Broad band free jet absorption mm-wave spectrum of 3-phenyl-1-propanol Assimo Maris, Paolo Ottaviani, Barbara M. Giuliano, Sonia Melandri, Walther Caminati ⇑ Dipartimento di Chimica ‘‘G. Ciamician’’ dell’Università, Via Selmi 2, I-40126 Bologna, Italy article info Article history: Available online 7 August 2012 Keywords: 3-Phenyl-1-propanol Conformational equilibria Free jet absorption spectroscopy Rotational spectra Ab initio calculations abstract The rotational broad band spectra of the OH and OD isotopologues of 3-phenyl-1-propanol have been investigated by broad band free-jet absorption millimeter-wave spectroscopy in the 60.0–78.3 GHz fre- quency range. The spectra of the GGt and of the TGt conformers, where the three labels refer to the tor- sions of the benzyl, phenylethyl and hydroxyl groups, respectively, have been assigned. Ab initio calculations, performed at the MP2/6-311++G level, were used to characterize the minima of the con- formational potential energy surface. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Alcohols formed by an alcoholic group and a benzene ring at- tached to an aliphatic chain are of considerable interest in spec- troscopy because they give rise to complex conformational equilibria, are often chiral molecules, and can be studied by a vari- ety of spectroscopic methods. The benzene ring is a chromophoric group, which allows studies of electronic fluorescence, while the permanent dipole moment allows the detection of the rotational spectra. Several alcohols of this kind have been investigated by rotational spectroscopy. The simplest one is benzyl alcohol, the spectrum of which is considerably complicated by the concerted internal rotation of the OH and benzyl group, which connects 4 equivalent minima. Its rotational spectrum and internal dynamics have been reported [1]. Phenyl-ethanol and phenyl-propanol can exist as two and three chemical isomers, respectively. Several spectroscopic studies have been dedicated to 1-phenyl-1-ethanol, 1-phenyl-1-propanol and 2-phenyl-1-propanol, probably because they are chiral species, and to their complexes with chiral mole- cules, in conjunction with chiral recognition studies. A wealth of results obtained by resonant-two-photon-ionization (R2PI) excita- tion spectra of the S 1 S 0 transition of mass resolved hydrogen bonded molecular complexes have been reported [2–8]. The rota- tional spectra have been reported for 1-phenyl-1-propanol [9] and 1-phenyl-2-propanol [10]. Less attention has been dedicated to 2-phenyl-1-ethanol and to 3-phenyl-1-propanol, which are non-chiral molecules, although the rotational spectrum of 2-phenyl-1-ethanol has been reported [11]. The conformational composition of 3-phenyl-1-propanol (3PP) appears quite complex and interesting, in relation to the values that the four dihedral angles s 0 , s 1 , s 2 and s 3 , shown in Fig. 1, can assume. In all conformers s 0 is close to 90 deg, so it is not con- sidered in the following labelling. We have then to consider 3 3 = 27 conformers which may be labeled with three consecutive letters ‘‘XXx’’ where X is G, T or G 0 according to the configuration gauche, trans or gauche 0 taken by the benzyl, phenyl-ethyl and hydroxyl groups. Thirteen conformers, labeled as TTg, GTt, GTg, GTg 0 , GGt, GGg, GGg 0 , TGt, TGg, TGg 0 , GG 0 t, GG 0 g and GG 0 g 0 are expected, in- deed. They are spectroscopically and energetically equivalent to their mirror images TTg 0 ,G 0 Tt, G 0 Tg 0 ,G 0 Tg, G 0 G 0 t, G 0 G 0 g 0 ,G 0 G 0 g, TG 0 t, TG 0 g 0 , TG 0 g, G 0 Gt, G 0 Gg 0 and G 0 Gg, which means that all conformers have statistical weight equal to 2, whereas a 14th conformer, TTt, has a statistical weight 1. In Fig. 2 the sketches of the 14 non- equivalent conformers are shown. The conformational equilibrium of 3PP has been already studied with many techniques. In his early NMR study, Snyder [12] found that the conformer where the phenyl and –CH 2 OH groups are trans disposed (TXx) is more stable than the gauche rotamer (GXx) by ca. 170 cm 1 in non-polar solvents and by ca. 200 cm 1 in polar sol- vents. More recently Elks et al. [13] studied 3PP in a jet-cooled environment, using the laser induced fluorescence and mass se- lected R2PI excitation spectroscopy of the S 1 S 0 electronic tran- sitions. Several conformer origins were observed and the two most intense peaks were assigned to the GG and TG conformer. The relative intensity of the GG peak was found to be about three times that of the TG peak. Successively Guchhait et al. [14] ob- served six conformers in the jet-cooled infrared-ultraviolet dou- ble-resonance spectrum of 3PP. On the basis of these data Mons et al. [15] could find that the hydroxyl hydrogen atom prefers the trans positions in both previously assigned conformers (GGt, TGt) and, moreover they 0022-2852/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2012.07.016 ⇑ Corresponding author. Fax: +39 051 2099456. E-mail address: [email protected](W. Caminati). Journal of Molecular Spectroscopy 280 (2012) 145–149 Contents lists available at SciVerse ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Transcript
Journal of Molecular Spectroscopy 280 (2012) 145–149
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Spectroscopy
journal homepage: www.elsevier .com/ locate / jms
Broad band free jet absorption mm-wave spectrum of 3-phenyl-1-propanol
Assimo Maris, Paolo Ottaviani, Barbara M. Giuliano, Sonia Melandri, Walther Caminati ⇑Dipartimento di Chimica ‘‘G. Ciamician’’ dell’Università, Via Selmi 2, I-40126 Bologna, Italy
a r t i c l e i n f o a b s t r a c t
Article history:Available online 7 August 2012
Keywords:3-Phenyl-1-propanolConformational equilibriaFree jet absorption spectroscopyRotational spectraAb initio calculations
0022-2852/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jms.2012.07.016
The rotational broad band spectra of the OH and OD isotopologues of 3-phenyl-1-propanol have beeninvestigated by broad band free-jet absorption millimeter-wave spectroscopy in the 60.0–78.3 GHz fre-quency range. The spectra of the GGt and of the TGt conformers, where the three labels refer to the tor-sions of the benzyl, phenylethyl and hydroxyl groups, respectively, have been assigned. Ab initiocalculations, performed at the MP2/6-311++G�� level, were used to characterize the minima of the con-formational potential energy surface.
� 2012 Elsevier Inc. All rights reserved.
1. Introduction
Alcohols formed by an alcoholic group and a benzene ring at-tached to an aliphatic chain are of considerable interest in spec-troscopy because they give rise to complex conformationalequilibria, are often chiral molecules, and can be studied by a vari-ety of spectroscopic methods. The benzene ring is a chromophoricgroup, which allows studies of electronic fluorescence, while thepermanent dipole moment allows the detection of the rotationalspectra. Several alcohols of this kind have been investigated byrotational spectroscopy. The simplest one is benzyl alcohol, thespectrum of which is considerably complicated by the concertedinternal rotation of the OH and benzyl group, which connects 4equivalent minima. Its rotational spectrum and internal dynamicshave been reported [1]. Phenyl-ethanol and phenyl-propanol canexist as two and three chemical isomers, respectively. Severalspectroscopic studies have been dedicated to 1-phenyl-1-ethanol,1-phenyl-1-propanol and 2-phenyl-1-propanol, probably becausethey are chiral species, and to their complexes with chiral mole-cules, in conjunction with chiral recognition studies. A wealth ofresults obtained by resonant-two-photon-ionization (R2PI) excita-tion spectra of the S1 S0 transition of mass resolved hydrogenbonded molecular complexes have been reported [2–8]. The rota-tional spectra have been reported for 1-phenyl-1-propanol [9]and 1-phenyl-2-propanol [10].
Less attention has been dedicated to 2-phenyl-1-ethanol and to3-phenyl-1-propanol, which are non-chiral molecules, althoughthe rotational spectrum of 2-phenyl-1-ethanol has been reported[11].
ll rights reserved.
inati).
The conformational composition of 3-phenyl-1-propanol (3PP)appears quite complex and interesting, in relation to the valuesthat the four dihedral angles s0, s1, s2 and s3, shown in Fig. 1,can assume. In all conformers s0 is close to 90 deg, so it is not con-sidered in the following labelling. We have then to consider 33 = 27conformers which may be labeled with three consecutive letters‘‘XXx’’ where X is G, T or G0 according to the configuration gauche,trans or gauche0 taken by the benzyl, phenyl-ethyl and hydroxylgroups. Thirteen conformers, labeled as TTg, GTt, GTg, GTg0, GGt,GGg, GGg0, TGt, TGg, TGg0, GG0t, GG0g and GG0g0 are expected, in-deed. They are spectroscopically and energetically equivalent totheir mirror images TTg0, G0Tt, G0Tg0, G0Tg, G0G0t, G0G0g0, G0G0g, TG0t,TG0g0, TG0g, G0Gt, G0Gg0 and G0Gg, which means that all conformershave statistical weight equal to 2, whereas a 14th conformer, TTt,has a statistical weight 1. In Fig. 2 the sketches of the 14 non-equivalent conformers are shown.
The conformational equilibrium of 3PP has been already studiedwith many techniques. In his early NMR study, Snyder [12] foundthat the conformer where the phenyl and –CH2OH groups are transdisposed (TXx) is more stable than the gauche rotamer (GXx) by ca.170 cm�1 in non-polar solvents and by ca. 200 cm�1 in polar sol-vents. More recently Elks et al. [13] studied 3PP in a jet-cooledenvironment, using the laser induced fluorescence and mass se-lected R2PI excitation spectroscopy of the S1 S0 electronic tran-sitions. Several conformer origins were observed and the twomost intense peaks were assigned to the GG and TG conformer.The relative intensity of the GG peak was found to be about threetimes that of the TG peak. Successively Guchhait et al. [14] ob-served six conformers in the jet-cooled infrared-ultraviolet dou-ble-resonance spectrum of 3PP.
On the basis of these data Mons et al. [15] could find that thehydroxyl hydrogen atom prefers the trans positions in bothpreviously assigned conformers (GGt, TGt) and, moreover they
Fig. 2. Molecular sketches of the 14 possible conformational forms of 3
146 A. Maris et al. / Journal of Molecular Spectroscopy 280 (2012) 145–149
attributed the lowest OH stretch frequency to the GG0g conformer,in which an OH� � �p intermolecular hydrogen bond takes place.
Since no rotational spectroscopy studies are available on 3PP,we decided to investigate its free-jet absorption millimeter-wavespectrum.
2. Experimental methods
The Stark modulated free-jet absorption millimeter-wave spec-trometer used in this study has already been described elsewhere[16]. Briefly, the apparatus consists of a radiation source workingin the 53–78 GHz region (Kvarz-Nizhny Novgorod, Russia), ahorn-dielectric lens system (Montech-Clayton, Australia), whichfocuses the radiation in the central part of the chamber with awaist of about 2 cm, and a Schottky diode detector (MillitechDXW10). The MW radiation and the free jet are arranged perpen-dicularly and a high voltage Stark modulation (up to 3 kV at a fre-quency of 33 kHz) is applied to the molecular sample. Afterdetection, the modulated signal is fed into lock-in amplifier. Glob-ally the system works in the 60–78 GHz region and the accuracy ofthe frequency measurements is better than 50 kHz.
A gas mixture of 3PP (ca. 1.5%) in argon was expanded from apressure of 200 mbar to about 5 � 10�3 mbar through a nozzle
GG'g GG't
GTg GTt
GGg GGt
TGg TGt
tTT
PP. The labels for two observed species (GGt and TGt) are in bold.
Table 1Ab initio (MP2/6-311++G��) relative energies and spectroscopic parameters of the 14 conformers of 3-phenyl-1-propanol.
Fig. 3. MP2/6-311++G�� relative energies of the 14 conformers of 3PP. The labels for two observed species (GGt and TGt) are in bold.
A. Maris et al. / Journal of Molecular Spectroscopy 280 (2012) 145–149 147
with a diameter of 0.35 mm at 90 �C. The –OD isotopic species wasprepared by fluxing D2O over the sample.
3. Computational calculations
HF/6-31G� and MP2/6-31G� ab initio calculations were alreadyavailable [13], suggesting the TTt, TGt, TGg, GGt, GGg0, GGg, GG0gspecies to be the seven more stable conformers. However, in orderto have an overview of the energies of the conformers and to ob-tain an estimation of some useful spectroscopic parameters duringassignment, we optimized all 14 plausible structures at the MP2/6-311++G�� level. The obtained results are summarized in Table 1and the relative energies are also visualized in Fig. 3.
Gaussian2003 software package [17] was used to optimize thegeometry of each possible conformer of 3PP and to calculate theharmonic vibrational frequencies. All calculations were performedat the MP2/6-311++G�� level and all the structural parameterswere freely optimized. The free energy of each conformer, in the
pre-expansion conditions, was evaluated in the harmonic andpolyatomic ideal-gas approximation.
4. Rotational spectra
The jet-cooled rotation spectrum provided us several weaklines, listed in Tables 2 and 3, which were found to belong totwo different conformers. The initial assignments were based onthe identification of high J (from 9 to 29) high K�1 (from 7 to 15)transitions. These transitions appear as a single line resulting fromthe coalescence due to near-prolate degeneracy of the involvedlevels of two lc- and two lb-R-type rotational lines. It was thenpossible to measure transitions with lower K�1, resolved in quar-tets made of two lc- and two lb-R-type lines. The lc-componentswere stronger in one set, while the lb-components were more in-tense in the second set. Both series of transition frequencies werefitted using Watson’s semirigid Hamiltonian in the ‘‘S’’ reduction
Table 2Measured transition frequencies (MHz) of the GGt-3PP conformer and of its hydroxyldeuterated form.
148 A. Maris et al. / Journal of Molecular Spectroscopy 280 (2012) 145–149
Table 4Experimental spectroscopic parameters (S-reduction, Ir representation) of the GGtand TGt conformers of 3PP and of their hydroxyl deuterated forms.
a Transitions fourfold overlapped due to near prolate degeneracy of the involvedlevels. Only Ka is given.
A. Maris et al. / Journal of Molecular Spectroscopy 280 (2012) 145–149 149
and Ir representation [18], obtaining the spectroscopic parametersreported in Table 4.
In order to determine the position of the hydroxylic hydrogenatom we measured the spectra of the hydroxyl deuterated forms.The fits of the measured transition frequencies were performed
as for the normal species, and the obtained spectroscopic constantsare also reported in Table 4.
From the combinations of the OH and OD rotational constantswe calculated the substitution coordinates of the hydroxyl hydro-gen atom with the Kraitchman’s method [19]. The obtained valuesare reported in Table 5. The experimental rs coordinates are in goodagreement with the calculated re coordinates of the GGt and TGtconformers.
As found in previous jet cooled experiments [13,15], the GGtand TGt conformers are the most stable ones. According to theintensity of the observed transition lines and to the ab initio dipolemoment components, and neglecting any conformational coolingeffect in the supersonic expansion, the two conformers seem tohave almost the same energy.
5. Conclusions
The rotational spectra of two conformers of 3PP have been as-signed in the free jet broadband mmw spectrum. In order to pre-vent conformational relaxation and then to observe linesbelonging to other conformers, we used helium instead argon ascarrier gas, but we did not observe residual rotational lines, strongenough to assign further conformers. As in other cases, with thistechnique we observed a smaller number conformers with respectto other spectroscopic methods, but we could determine precisesets of spectroscopic data, including centrifugal distortion con-stants. In addition, we could determine rather precisely the relativepopulation of the two observed species.
Acknowledgments
We thank the Italian MIUR (PRIN08, Project KJX4SN_001) andthe University of Bologna (RFO) for financial support.
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