvibrational degrees of freedom lead to two bending and twostretching modes, of which two are doubly degenerate. The twobending modes (e and a2) are infrared-active but have lowintensity. The symmetric stretching vibration 1 (a1) is notinfrared-active, while the degenerate 3 (e) antisymmetricstretching vibration is infrared-active. The vibrational predis-sociation spectrum of NO3- Ar reveals only a single band at1349 cm-1, which was assigned to the 3 mode, suggesting thatthe nitrate ion retains its three-fold symmetry in the Ar complex.

    In contrast, this degeneracy does not persist in extended solidand liquid environments. Solvent as well as counterion interac-tions in aqueous media are sufficiently strong to perturb theelectronic structure of NO3-, resulting in a distortion from theD3h symmetry of the bare anion. These effects lift the degeneracyof vibrational transitions, and render all six vibrational modesboth Raman- and IR-active.8-10 For finite systems, eitheraqueous clusters or systems with extended interfaces, moleculardynamics simulations have predicted that nitrate ions, due totheir relatively large molecular polarizability, prefer interfacialrather than bulk solvation.11 However, vibrational sum-frequencyspectroscopy,12 in combination with more recent calculations,13,14

    suggests that acid dissociation is thermodynamically disfavoredat the water/air interface, so that nitrate ions are bulk-solvatedand molecular HNO3 is present at the surface.

    Several experiments have been performed on microhydratedNO3- ions, that is, small NO3-(H2O)n clusters. Lee et al.15

    measured the sequential hydration energies for NO3-(H2O)n upto n ) 3, obtaining values of 61 (1 f 0), 60 (2 f 1), and 58kJ/mol (3 f 2). Wang and co-workers16 measured the photo-electron spectra for these ions up to n ) 6. Aided by B3LYP/aug-cc-pVXZ (X ) D or T) calculations, they concluded thatthe first three water molecules form the first solvation shell,giving rise to a highly symmetric NO3-(H2O)3 cluster. The nextthree water molecules form a second solvation shell withoutdirect contact to NO3-. More recently, Pathak et al.17 usedB3LYP/6-311++G** calculations to investigate the energiesfor a large number of different isomers up to NO3-(H2O)8. Theirfindings are distinctly different from previous findings; startingwith n ) 3, interwater H-bond formation is competitive withnitrate-water binding, leading to the formation of water ringswith nitrate preferring a surface position. With the exceptionof the smallest ions, they predict different structures comparedto those obtained by Wang et al.16 Differences from theory forhydrated NO3-, even in the case of small ions, are well-documented. Several publications suggest either a symmetricC2V isomer for the monohydrated ion with two equivalentH-bonds17-19 or a slightly asymmetric variant of Cs symmetry

    Part of the Robert Benny Gerber Festschrift.* To whom correspondence should be addressed. E-mail: [email protected]

    fhi-berlin.mpg.de (K.R.A.); [email protected] (D.M.N.). Fritz-Haber-Institut der Max-Planck-Gesellschaft. University of California. Lawrence Berkeley National Laboratory. Current address: Department of Chemistry, University of Arizona,

    Tucson, Arizona 85721-0041.

    J. Phys. Chem. A 2009, 113, 758475927584

    10.1021/jp9017103 CCC: $40.75 2009 American Chemical SocietyPublished on Web 05/15/2009

  • with two nonequivalent H-bonds.16,20-23 These variations appearto be dependent on the level of theory and basis sets used inthe calculation. Also, for larger clusters, for example, n ) 316,17,22and 6,16,17,20 both highly symmetric and asymmetric structureshave been proposed as ground-state isomers.

    Clearly, many open questions remain regarding the hydrationof nitrate ions at the molecular level. Here, we use vibrationalspectroscopy of gas-phase clusters to address some of theseproblems. We measure IR spectra of mass-selected nitrate-waterclusters from 600 to 1800 cm-1, the spectral region ofcharacteristic vibrational modes of the ion and the solventmolecules, at low temperature and in the absence of a counte-rion. This method has proven very useful in recent studies onthe microhydration of anions, for example, SO42- and SF6-,24-26

    and on water cluster anions.27,28 Similar to SO42-, NO3- is ofsufficiently high symmetry to support degenerate vibrationallevels. This degeneracy can be lifted upon asymmetric solvation,leading to a splitting of vibrational levels and additional bandsin the experimental IR spectra, thereby directly probing thehydration shell environment.

    II. Experimental Methods

    Infrared multiple photon photodissociation (IRMPD) experi-ments29-31 were carried out on a previously described ringelectrode trap-time-of-flight mass spectrometer,32 which wastemporarily installed at the Free Electron Laser for InfraredeXperiments (FELIX) user facility33 in the FOM Institute,Rijnhuizen (The Netherlands).

    Microhydrated NO3- ions were produced by electrospray ofa 5 mM solution of ammonium nitrate in a 50:50 mix of water/acetonitrile using a modified Waters z-spray source. Thenegative ions were transferred into the high-vacuum system,sampled by a 3 mm diameter skimmer, and focused into a radiofrequency (RF) linear ion guide. Ions of interest were mass-selected in a quadrupole mass filter, deflected 90 by anelectrostatic quadrupole ion deflector, and focused into the linearring electrode RF ion trap. The ion trap was filled with He buffergas and cooled to 10 K by means of a closed cycle He cryostat.Collisions with the He buffer gas cooled the ions close to theambient temperature.

    IRMPD spectra were obtained by accumulating ions for 195ms, extracting them into the extraction region of a time-of-flightmass spectrometer, and irradiating them with pulsed radiationfrom FELIX prior to the application of the high-voltage pulsesfor the measurement of the TOF mass spectra. FELIX macro-pulses were produced at 5 Hz with a pulse length of 5 s, awavelength bandwidth of 0.25% rms over the range from 600to 1800 cm-1, and typical pulse energies of up to 50 mJ.

    III. Computational Details

    Electronic structure calculations were performed using theGaussian 98 and 03 programs.34,35 Geometry optimizations wereperformed for a large number of different isomers ofNO3-(H2O)1-6 clusters using density functional theory, employ-ing the B3LYP hybrid exchange-correlation functional36,37 andthe 6-31+G* basis set. Vibrational frequencies were calculatedto ensure that the structures corresponded to local minima. Thelowest-energy isomers located from this search were thenoptimized using the augmented correlation-consistent polarizedvalence basis set aug-cc-pVDZ.38,39 In some instances, smalldifferences in structures between isomers were observed, forinstance, a H-atom pointing up or down. In such a case thelowest-energy form was used, but for the purposes of our study,these were not considered distinct isomers, helping to reduce

    the computational load. Simulated linear absorption spectra werederived from B3LYP/aug-cc-pVDZ scaled harmonic frequencies(scaling factor: 0.97)40 and intensities. In order to fac


Recommended