5761
J. Phys. Chem. 1995,99, 5761-5764
Evidence for the Cyclic Form of Phenol Trimer: Vibrational Spectroscopy of the OH Stretching Vibrations of Jet-Cooled Phenol Dimer and Trimer Takayuki Ebata," Takeshi Watanabe, and Naohiko Mikami Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980-77, Japan Received: December 12, 1994; In Final Form: February 28, 1995@
The OH stretching vibrations of the phenol dimer and trimer prepared in supersonic jets have been observed by IR-UV double-resonance and stimulated Raman-UV double-resonance spectroscopies. In the trimer, all the three OH stretching vibrations showed large red shifts from that of bare phenol. Evidence of the cyclic structure was obtained from an intensity altemation between IR and Raman spectra and by the measurement of the Raman depolarization ratios. In the dimer, it was found that the IR intensity of the OH stretching vibration of the proton-donating phenol is 4 times larger than that of the proton-accepting phenol.
Introduction Very recently, infrared (IR) and Raman spectroscopic studies of molecular clusters prepared in supersonic jets have become possible by using combined detection methods based on sensitive resonance-enhanced ionization (REMPI) spectroscopy. In our previous paper, by using IR-UV double-resonance spectroscopy or ionization detected IR (IDIR) spectroscopy,] we presented IR spectra of the OH stretching vibrations of phenol-(HzO), (n = 1-3) hydrogen-bonded clusters generated in supersonic free jets. In the spectrum, not only the vibrations of the phenol site but also the water site was clearly identified. We emphasized that the structures of the clusters are such that a hydrogen bond is formed between phenol and water cluster. Very recently, we found characteristic vibrations that are assigned to a hydronium ion in the cluster with n I4.2 In the present paper, we report the IR and Raman spectra of the OH stretching vibrations of the phenol dimer and the trimer generated in supersonic jets. The phenol dimer and the trimer are thought to be basic units of the phenol crystal whose structure is known to be pseudoorth~rhombic.~ The hydrogenbonding network of phenol is a key feature of the crystal structure. Therefore, knowledge of the structures and dynamics of the dimer and the trimer provides important information for the studies of crystal growth and vibration-phonon coupling in the crystal. For the spectroscopic study of phenol clusters in supersonic jets, Fuke and Kaya first reported the electronic spectra of the phenol dimer and the trimer.4 Later, Connell et al. obtained the rotational constants of the dimer by rotational coherence spectroscopy and reported that two phenols are bound by a single hydrogen bond.5 Although Fuke and Kaya suggested the cyclic structure for the trimer based on the electronic spectrum, no direct evidence of the cyclic structure has yet been given. As was demonstrated by stimulated Raman-W doubleresonance spectroscopy presented by Hartland et a1.,6 the investigation of the OH stretching vibration is of special importance, since the hydrogen bonding is directly reflected in this vibrational spectrum. If the structure of the trimer is of cyclic form, it will belong to the C3 point group. Therefore, not only will all three OH stretch vibrations show large red shifts from that of free phenol but also clear intensity altemation between IR and Raman spectra will be observed for the three vibrations. Furthermore, a measurement of the depolarization @
Abstract published in Advance ACS Abstracts, April 1, 1995.
0022-365419512099-5761$09.0010
ratios for these vibrations in the Raman spectra is also very useful to confirm the symmetric structure of the trimer. The structure of the clusters may also be reflected in the intensity change of the IR absorption. We investigated how the OH stretch IR absorption intensity changes upon hydrogen bond formation in the phenol dimer.
Experimental Section Both IR and Raman spectra were observed for the dimer and trimer of phenol by I R - W double-resonance spectroscopy and stimulated Raman-W double-resonance spectroscopy. In both spectroscopies, the depletion of the ground state population induced by IR absorption or stimulated Raman pumping was monitored through the ion current generated by resonanceenhanced multiphoton ionization (REMPI) via the S I state. The experimental setup and excitation scheme for the IR-UV double-resonance spectroscopy, called ionization detected infrared spectroscopy (IDIRS), is the same as was described previously.' The setup of the stimulated Raman-UV doubleresonance spectroscopy, called ionization detected stimulated Raman spectroscopy (IDSRS), was also described previously.' The only difference from the previous scheme is that we measured ionization loss spectra, which was described by Felker and co-workers.6x8 The IR laser beam or stimulated Raman pumping beams were introduced 50 ns before the ionization (UV) laser beam. When the IR laser frequency or Raman difference frequency is resonant with the vibrational transition, the ground state clusters are pumped to the vibrational level, resulting in the depletion of the REMPI signal which monitors the ground state population. Therefore, by scanning the IR laser frequency or Raman difference frequency while monitoring the ion signal, we obtain an ionization detected IR (IDIR) spectrum or an ionization detected stimulated Raman (IDSR) spectrum.
Results and Discussion
+
Figure 1 shows the origin region of the SI-SO (1 1) REMPI spectra of free phenol, phenol dimer, and the trimer. The S,SO electronic spectrum of phenol dimer has been investigated by several worker^,“.^ and its structure is well understood. The (0,O)band of the proton donor site of the dimer (36 044 cm-I) is red-shifted by 304 cm-l from that of free phenol. The peak at 36 164 cm-' is assigned to the intermolecular stretching vibration of the dimer. Other peaks have been assigned by 0 1995 American Chemical Society
5762 J. Phys. Chem., Vol. 99, No. 16, 1995
Letters 36348 cm-1 0-0
Phenol (Phenol)2 36044 cm-1 0-0
(Phenol)3
36000
36100
36200
36300
36400
Wavenumber I cm" Figure 1. Mass-selected (1 + 1) resonance-enhancedMPI spectra of bare phenol, the phenol dimer, and the trimer. A peak marked by an asterisk in the dimer and the trimer spectra is assigned to the monomer band, and the peaks marked by a dot in the trimer spectrum are assigned to the dimer bands which appeared due to insufficient mass separation. Dopfer et aL9 to the intermolecular vibrations and torsional vibrations of phenyl group. For the phenol trimer, no spectral analysis has been given nor has the SI-SO frequency of the (0,O) been reported yet. The SI-SO electronic spectrum of the phenol trimer shows a characteristic feature: the (0,O) band at 36 202 cm-' is very weak, and a long progression of 12 cm-l is seen with its maximum. intensity at v = 5. In addition to the main progression, several satellite peaks are observed. All these structures may be assigned to the torsional motion of phenyl groups. The weak (0,O) band indicates that the equilibrium torsional angle of phenyl group drastically changes in the SI state. A detailed vibrational analysis is in progress. Figure 2 shows the IDIR spectrum of the OH stretching vibrations of (a) bare phenol, (b) phenol dimer, and (d) phenol trimer. In this measurement, the UV laser frequency was fixed at several peaks in Figure 1, and the same spectra as those of Figure 2 were obtained. In the phenol dimer, two OH stretching vibrational bands were observed: one located at 3654 cm-' and the other at 3530 cm-I. The red shifts of the frequency of these vibrations from that of the bare phenol (3657 cm-I) are 3 and 127 cm-I, respectively. The observed frequencies agree well with those observed by Hartland et al. with IDSR spectroscopy.6 The former band is assigned to the OH stretching vibration of the proton accepting phenol and the latter band to that of the proton donating phenol. As can be seen in Figure 2b, the absorption intensity of the OH stretching vibration of proton donating phenol is stronger than that of the proton accepting phenol, which will be discussed later. In the IDIR spectrum of the trimer (Figure 2d), two bands are observed at 3441 and 3449 cm-I. The latter band is seen at a shoulder of the former band. The red shifts of the two bands are 216 and 208 cm-I, respectively, which are further red shifted from those of the dimer. The other noticeable feature of the spectrum is that no band is seen in the region near 3660 cm-I, which is the frequency of the OH stretching vibration free from hydrogen bonding. This result indicates that all the
(a) Phenol
IR
(b) (Phenol)z IR
( c ) (PhenoUz
Raman
, 3350
3400
I
,
3450 3500 3550 3600 Vibrational Energy / cm-l
I
3650
3700
Figure 2. Ionization detected IR (IDIR) spectra of the OH stretching region: (a) bare phenol, (b) phenol dimer, and (d) phenol trimer. Ionization detected stimulated Raman (IDSR) spectra: (c) phenol dimer and (e) IDSR spectrum of phenol trimer. The proposed structure of the phenol trimer is also drawn.
three bands are involved in the hydrogen bond in the trimer, suggesting the cyclic trimer. If the trimer has a cyclic structure in which all the three OH groups are involved in the hydrogen bond, the structure would
Letters
J. Phys. Chem., Vol. 99, No. 16, 1995 5763 1.4-
-P
m 1.2 n
tz
0
-
Donor
e Acceptor
1.00.8-
3360
3380
3400
3420
3440
3460
Vibrational Energy I cm'
Figure 3. IDSR spectrum of phenol trimer measured at two laser polarization geometries (-, El I IE2; EllE2). The spectra are modified by using eq 2 (see text).
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
IR laser power / pJ
---
Figure 4. IR laser power dependence of the dip intensities of the OH stretching vibration of proton donating site and proton accepting site of the phenol dimer (see text).
be expressed by the C3 point group assuming that the orientation of the aromatic ring does not affect significantly the OH vibration. In Figure 2 is shown the proposed srtucture of the trimer. In this structure, the three OH stretching vibrations are in-plane vibrations and are classified into A and E symmetry species with the A symmetry vibration most stabilized. The in-plane A symmetry vibration is strongly Raman active but IR inactive. The E symmetry vibrations are weak in Raman but active in IR. Therefore, to obtain conclusive evidence for the cyclic form, we measured the Raman spectrum by using IDSR spectroscopy. Figure 2 also shows the IDSR spectra of the phenol dimer (c) and the trimer (e). As can be seen in Figure 2e, a new intense band has appeared at 3394 cm-', while the two bands observed in the IDIR spectrum have become weak in the IDSR spectrum. These results agree very well with the expectation for the cyclic trimer. Therefore, it is concluded that the phenol trimer is cyclic, the 3394 cm-' vibration observed in the Raman spectrum is the symmetric (A species) OH stretching vibration, and the vibrations of 3441 and 3449 cm-I observed in the infrared spectrum are the degenerate (E species) OH stretching vibrations. To confirm the assignments of the symmetry given above, we further measured Raman depolarization ratios for the three OH stretching vibrations of the trimer. The measurement of the Raman depolarizationratio for the jet-cooled molecules was performed by Ebata et al. for the benzene and its dimer.7 The depolarization ratio (e) is equal to the ratio of the Raman intensities (Il/Ill) measured when the directions of the two linearly polarized Raman pumping beams are perpendicular (EllE2) and parallel ( EIIIE~).To change the direction of the polarization, a Babinet Soleile compensator was introduced in one of the Raman pumping (second harmonics of Nd:YAG laser) beam. The measurements were performed under the condition that the spectra were not saturated by Raman pumping lasers. Also, care was taken to correct for the reflection efficiency of the beam combiner for the two laser beams with different polarization. The evaluation of the Raman intensity from the IDSR spectrum will be described below, because the Raman bands in the IDSR spectrum are observed as absorption bands. The ratio (e = Il/Ill) is 0 5 < 0.75 for totally symmetric vibration and e = 0.75 for the non-totally symmetric vibration. Figure 3 shows the IDSR spectra measured at different laser polarizations, where input laser intensities are normalized. The obtained values of were 0.17 f 0.05 for the 3394 cm-' vibration and 0.7 f 0.1 for the 3441 and 3449 cm-' vibrations. By considering the experimental uncertainty, the latter value of can be assumed to be 0.75; the results
indicate that the 3394 cm-I band is classified as a totally symmetric OH stretching vibration, and the 3441 and 3449 cm-' vibrations are classified as non-totally symmetric OH stretch vibrations. Therefore, the cyclic structure of the trimer is confirmed by the measurement of the Raman depolarization ratios. Another interesting feature of the IR spectrum is the absorption intensity in the dimer. As can be seen in the Figure 2b, the absorption intensity of the OH stretching vibration of the proton donating phenol is much stronger than that of the proton accepting phenol. So, we obtained the relative absorption cross sections by measuring the dip intensities corrected by the IR laser power. From a simple rate equation model, the relationship between the IR (Raman) transition cross section and the ion intensity can be expressed as follows:
Here, OIR represents the IR absorption cross section and ZIRis the IR laser intensity. OR,,, represents the cross section of the Raman transition, and ZI and 1 2 are the Raman pumping laser powers. Con and Coffare the ion intensities when the IR (Raman) laser frequency is resonant and off-resonant with the vibrational transition, respectively. To derive this equation, we assumed that the stimulated emission rate from the vibrational level to the ground vibrational level is much smaller than the relaxation rate. This assumption is quite reasonable, since the decay rate of the OH stretching vibration is reported to be s-l from the line width measurement: while the stimulated emission pumping rate in the present experimental condition is estimated to be lo9 s-l. Figure 4 shows the plot of - ln(Con/Coff)on the IR laser power for the two OH bands. Straight lines were obtained for both of the bands, and the ratio of the two slopes, which corresponds to the ratio of the absorption cross sections, was obtained to be 4.1. Therefore, in the dimer, the IR absorption intensity of OH vibration of the proton donating phenol become 4.1 times larger than that of the proton accepting phenol. Since the change of the OH stretching vibration of proton accepting phenol is small as can be seen from the small red shift, this means that the IR absorption intensity of the OH stretch of the proton donating phenol is 4.1 times enhanced compared to that of bare phenol. The enhancement of the OH absorption intensity indicates the increase of the derivative of the dipole moment against the OH stretching vibration, and this is due to the fact that the charge distribution of the OH group
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5764 J. Phys. Chem., Vol. 99, No. 16, 1995
of the proton donating phenol drastically changes upon hydrogen bond formation. Such intensity enhancement of OH stretching vibrations by hydrogen bond formation is well-known in solution,I0 though it was rarely quantified in the bulk systems. Therefore, the present result can be quantitatively related to the structure by theoretical calculation. Similar enhancement of the OH stretch absorption intensity in hydrogen-bonded dimers has been reported for the methanol dimer.” Finally, we consider the lifetime of the OH stretching vibrational level of the dimer and the trimer. Hartland et aL6 measured the bandwidth of the OH stretching vibration of phenol in the phenol-HZ0 (1:1) cluster to be 1.7 cm-I. In the phenol dimer, the width of the OH stretching band of the proton donating phenol is estimated to be about 2 cm-I. Therefore, in the dimer, the vibrational relaxation is also on the order of a picosecond if the widths are entirely due to homogeneous broadening. In the trimer, the bandwidth is about 8 cm-I, and therefore the vibrational relaxation rate of the trimer is 4 times larger than that of the phenol dimer. The dynamics after vibrational excitation are very interesting. The input energy of 3400-3660 cm-’ is larger than the binding energy of the clusters, and it will be interesting to investigate how the input energy is released in the clusters. Such work is planned for the future.
Acknowledgment. This work was supported in part by the grant-in-aid on Priority-Area Research on “Photoreaction Dynamics” supported by the Ministry of Education, Science and Culture, Japan. References and Notes (1) Tanabe, S.; Ebata, T.; Fujii, M.; Mikami, N. Chem. Phys. Lett. 1993, 215, 347. (2) Ebata, T.; Watanabe, T.; Mikami, N. Chem. Phys. Letr., submitted. (3) Sheringer, C.; Wehehahn, 0. J.; Stackelberg, M. V. 2. Electrochem. 1960, 64, 381. Groth, P. Chem. Kristallogr. 1971, 4, 72. (4) Fuke, K.: Kaya, K. Chem. Phys. Lett. 1982,91,311; 1983,94,97. (5) Connell, L. L.; Ohline, S. M.; Joireman, P. W.; Corcoran, T. C.; Felker, P. M. J. Chem. Phys. 1992, 96, 2585. (6) Hartland, G. V.; Benson, B. F.: Venturo, V. A.; Felker, P. M. J. Phys. Chem. 1992, 96, 1164. (7) (a) Ebata, T.: Hamakado, M.; Moriyama, S.; Morioka, Y.; Ito, M. Chem. Phys. Len. 1992, 199, 33. (b) Ebata, T.; Ishikawa, S.; Ito, M.; Hyodo, S. Laser Chem. 1994, 14, 85. (8) (a) Venturo, V. A.; Felker, P. M. J. Chem. Phys. 1993, 99, 748. (b) Venturo, V. A.; Felker, P. M. J. Phys. Chem. 1993, 97, 4882. (c) Schaeffer, M. W.; Maxton, P. M.; Felker, P. M. Chem. Phys. Lett. 1994, 224, 544. (9) Dopfer, 0.; Lembach, G.; Wright, T. G.; Muller-Dethlefs, K. J. Chem. Phys. 1993, 98, 1933. (10) Pimentel. G. C.: McClellan. A. L. The Hvdrown Bond Freeman: New York, 1960. (1 1) Huisken, F.; Kulcke, A. J. Chem. Phys. 1991, 95, 3924. i
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