1164
J. Phys. Chem. 1992,96, 1164-1173
infusion with 0.05 g/L KSCN in methanol. The corrugation in these pictures is about twice the mean thickness of the gold layer. Thus, some of the deepest valleys may, and probably do, reach the underlying oxide layer. The width of these penetrations, however, is probably quite narrow since we see no evidence of the increased noise expected as the tunneling tip traverses the alumina layer. Further, there is no evidence that infusion from methanol increases the roughness of the surface in a substantial way. This picture is unlike that for lead, as developed by Nelson and coworkers.16 Their TEM images show large openings (of the order of 20 nm wide and hundreds of nm long) in their 15 nm thick lead films that substantially increase in size with exposure to water vapor. Our pictures of the gold surface are also consistent with the fact that these films are only slightly permeable to formic acid in the vapor phase. The observed infusion may be, in part, due to a specific chemical reaction between gold and thiocyanate. We are currently exploring this possibility.
Conclusiom We show for the first time inelastic electron tunneling spectra of gold-topped postfabricationdoped tunnel diodes. We find that the thiocyanate ion readily infuses into these Al-Al,OrAu junctions without the presence of water. These infused devlces
give a strong and distinct thiocyanate spectrum over more than 4 orders of magnitude variation in KSCN concentration; the band near 330 cm-I is still easily identified from junctions infused with lo-' mol/L KSCN in methanol. Comparison of tunneling spectra obtained by conventional doping of KSCN in Au- and Pb-topped junctions indicates that there is a thiocyanate-gold interaction resulting in the formation of a metal complex of some kind. This complex has an especially strong vibrational band(s) near 330 cm-'that signals the presence of SCN- over a wide concentration range. Tunneling spectra taken from infused and conventionally doped Au-topped junctions are essentially the same, indicating that the gold-thiocyanate reaction is not driven by the thermal energy of the gold as it is deposited. We tentatively conclude that gold-topped tunnel junctions may have especially high spectral sensitivity for thiocyanate because of this complex formation. STM images of the junctions before and after infusion suggest that the gold layer studied here is a much denser structure than the infusible lead layers studied previously.
Acknowledgment. This work was supported by the Environmental Protection Agency under Grant R-816329-01-0. Their assistance is gratefully acknowledged.
Ionlratlon-Locrs Stimulated Raman Spectroscopy of Jet-Cooled Hydrogen-Bonded Complexes C o n t a w Phends Gregory V. Hartland,+ Bryan F. Heason, Vincent A. Venturo, and Peter M. Felker*J Department of Chemistry and Biochemistry, University of California. Los Angeles, California 90024- 1569 (Received: August 30, 1991)
Stimulated Raman spectra of a variety of hydrogen-bonded, one-to-one complexes of phenol and substituted phenols are presented and discussed. The Raman spectra were recorded with ionization-lossstimulated Raman spectroscopy on supersonic molecular beam samples. Complexation-induced vibrational frequency shifts and vibrational line widths are reported for both the protondonating moieties (the phenols) and the proton-accepting moieties in the complexes. The vibrational frequency shifts are discussed in terms of the strength of the hydrogen bond and the usefulness of different vibrational modes as indicators of hydrogen bonding. particular attention is paid to the phenolic CO and OH stretch fundamentals. The line-width measurements suggest that vibrational modes closely associated with the hydrogen bond (Le., the phenolic CO and OH stretches) relax more efficiently than other modes in the complexes. Even so, the lifetimes of the excited OH stretches are at least a few picaseconds for several of the complexes. Evidence for mode selective dynamics in the OH stretch region of the substituted phenol-water complexes is also presented and discussed.
I. Introduction Hydrogen-bonded complexes involving phenol have been the subject of numerous molecular beam spectroscopy experiments.'+ The interest in phenol H bonding has several motivations. First, phenol is the simplest aryl alcohol. Its H-bonding interactions can serve as prototypes for the interactions of larger species. Second, the molecule undergoes a large increase in acidity upon electronic excitation into its SImanifold. This increase allows, under some circumstances, for the Occurrence of excited-stated proton transfer to proton-accepting solvent species.6.' A detailed understanding of such processes requires information conceming the solvation of phenol by H-bonding solvents. Third, the study of the H-bonding properties of phenol (and derivatives) can shed light on the analogous properties of tyrosine residues in proteins.l0 Thus, important information relevant to intraprotein and protein-solvent interactions may be obtained by studying the details of phenol H bonding. Fourth, H bonding involving phenol and phenol derivatives has been studied extensively in the condensed Current address: Department of Chemistry, University of Pennsylvania, Philadelphia. PA 19104-6323. 'NSF Presidential Young Investigator 1987-92.
0022-3654/92/2096-1164$03.00/0
phase." The potential exists, therefore, for making connections between results from these condensed-phase studies and results (1) (a) Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1982,86, 1768. (b) A h , H.; Mikami, N.; Ito, M.; Udagawa, Y. J . Phys. Chem. 1982,86,2567. (c) Abe, H.; Mikami, N.; Ito, M.; Udapwa, Y. Chem. Phys. Lett. 1982,93, 217. (d) Oikawa, A.; A h , H.; Mikami, N.; Ito, M. J. Phys. Chem. 1983,87, 5083. (e) Gonohe, N.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1985, 89,3642. (f) Ito, M.J. Mol. Srmr. 1988,177, 173. (g) Ebata, T.; Furukawa, M.; Suzuki, T.; Ito, M. J . opt. Soc. Am. B 1990, 7, 1890. (2) (a) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1982, 91, 311. (b) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1983,9497. (c) Fuke, K.; Yoshiuchi, H.; Kaya, K.; Achiba, Y.; Sato, K.; Kimura, K. Chem. Phys. Lett. 1984,108, 179. (3) Sur, A.; Johnson, P. M. J . Phys. Chem. 1986,84, 1206. (4) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1987,87, 115.
(5) (a) Lipert, R. J.; Bermudez, G.; Colson, S.D. J . Phys. Chem. 1988, 92,3801. (b) Lipcrt, R. J.; Colson, S.D. J. Chem. Phys. 1989,89,4579. (c) Lipert, R. J.; Colson, S.D. Chem. Phys. Lett. 1989, 161, 303. (6) Solgadi, D.; Jouvet, C.; Tramer, A. J . Phys. Chem. 1988, 92, 3313. (7) Steadman, J.; Syage, J. A. J. Chem. Phys. 1990, 92,4630. (8) Stanley, R. J.; Castleman, A. W. J . Chem. Phys. 1991, 94, 7744. (9) Reiser, G.; Dopfer, 0.;Lindncr, R.; Henri, G.; Mllller-Detlefs, K.; Schlag, E. W.; Colson, S. D. Chem. Phys. Lett. 1991, 181, 1. (10) Takeuchi, H.; Watanabe, N.; Satoh, Y.; Harada, I. J. Romon Specrrosc. 1989, 20, 233.
0 1992 American Chemical Society
Hydrogen-Bonded Complexes Containing Phenols
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1165
respectively)-ILSRS (see Figure 1) is especially promising in regard to studies of complexes and clusters. This is so for several r e a ~ 0 n s . IFirst, ~ ~ the technique can be implemented even when the vibrational state excited by the Raman process is short-lived. Second, ILSRS can probe the Raman-activevibrational transitions localized in all of the moieties of a cluster, not just the moiety through which the photoionization process proceeds. For example, in the phenol-water complex the vibrational level structure of the water moiety can be probed in addition to that of the phenol moiety. Third, being based on the Raman effect, the vibrational transitions accessible to ILSRS are often different than those that are accessible to other methods. For example, the phenolic OH stretching mode, one that is very important in regard to H bonding, is difficult to study by dispersed fluorescence or stimulated emission schemes because the S1 So Franck-Condon factors associated with L\v # 0 transitions of the mode are small. However, the mode does have appreciable Raman activity17and can be studied by ILSRS. Fourth, many of the vibrational resonances in ILSRS Figure 1. Energy level diagram for ILSRS. (la)] is the manifold of are characterized by very simple rotational structure, namely, an rotational levels in the ground vibronic state, (Ib)] is the manifold of overwhelmingly dominant Q-branch. This situation facilitates rotational levels in an excited vibrational level of the ground electronic the measurement of vibrational frequencies and line widths. Fifth, state, (In))is the manifold of rotational levels in an excited vibronic state, Q,,,,= (Eb - E,)/h, and the laser fields at frequenciesol, w2, and w3 are unlike stimulated emission spectroscopies such as ion dip, ILSRS the pump, Stokes, and probe laser fields, respectively. The population is relatively insensitive to the lifetimes of the excited vibronic states of the (la)) manifold is monitored by ionization induced by the probe involved in the process. Finally, ILSRS has the same laserlaser, which is resonant with the (In)) (la)]vibronic transition. When bandwidth-limited resolution and species-selectivitycharacteristics w1 - w2 = Qh,population is transferred from (la)] to (lb)]via stimulated that obtain in the ion-dip technique. Given these characteristics, Raman scattering. This produces a decrease in the ion signal. one expects that ILSRS might be fruitfully employed in a vifrom studies of the simpler H-bonded systems that obtain in brational spectroscopic study of H-bonded complexes involving molecular beam experiments. phenol and phenol derivatives. One aspect of jet-cooled phenol complexes that has received In this paper we present ILSRS results for a series of jet-cooled attention is their ground-state vibrational spectroscopy. The first complexes involving phenol and monomethyl- and monochlorosuch studieslb*cinvolved relatively low-resolution dispersed phenols in combination with water, methanol, ethanol, diethyl fluorescence measurements on a variety of one-to-one complexes ether, ammonia, benzene, NZ,methane, and argon. The measured involving phenol. More recent studies1gP8have employed the Raman spectra encompass four spectral regions: the CC stretching stimulated emission scheme known as ion-dip s p e c t r o ~ c o p y ~ ~ J ~region (800-1050 cm-l), the phenolic CO stretching region (near in experiments on phenol-(water),, complexes at subwavenumber 1260 cm-l), the aromatic CH stretching region (near 3070 cm-I), resolution. One of the principal strengths of all of these studies and the OH stretching region (-3500-3700 cm-l). The results is their success in obtaining information on the low-frequency pertain to two general issues. The first is the characterization intermolecular vibrational modes of the species. Such information of the shifts in intramolecular vibrational frequencies that occur is perhaps the most direct means by which to learn about interupon the formation of a complex. In this regard, we are interested molecular potential‘surfaces. In addition, results from the ion-dip in those “marker” vibrational modes in the phenolic moiety whose studies are important in that they shed light on the ground-state frequencies are especially sensitive to H-bonding interactions. The vibrational dynamics of the H-bonded species. behavior of such modes can provide clues as to the nature of In this laboratory we have been successful in employing intermolecular interactions, not only in the complexes, but in mass-selective, ionization-detected stimulated Raman spectroscondensed-phasesystems, as well.’ Two important examples are copies (IDSRS) in medium-resolution (0.05-0.3 cm-’) studies of the phenolic CO and OH stretching vibrations. The behaviors the ground-state vibrational structure of weakly bound complexes of such modes are very often monitored in condensed-phase studies and ~1usters.l~IDSRS techniques15J6are double-resonance as indicators of the extent and strength of H-bonding interactions.lOJlvl8 However, systematic results pertaining to their schemes in which the population changes induced by stimulated Raman processes are monitored by resonantly enhanced multifrequency shifts in isolated, one-to-one H-bonded complexes are photon ionization (REMPI). Coupled with mass analysis of the quite We are also interested in the frequency-shift photoions14IDSRS methods are sensitive, species-specific means behavior of the vibrational modes localized in the proton-accepting by which to measure the vibrational spectra of species in molecular “solvent” moieties of the complexes. Results of this sort could beams. eventually be valuable benchmarks in the evaluation of trial inOf the two variants of IDSRS-ionization-gain and ionizatermolecular potential energy surfaces. Moreover, one wonders tion-loss stimulated Raman spectroscopy (IGSRS and ILSRS, if some such modes might also be useful as H-bonding “marker” modes. (1 1) See, for example: (a) Pimentel, G. C.; McClellan, A. L. The HyThe second issue pertains to the dynamics of vibrationally drogen Bond; W. H. Freeman: San Francisco, 1960. (b) Pimentel, G. C.; excited states in the H-bonded complexes. Vibrational dynamics McClellan, A. L. Annu. Rev. Phys. Chem. 1971, 22, 347. (c) Vinogradov,
-
-
S. N.; Linnell, R. H. Hydrogen Bonding Van Nostrand Reinhold New York, 1971. (12) (a) Cooper, D. E.;Klimcak, C. M.; Wessel, J. E. Phys. Rev. Lett. 1981,46,324. (b) Cooper, D. E.; Wessel, J. E. J. Chem. Phys. 1982,76,2155. (13) (a) Suzuki, T.;Mikami, N.; Ito, M. Chem. Phys. Lett. 1985,120,333. (b) Suzuki, T.; Mikami, N.; Ito, M. J. Phys. Chem. 1986, 90, 6431. (c) Suzuki, T.; Hiroi, M.; Ito, M. J. Phys. Chem. 1988, 92, 3774. (14) (a) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Felker, P. M.J. Chem. Phys. 1989,91,2751. (b) Hartland, G. V.; Henson, B. F.; Venturo, V. A.; Hertz, R. A.; Felker, P. M. J. Opt. SOC.Am. B 1990, 7, 1950. (c) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Hertz, R. A.; Felker, P. M. Chem. Phys. Lett. 1991, 176,91. (1 5 ) (a) Esherick, P.; Owyoung, A. Chem. Phys. Lett. 1983,103,235. (b) Esherick, P.; Owyoung, A.; Pliva, J. J. Chem. Phys. 1985, 83, 3311. (16) Bronner, W.; Oesterlin, P.; Schellhorn, M. Appl. Phys. B 1984,34, 11.
(17) Wilson, H. W.; MacNamee, R. W.;Dung, J. R. J. Raman Spectrosc. 1981, 11, 252. (18) (a) Thijs, R.; Zeegers-Huyskens,T. Spectrochim. Acta 1984, 4 0 4 307. (b) Vanderheyden, L.; Vandenbrande, R.; Zeegers-Huyskens, T. Spectrosc. Lett. 1989, 22, 451. (19) There is, however, a body of data on the OH stretch fundamentals of phenol complexes in the gas phase at 150 OC and at a phenol partial pressure of about 280 Torr. See: Hussein, M. A.; Millen, D. J.; Mines, G. W. J. Chem. SOC.Faraday Trans. 2 1976, 72,686. (20) A considerableamount of data concerning the frequency shift of the HF stretch fundamental in small HF H-bonded complexes has been reported. See, for example: (a) Legon, A. C.; Millen, D. J. Chem. Rev. 1986,86,635. (b) Thomas, R. K. Proc. R. SOC.London, Ser. A 1971,352, 133. ( c ) Pine, A. S.;Lafferty, W. J. J. Chem. Phys. 1983, 78,2154, and references therein.
1166 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
1
Trigger
F v 2. Schematic diagram of the experimental apparatus used for ILSRS epxeriments. BS = beam splitter; SF = Supracil flat; MOL. BEAM = molecular beam apparatus; TOFMS = time-of-flight mass spectrometer; DL1 and DL2 denote Spectra-Physics PDL-2 (or PDL-3) dye lasers;GCR-3 and DCR-2A denote Spectra-PhysicsNd:YAG lasers. The pump laser field (1) was supplied by 20% of the second harmonic of the GCR-3. The Stokes laser field (2) was supplied by the output of DL1. The probe laser field (3) was supplied by the output of DL2 frequency-doubled by a Spectra-Physics wavelength extender (WEX).
can be addressed via the measurement of linewidths in ILSRS spectra, which of course yield lower limits on the lifetimes of the pertinent excited vibrational states. Of particular interest here are the lifetimes of vibrational states corresponding to modes in close proximity to the H bond-for example, the CO and OH stretches in proton-donatingphenols. Such modes might be expected to couple effectively with the low-frequency intermolecular vibrational modes in the complexes to give rise to fast predissociation and/or intramolecular vibrational energy redistribution (IVR) processes. Also of interest is the possibility that the sort of mode-specific dynamics that has been found to characterize proton-donating vs proton-accepting vibrational modes in smaller H-bonded complexesZ1might also be present in the larger species considered here.
II. ExpehtaISection A diagram of the experimental apparatus used for ILSRS is shown in Figure 2. The pump and Stokes laser fields in the ILSRS process (laser fields 1 and 2 in Figures 1 and 2) were obtained from an injection-sceded NdYAG laser (SpectraPhysics GCR-3) and associated dye laser (Spectra Physics PDL-2 or PDL-3). The pump field consisted of 20% of the second harmonic of the Nd:YAG, and the Stokes field consisted of the output of the dye laser. The probe laser field (laser field 3 in Figures 1 and 2) was generated by frequency doubling the output of a second dye laser (SpectraPhysics PDLZ), which was pumped by a second NdYAG laser (SpectraPhysic8 DCR-2). The firing of this second laser system was delayed relative to that of the fmt by about 15-20 ns. The pump and Stokes fields, which had parallel linear polarizations, were combined collinearly on a 50/50 beam splitter. The resulting two-wlor pulse train was then combined collinearly with that part of the probe field which reflected from the front face of a suprasil flat. The probe field reflection from the back face of the flat was used for signal normalization purposes as described in ref 14b. All excitation fields were focused with a 12 in. focal length lens into the ionization region of a time-of-fight mass spectrometer (TOFMS), at which point they intersected the molecular beam sample. The molecular beam apparatus and time-of-flight mass spectrometer have been described e1~ewhere.l~~ Briefly, H-bonded complexes were synthesized by expanding a mixture of the pertinent phenol and its complexing partner (both at several Torr vapor pressure) seeded in He (at about 2000 Torr) through the 200 pm diameter orifice of a solenoid-driven pulsed molecular beam valve whose firing was synchronized with the firing of the laser system. The preexpansion mixture of gases was generally prepared by bubbling the He carrier gas through the “solventn of interest and then passing this gas over the heated phenolic (21) See,for example: Miller, R. E. Acc. Chem. Res. 1990, 23, 10, and references therein.
Hartland et al. compound. For the more volatile complexing partners, however, He and the gas of interest were premixed to a given ratio, the mixture then serving as the carrier gas. The molecular beam was skimmed by a 1 mm diameter skimmer approximately 4 cm downstream from the expansion orifice, after which it entered the ionization region of the TOFMS. The ions produced by interaction with the laser fields were accelerated by a Wiley-McLaren deflection plate arrangementZ2into a l m long, differentially pumped flight tube. The ion signal was detected by a dual microchannel plate assembly, the amplied output of which was monitored with a fast oscilloscope and one or two boxcar integrators. The boxcar outputs were digitized and directed to a computer for storage. Prior to measuring the ILSRS spectra, the vibronic spectra of all the complexes were recorded with one- or two-color REMPI. The spectra obtained, in particular the values of the complexation-induced SI So frequency shifts A(SI-So),were in good agreement with published vibronic ~pectra,~~~J3-2* when the latter were available. To obtain ILSRS spectra the probe laser was tuned to the phenol-localized S, So 0; band of the complex to be studied. The probe laser intensity and the molecular beam conditions were adjusted to give the best possible ratio of resonant to nonresonant ionization. Then, while monitoring the signal due to the parent ion mass of the complex of interest, the Stokes laser frequency was scanned to produce an ILSRS spectrum. In most cases, the resolution in the spectra presented below is about 0.3 m-l, as determined by the bandwidth of the PDL-2 dye laser used as the Stokes source. To obtain some spectra, however, the Stokes source was the PDL-3 laser. These spectra are characterized by about 0.05 cm-I resolution and are denoted as such. Because of our interest in accurately characterizing vibrational frequency shifts due to complexation, the following frequency calibration procedures were employed. ILSRS spectra recorded in the 800-1300-Cm1 region (CC and CO stretching modes) were refercnd to phenol monomer ILSRS spectra that were measured directly before and/or after the measuremeht of the complex’s spectrum. The frequencies of phenol’s resonances in this region (see Table I) have been determined precisely by Fourier-transform IGSRS14b729measurements. ILSRS spectra recorded in the 3000-3650-~m-~ region (CH and OH stretching modes) were calibrated by calibrating the Stokes laser’s wavelength read-out to ILSRS spectra of the benzene u2 fundamental, whose absolute frequency was taken from the literature.M Ar, CH4, N2, and NH3 were obtained as high-pressure gases and were used without further purification. All other chemicals except water were reagent grade and were used without further purification. The water used was UCLA Chemistry Department distilled water,
-
-
III. Results A. Stimulated Raman Spectra of the Substituted wen01 Monomers. The observed Raman transitions and assignments for phenol are presented in Table I and those for the substituted phenols e,m-, and p c r a o l and m- and pchlorophenol in Table 11. The labeling of the vibrational modes follows that of V a ~ s h y i . (Note ~ ~ that this labeling differs in some respects from that used by others in the literature, e.g., refs 1 and 8.) The vibrational spectroscopy of the orthe and meta-substituted phenols (0- and m-cresol and m-chlorophenol) is complicated by the ex(22) Wiley, W. C.; McLaren, I. H. Reo. Sci. Instrum. 1955, 26, 1150. (23) Aota, T.; Ebata, T.; Ito, M. J. Phys. Chem. 1989, 93, 3519. (24) Oikawa, A.; A h , H.; Mikami, N.; Ito, M. Chem. Phys. Left. 1985, 116, 50. (25) (a) Oikawa, A.; A h , H.; Mikami, N.; Ito, M. J. Phys. Chem. 1984, 88, 5180. (b) Ito, M.; Oikawa, A. J. Mol. Strucr. 1985, 126, 133. (26) Mizuno, H.; Okuyama, Ebata, T.; Ito, M. J . Phys. Chem. 1987,91, 5589. (27) Appel, I.; Kleinermanns, K. Ber. Bunsenges. Phys. Chem. 1987,91, 140.
(28) Song, K.; Hayes, J. M. J . Mol. Spectrosc. 1989, 134, 82. (29) Hartland, G. V.; Henson, B. F.; Connell, L. L.; Corcoran, T. C.; Felker. P. M. J . Phys. Chem. 1988, 92, 6877. (30) Thakur, S. N.; Goodman, L.; Ozkabak, A. G. J . Chem. Phys. 1986, 84, 6642. (31) VarsLnyi, G. VibrufionalSpectra of Benzene Deriuatiues; Academic Press: New York, 1969.
Hydrogen-Bonded Complexes Containing Phenols
TABLE I: Observed R a w Resonances rad Assignments for Jet-Cooled P b l freq,” cm-’ assignmentc 999.31b VI 2 1009.47b 2Y16b 1021.9 Yl8a 1026. 14b 1253.4 1261.7 “la 1263.3 1264.6 3060.9
freq,” cm-I 3072.7 3074.5 3075.5 3076.5 3086.6 3098.9 3103.6 3656.7
assignment‘ YZ
(shoulder) ~20.
”Estimated uncertainty of 1 0 . 1 cm-l unless otherwise marked. *Estimated uncertainty of 10.05 cm-I. ‘Based on refs 17, 31, 33, and 34.
istence of two rotational isomers that arise from the OH proton being either cis or trans to the substituent.2e27 For the metasubstituted phenols the electronic origins of the isomers are s e p arated by approximately 100 cm-’ and can be cleanly resolved in a molecular beam. Hence, because of the Raman-UV double resonance condition of ILSRS, the stimulated Raman spectrum of one isomer, or the other, can be unambiguously obtained. The assignment of the vibronic spectroscopy of the two rotational isomers of m-cresol and m-chlorophenol is uncertain in the literat~re.”’~ Thus,for these species, we have labeled the rotational isomer with the lower energy 0; transition as the a isomer and the other as the b isomer. For o-cresol we were only able to obtain ILSRS spectra from the rotational isomer with the lower energy 0: transition. This species has been assigned to the trans isomer by Ito et al.23 For the phenol monomer, vibrational assignments were made for the uI2 mode (a CC breathing mode), 2u16b(the overtone of a CC bending vibration), ul& (a CH bend), u7a (the CO stretching mode), u2 (the CH symmetric stretch), uZoa (a CH stretch), and uOH (the OH stretch), as indicated in Table I. The observed frequencies of these resonances are in good agreement with literature values.17~33.34 ILSRS spectra for the 0-, m-, and pcresol monomers were only obtained in the CC stretching region. No sharp depletions could be observed for higher energy resonances. Our inability to record ILSRS spectra of the higher energy cresol transitions might reasonably be attributed to spectral congestion caused by IVR involving methyl rotor torsional l e ~ e l s ~ ~ s congestion uch reduces the Raman intensity in the ILSRS spectra by distributing oscillator strength over many transitions. For tram-0-cresol and the a and b rotational isomers of m-cresol only one Raman transition was observed,see Table 11. For pcresol two Raman transitions were observed. The strongest of these was assigned to the u1 mode (a CC stretching vibration). In Figure 3 the ILSRS spectra of the a and b rotational isomers of m-chlorophenol are shown for the CC stretching region (top) and the CH stretching region (bottom). Figure 3 demonstrates the ability of ILSRS to distinguish between isomers of a given species. It further shows that there are considerable differences between the stimulated Raman spectra of the two rotational isomers of mchlorophenol. For pchlorophenol Raman transitions were observed in the CC stretching region and for the uOH fundamental, as indicated in Table 11. B. Stimulnted Raman Spectra of Weakly B o d Complexes. In this subsection we consider separately the Raman resonanca aso6ated with the phenol moiety and the complexing partner in the one-to-one complexes. For all the complexes in which H bonding would be expected we assume that the phenolic moiety is the proton-donating one. Such a situation would be expected (32) Johnson, J. R.; Jordan, K. D.; Plusquellic, D. F.; Pratt, D. W. J. Chem. Phys. 1990,93,2258. (33) Bist, H. D.; Brand, J. C. D.; William, D. R. J. Mol. Specrrosc. 1%7, 24, 402. (34) Green, J. H. S.; Harrison, D. J.; Kynaston, W. Specrrochim. Acra 1971, 27A, 2199. (35) Parmenter, C. S.; Stone, B. M. J . Chem. Phys. 1986, 84, 4710.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1992 1167 TABLE II: Observed RMIll Resonances rad Assignments for Jet-Cooled Substituted Phenols species frw? cm-’ assignment o-cresol 1041.8 m-cresol (a isomer) 1000.9 VI 2 m-cresol (b isomer) 1000.6 VI2 p-cresol 854.6 VI 860.9 m-chlorophenol (a isomer) 997.7 998.5 1001.5 Y12 3062.4 309 1.4 3665.9 Yon m-chlorophenol (b isomer) 1000.3 VI2 307 1.6 3075.2 3084.3 3085.2 3093.9 3096.8 (shoulder) 3097.9 (strong) 3098.8 (shoulder) 3666.5 Yon p-chlorophenol 833.1 847.3 853.6 3658.6 Yon “Estimated uncertainty of 10.1 c d ,
I
I
3080
I
I
3070
3080
I 3090
I
31W
Frequency (cm”) Figure 3. ILSRS spectra of the a and b isomers of the m-chlorophenol monomer in the 1000-cm-l (top) and the 3OOO-cm-l (bottom) spectral regions. The spectra were recorded by monitoring the m-chlorophenol parent ion signal and tuning the probe laser to either the a m-chloro: transition or the b m-chlorophenol 0 : transition. phenol 0
based on H-bonding information from condensed-phase studies.” Furthermore, it is supported by results pertaining to vibronic frequency shifts in the complexe~.’”**,~~-~~ And, it is entirely consistent with the Raman results reported below. 1. Proton Donor-Localized Vibrational Transitions. ILSRS spectra of the phenol H-bonded complexes and the phenol-Ar and phenolXH, van der Waals complexes are shown in Figures 4-6. Figure 4 shows the CC stretching region. Figure 5 shows the CO stretching region. Figure 6 shows the region of the OH stretch
1168 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Hartland et al.
- Ar 7N2
0 C
.-cn In C
.-0
-C6H6
4-
C
g!
7slycrs-
Q
a
H20
r ---)f-
1
- CH30H
-C~H~OH
- O(C2H&
990
1000
1010
1020
1030
1040
Frequency ( c m - ' ) Figure 4. ILSRS spectra of hydrogen-bonded and van der Waals complexes of phenol in the loOO.cm-' region. The spectra are arranged, from topt&ottom, in order to increasing red shift of the complex 0: transition from the phenol monomer. For the phenol dimer the transitions labeled with an (*) were assigned to the proton-donating phenol. For the phenolC6H6 complex the transition at -991.2 cm-l was assigned to the benzene localized v I mode.
TABLE IIk Electronic and Phenol-LocalizedVibrational Frequency
I
I
1240
1250
-H2O
T
7
7T-Y 1260
1270
1280
1290
Frequency (cm") Figure 5. ILSRS spectra of hydrogen-bonded and van der Waals complexes of phenol in the CO stretching mode region. The spectra are : transition from arranged in order of increasing red shift of the complex 0 the phenol monomer. For the phenol monomer the strongest ILSRS transition, at -1261.7 cm-', was assigned to the CO stretching fundamental In the phenol dimer spectrum the transition at -1271.0 cm-'was assigned to the proton-donating phenol v7a fundamental and the transition at 1244.9 cm-l was assigned to the proton-accepting phenol v7a fundamental.
-
Shifts (in cm-') for Phenol Compkxes species phenol -Ar -CHI -N2 C6H6
-phenol proton donating -phenol proton accepting -H,O CzH5OH CH30H -(CzH5)20 -NH3
electronic shift" 36360 -3 3 -58 -99 -146 -304
2v16b
'Ian
v7a
0.2 0.8 0.4 0.8 1.1
0.0
0.0
0.6 0.5 2.1 3.4
0.5 0.7 -1.0 -0.6
0.2 0.5 3.9 4.6 9.3
-78.1 -126.4
+353c
-1.5
-0.9
2.9
-16.8
-2.0
-353 -400 -414 -425 -634
1.2 1.2 1.1 1.0 1.1
3.6
-0.4 -0.2 0.0 -0.6 -1.1
11.8
-134.5
vibrational shiftsb VI2
5.4 5.5
3.6
13.9 10.3 17.8
VOH
-
"Shifts relative to the observed frequency of the phenol SI So 0: band. Estimated uncertainties are 1 cm-l. Except where noted, the values presented were measured in this work. Reported values preceding many of our values may be found in refs 1-6 and 8. bShifts relative to the values in Table I. Estimated uncertainties are about iO.1 cm-I. cFrom ref 2b.
fundamental. In the latter region we were only able to obtain good ILSRS spectra for the phenol-C6H6, (phenol),, and phenol-H,O complexes. Thus, these are the only species represented in Figure 6. The frequency shifts of the vibrational modes of phenol in the different complexes are given in Table 111. Also included in Table I11 are the frequency shifts of the S1 So 0; transitions of the complexes from that of bare phenol. From F v 4-6 and Table I11 it can be seen that the v,,, 2vlbbr and v7r resonances are blue-shifted and vOH is red-shifted when phenol acts as a proton donor, while the v18s fundamental can be either red- or blue-shifted depending on the complex. The magnitude of the frequency shift is greatest for the v7a and vOH
-
I
I
I
I
I
I
I
3520
3540
3560
3580
3WO
3620
3640
3660
Frequency (cm") Figure 6. ILSRS spectra of the vOH mode of phenol, phenOkc&, (phenol),, and phenol-H,O. In the phenol dimer spectrum, (A) is the proton-accepting phenol and (D) is the proton-donating phenol.
fundamentals, which are modes that are directly involved in the hydrogen bond. In the ILSRS spectra of phenol dimer two sets of Raman transitions were observed: one due to the proton-donating phenol and one due to the proton-accepting phenol." Those that appear (36)The vibronic spectroscopyof ref 2 suggests that in phenol dimer there is a proton donor phenol and a proton acceptor phenol. We have obtained
other evidence that this is so by rotational coherence spectroscopy: Connell, L.L.; Ohline, S.M.;Joireman,P.W.; Corcoran, T. C.; Felker, P.M.J. Chem. Phys., in press.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1169
Hydrogen-Bonded Complexes Containing Phenols
TABLE W. Ekbonic d Wenouc Vibntioml Frequency Shirts (in c d ) for suktitptcd Phenol Complexes electronic species shift' 36113 0-cresol (a isomer) C6H6
-H20 -NH, m-cresol (a isomer) C6H6
-H2O -NH, m-cresol (b isomer) C6H6
-H20 -NH3 pcresol C6H6
-H20 -NH, m-chlorophenol (a isomer) X6H6 -H20 -NH3 m-chlorophenol (b isomer) -H20 -NH3 pchlorophenol
