J . Phys. Chem. 1984,88, 1298-1302
1298
etc. about the local normal to the metal surface in order to take into account the fact that there is no preferred molecular orientation in the surface. In expressions that follow, axx,a,,,,, etc. refer to rotationally averaged values of the polarizability. The rotationally averaged a is related to a above through the similarity transformation U’aU where U is a simple rotation tensor about the z axis. For A-type modes we will consider the simplified case of a,, = a,,,,. In that event
(p:) Sa,2
a:
(a,
- 71f’~,J2
+ 477’ax,az2 + 31f27’2az,2
(Pz2) The depolarization ratio p = ( P : ) / ( P z 2 ) will therefore vary between its gas-phase value when 77’ is approximately unity to 0.33 when 77’is very large, Le., for excitation frequencies at which the normal component of the surface field exceeds the tangential component substantially. Hence A-type vibrational modes whose innate spontaneous Raman depolarization ratios are less than 1/3 will appear to be somewhat depolarized as a result of adsorption onto a metal sphere.
For B-type modes one obtains p = 3/4.
Note Added in ProoJ: CreightonIg has recently published an analysis of the depolarization ratios of the SERS spectrum of pyridine which is in many ways similar to that presented in this paper. Registry No. Ag, 7440-22-4;phthalic acid, 88-99-3;isophthalic acid, 121-91-5;terephthalic acid, 100-21-0; maleic acid, 110-16-7; fumaric acid, 110-17-8; urea, 57-13-6. Supplementary Material Available: Additional figures which contain the Raman spectra of the dicarboxylic acids studied along with the proposed geometries of the ions on silver are available. These are identified in the text by an S before the figure number (21 pages). Ordering information is given on any current masthead page. (18) D. A. Long, “Raman Spectroscopy”, McCraw-Hill, New York, 1977. (19) J. A. Creighton, Surf. Sci., 124, 209 (1983).
Gas-Phase Overtone Spectral Investigation of Inequivalent Aryl and Alkyl CH Bonds in Toluene and the Xylenes Kathleen M. Gough and Bryan R. Henry* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received: May 24, 1983; In Final Form: August 9, 1983)
The gas-phase overtone spectra of toluene, o-xylene, m-xylene, and p-xylene are measured in the region of Au = 3 and 4. The relationship between the shift in the position of the overtone peaks relative to benzene and the change in CH bond length is used to predict both aryl and alkyl rCH.Thus, the aryl CH bonds ortho to the substituent(s) are predicted to be 0.0014.002 A longer than those in the meta or para positions. The latter are essentially the same as in benzene. These results are in agreement with reported values of ~ C Hfrom recent 4-21 ab initio optimized geometry calculations. The conformationally inequivalent methyl CH bonds in o-xylene are clearly resolved. The methyl regions in the overtone spectra of toluene and of m- and p-xylene are more complex. A tentative assignment of the latter is made on the basis of the assumption that absorptions are observed from methyl groups below and above the barrier to internal rotation. The minimum-energy conformation of the methyl group appears to be “planar”, that is, with one CH bond in the plane of the ring. The CH bond is shortest in this position, increasing to a maximum when it is at 90’ to the ring plane. Calculations at the 4-21G level, using reported optimized geometries, are performed and show an overall increase in antibonding interaction between the methyl hydrogen and the ring as the position is changed from 0 to 90’ to the ring. This accounts for the increase in the methyl CH bond length.
Introduction Higher overtones of CH-stretching vibrations have been found to exhibit properties corresponding to the local environment of the oscillator.’ In this local mode model, the C H bonds behave as uncoupled anharmonic oscillators, and the positions of the peak maxima are given by hE (cm-’) = uCHv+ XcHv2
where u refers to the number of CH-stretching vibrational quanta. Conformationa12-sand steric differences6 between bonds, as well as structurally inequivalent C H bonds7 (methyl, aryl, acetylenic, etc.), are frequently resolvable in the spectra. In terms of the local-mode parameters, ucH,the local mode stretching frequency, (1) B. R. Henry, Vib. Spectra Struct., 10, 269 (1981). (2) B. R. Henry, I-Fu Hung, R. A. MacPhail, and H. L.Strauss, J . Am. Chem. SOC.,102, 515 (1980). (3) B. R. Henry and W. R. A. Greenlay, J. Chem. Phys., 72,5516 (1980). (4) B. R. Henry and M. A. Mohammadi, Chem. Phys., 55, 385 (1981). (5) J. S.Wong and C. B. Moore, J . Chem. Phys., 77, 603 (1982). (6) B. R. Henry, M. A. Mohammadi, and J. A. Thomson, J . Chem. Phys., 75, 3165 (1981). (7) W. R. A. Greenlay and B. R. Henry, J. Chem. Phys., 69, 82 (1978).
shows a dependence on bond length5-8,9while XCH,the diagonal local mode anharmonicity constant, is also sensitive to steric influences.6 The molecular structure of toluene, and in particular the lengths of the C H bonds, have been the subject of numerous investigations. Electron diffraction studieslOJ1 produced conflicting results. Detailed microwave analysis’2 failed to provide an unambiguous structure because of the complexity of the spectra. An ab initio calculation at the 4-21 level13 predicted ring asymmetry, with variations of up to 0.0014 8, among the aryl CH bonds. The methyl CH bonds were calculated to be 0.01 1 8,longer and were predicted to vary by 0.003 8, during internal rotation. It has been shown that a change in C H bond length of 0.001 8, corresponds to a frequency shift of 10 cm-’ in the fundamental CH-stretching frequency14 and 69 cm-’ at the fifth (8) Y. Mizugai and M. Katayama, Chem. Phys. Lett., 73, 240 (1980). (9) 8 . R. Henry and K. M. Gough, Laser Chem., in press. (10) T. Iijima, Z. Naturforsch., A , 32A, 1063 (1977). (1 1) R. Seip, Gy. Schultz, I. Hargittai, and Z. G. Szabo, Z . Naturforsch., A, 32A, 1178 (1977). (12) W. A. Kreiner, H. D. Rudolph, and B. Tan, J . Mol. Spectrosc., 48, 86 (1973). (13) F. Pang, J. E. Boggs, P. Pulay, and G. Fogarasi, J . Mol. Struct., 66, 281 (1980).
0022-3654/84/2088-1298$01.50/0 0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1299
Gas-Phase Overtone Spectra of Toluene and Xylenes
TABLE I: Deconvoluted Peak Positions (cm-')and Peak Areasa at A u = 3-5 for Benzene,b Toluene, and the Xylenes Av
3
assignt aryl m,P, m+p 0,o+m, o+p
benzene 8786
toluene 8806 (2.9) 8734 (2.0)
o-xylene 8805 (1) 8740 (1)
rn-xylene 8797 (1.0) 8754 (1.8) 8703 (1.0) 8500 (1.0) 8443
O f 0
4
methyl CH (in plane) CH (free)" CH (out of plane) aryl m, P, m+p 0,o+m, o+p
11498
8506 (1.0) 8445 8401 (1.5) 1 I 496 (2.9) 11423 (2.0)
8498 (1.0) 8399 (1.8) 11 405 (1.0) 11502 (1.1)
}lP::1:go; 11 349 (1.0) 11091 (1.0) 11 020
o+ 0
methyl CH (in plane) CH (free)c CH (out of plane) aryl m, P
a
11087 (1.0) 11098 (1.0) 11018 10959 (1.0) 10951 (1.8) 10957 (1.4) 14 072 14 061 (3) 0 13 962 (2) methyl CH (in plane) 13 555 CH (free)c 13 473 CH (out of plane) 13 409 Relative areas for peaks of a given bond type are given in parentheses From ref 21. See text.
00
88CO
8400 Y
(cm-l)
Figure 1. The overtone spectra of gas-phase toluene and o-xylene at 86 ' C in the region of Au = 3.
The consistency of these results for such a small change in bond length suggests that high-overtone CH-stretching vibrations can be a powerful aid in resolving structural problems. The liquidphase spectra of toluene and the xylenes have been reported p r e v i o u ~ l y . ~ Intermolecular ~-~~ interactions cause significant line broadening which obscures the small frequency shifts expected from the 4-21 studies. In the gas phase, the lines are much sharper and differences as small as 0.001 h; should be resolvable. In this paper, the results of a study of the gas-phase overtone spectra of toluene and the xylenes are presented. Also, ab initio molecular orbital calculations on toluene at the 4-21G level are reported and the results are compared with the overtone spectra.
Experimental Section The compounds were obtained commercially at 99+% purity and used without further purification. A multiple path length gas cell (Wilks Scientific Corp., South Nonvalk, CT, Model 5720) with a heating jacket was evacuated, and the temperature was kept constant at 86 "C. The sample was placed in a 50-mL flask fitted with a stopcock and Swagelok connections and degassed. Then the flask was attached to the gas cell, and the sample was heated in a water bath to 75 OC, with stopcock and inlet valves open, until sufficient vapor was introduced into the gas cell to give an absorption signal between 0.5 and 1.0 at Av = 2. Prior to recording the spectra at CW = 3 and 4, more sample was introduced into the gas cell. The path length was also increased for the higher (14) D. C. McKean, Chem. Sor. Reu., 7, 399 (1978).
(15) R. J. Hayward and B. R. Henry, Chem. Phys., 12, 387 (1976). (16) R. Nakagaki and I. Hanazaki, Chem. Phys. Letf.,83, 512 (1981). (17) R.Nakagaki and I. Hanazaki, Chem. Phys., 72, 93 (1982).
I
1
880C
p-xylene 8731 8499 (1.0) 8442 8400 (1.6) 11411 11080(1.0) 11 016 10950 (1.3)
I
8400
Y (cm-ll Figure 2. The overtone spectra of gas-phase m-xylene and p-xylene at 86 O C in the region of Au = 3.
001
oc 0 Y
(ern-')
Figure 3. The overtone spectra of gas-phase toluene and o-xylene at 86 ' C in the region of Au = 4.
overtones to a maximum of approximately 9 m at Au = 4. The spectra were recorded with a Beckman 5270 spectrophotometer, and the digitized signal was transferred to a Nicolet 1280 computer for analysis. The spectra of the xylenes, particularly at Au = 4, were very weak, so several runs were recorded and signal averaged. In all cases, the background spectrum was recorded from the evacuated cell at each path length and subtracted from the sample spectrum. Spectra were deconvoluted with the Nicolet curve analysis program, which fitted Lorentzian peaks to the experimental band. The calculated and experimental curves were plotted and compared to check the quality of the deconvolution fit. Molecular orbital calculations were performed on an IBM 370, with MONSTERGAUSS,'~ a modified version of the GAUSSIANBO (18) M. R. Peterson and R. A. Pokier, MONSTERGAUSS, Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, 198 1.
1300 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Gough and Henry
TABLE 11: Calculated and Experimental CH Bond Lengths (in A) for Benzene, Methane, and Toluene 4-2 molecule assignt
electron diffrctn
1'8
re 1.077 1.086
rcalcd
1.0721 1.0815 1.0728 1.0723 1.0719 1.0724 1.0733
e
YEC
YO
rgd
Raman
roe
YO
1.084 1.093 1.085 1.084 1.084 1.084 1.085
I
1.11 i 0.01
1
p-XYLENE
1.117 ?: 0.005
Reference 22.
, I1600
1
I1200
1
I
I0800
v (cm-ll
Figure 4. The overtone spectra of gas-phase rn-xylene and p-xylene at 86 OC in the region of Au = 4.
program.I9 The optimized geometries for both the planar and orthogonal conformations were taken from the results of Pang et al.I3 The 4-21G basis set in our program uses the exponents and coefficients obtained by Binkley et which differ slightly from those used in the geometry optimization."
Results The gas-phase spectra of toluene and the xylenes at Av = 3 and 4 are shown in Figures 1-4. The deconvoluted peak positions and the relative areas, where appropriate, are listed in Table I along with the peak assignments. Table I1 gives CH bond lengths in benzene and toluene (roLM)obtained from overtone peak positions through the correlation of a change in bond length of 0.001 A with a frequency shift of 33 cm-' at Au = 3 and 44 cm-' at Av = 4. This correspondence is based on the previous work referred to in the I n t r o d u c t i ~ n . ~ These * ~ J ~ values are compared to values obtained from geometry-optimized MO calculation^^^ and to values obtained from various experimental techniques. It was not possible to obtain Av = 5 because of the sensitivity of our experiment. However, the photoacoustically determined gas-phase overtone spectrum of toluene at Av = 5 has been reported.21 The data derived from this spectrum, as well as our data for Av = 3 and 4, were used to calculate the local-mode parameters for toluene from eq 1. These parameters were de(19) R. Krishnan, R. Seeger, D. J. DeFrees, H. B. Schlegel, s. Topiol, L. R. Kahn, and J. A. Pople, GAUSSIANBO,Department of chemistry, CarnegieMellon University, Pittsburg, PA, 1980. (20) J. S. Binkley, J. A. Pople, and W. J. Hehre, J . Am. Chem. SOC.,102, 939 (1980). (21) K. V. Reddy, D. F. Heller, and M. J. Berry, J. Chem. Phys., 76, 2814 (1982). (22) P. Pulay, G. Fogarasi, and J. E. Boggs, J . Chem. Phys., 74, 3999 (1981). (23) D. C. McKean, J . Chem. SOC.D,1373 (1971). (24) A. Langseth and B. P. Stoicheff, Can. J . Phys., 34, 350 (1956). (25) J. Herranz and B. P. Stoicheff, J . Mol. Spectrosc., 10, 448 (1963).
ra
1.054 1.092
LM
1.084
1.086 1.084 1.084 1.084 1.086
Reference 10.
Reference 11.
TABLE 111: Local-Mode CH-Stretching Frequencies and Diagonal Local-Mode CH-Stretching Anharmonicity Constants of Toluene aryl CH (ortho) CH (meta, para) methyl CR (in plane) CH (free)' CH (60" to plane) 'See text.
00
roiso f,g
local mode
1.078 f 0.002 1.083 -i. 0.002 1.074 t 0.002 1.083 i 0.002 1.078 * 0.002
assignt
m-XYLENE
IR
1.084 ?: 0.005h 1.0939,'
1.0818 1.0839 1.0886 1.096 ' For the "planar" conformation. The numberin of atoms is taken from ref 13. Reference 12. Reference 4. Reference 23. 'Reference 24. Reference 25.
I
microwave
WCH,
cm-'
3091 i. 9.2 3120.0 i 0.6 3021 f 3 2996.5 i 0.7 2978 t 3
XCH,cm-' -59.5 i 2.2, -61.6 i 0.1 -62.2 + 0.8 -60.2 i 0.2 -59.3 t 0.8
TABLE IV: Mulliken Electron Populations' between Methyl Hydrogens and Ring Carbons in Toluene at the 4-21G Level ordeg car- bitbon a l 0 30 60 90 C(1) u -0.049 85 -0.047 75 -0.042 72 -0.040 35 71 0.0 -0.00265 -0.008 65 -0,011 84 tot -0.04985 -0.05040 -0.051 37 -0.05219 C(2) u -0.00022 -0.00035 0.00003 0,00108 71 0.0 - 0.000 55 -0.001 54 -0.001 69 tot -0.00022 -0.00090 -0.00151 -0.00061 0.000 15 0.00003 -0.00004 C(3) u 0.00021 t 0.0 0.000 02 0.000 03 0.000 01 tot 0.00021 0.000 17 0.000 06 -0.000 03 C(4) u 0.0 0.0 0.0 0.0 t 0.0 0.0 0.00001 0.00001 tot 0.0 0.0 0.000 01 0.000 01 -0.000 06 -0.000 06 -0.000 06 -0,000 04 C(5) u 71 0.0 0.0 0.0 0.00001 tot -0.000 06 -0.000 06 -0.000 06 -0.000 03 0.00267 0.00250 0.00199 0.001 10 C(6) u t 0.0 -0.00025 -0.00095 -0,001 69 tot 0.002 67 0.002 25 0.001 04 -0.00059 Total population shared will be double these values. e = dihedral angle H(Me)C(Me)C(l)C(2) between the CH bond and the aromatic ring plane. termined for the inequivalent C H bonds and are listed in Table 111. The spectra at Av = 2 were also recorded but are not shown. Nonbonded interactions between the methyl hydrogens and the ring carbon atoms in toluene were determined from the 4-21G calculations and are listed in Table IV.
Discussion Aryl CH Bonds. The 4-21 calculation^'^ predict that the two C H bonds ortho to the methyl substituent will be about 0.001 8, longer than those in the meta and para positions. This is a manifestation of the substituent effect, which has been examined previously with respect to q,the inductive part of the Hammett g?628 The methyl group acts as a donor of both cr and x electrons. (26) K. M. Gough and B. R. Henry, J . Phys. Chem., 87, 3433 (1983).
Gas-Phase Overtone Spectra of Toluene and Xylenes These effects are apparent in our 4-21G calculation as shown by changes in the electron populations of the ring atoms. Both factors influence the CH bond strength primarily at the ortho position.29 The moderately good correlation of frequency shift in the liquid phase with uI for monosubstituted benzenes2’ evidently reflects the lesser changes at the meta and para positions, distorted by the presence of unresolved ortho C H bonds. o-Xylene has two C H bonds experiencing (ortho + meta) influence and two with (meta + para) influence. m-Xylene should exhibit three bond types, in the ratio of 1:2:1, from (meta + para), (ortho para), and (ortho ortho) influenced positions, respectively. p-Xylenepossesses only a single aryl C H bond type, (ortho meta). The aryl C H bonds should not change noticeably with internal rotation of the methyl group (ArCH C 0.0005 Meta and para effects should be small and approximately equivalent. The aryl region of the overtone bands of toluene and the xylenes displays precisely the patterns predicted (Figures 1-4, Table I). The toluene and o-xylene spectra at Av = 3 and 4 have doublets for which the ratios of the deconvoluted areas are approximately 3:2 and 1:1, respectively. The m-xylene spectrum at Av = 3 has a partially resolved triplet in the ratio 1:1.8:1. The p-xylene spectra at Av = 3 and 4 have only single peaks. The frequency shifts relative to benzene for all four molecules indicate that the bonds meta, para, and (meta para) to the methyl group(s) are comparable in length to benzene. This is compatible with both the low value of uI and our 4-21G toluene molecular orbital calculations. On the basis of the frequency shifts, the bonds which are ortho, (ortho meta), or (ortho para) are predicted to be longer by 0.001-0.002 A. The (ortho + ortho) bond in m-xylene is predicted to be still longer by 0.001 8,. The local-mode parameters for toluene (Table 111) indicate that the C H bonds ortho to the methyl group are less anharmonic than those meta or para to it. The ordering is subject to some uncertainty because of the relatively large error associated with the ortho CH anharmonicity. (The ortho C H peak at Av = 4 is not well resolved.) However, this is the expected ordering based on the same sort of steric hindrance observed in the methyl-substituted pentanes and the halogen-substituted benzenes.6s26 Methyl C H Bonds. The methyl group in toluene, and presumably in m- and p-xylene, is nearly a free rotor. The 6-fold barrier in toluene is 0.059 kJ/mol.12 The 4-21 ab initio calcul a t i o n ~ predict ’~ that the “orthogonal” form, with one C H bond perpendicular to the ring plane, is 0.013 kJ/mol more stable than the “planar” form with one C H bond in the ring plane. The C H bond is predicted to be shortest when in the ring plane and is predicted to increase by about 0.003 A to a maximum length when at 90° to the ring.13 Using optimizedI3 planar and orthogonal geometries and the 4-21(Y9 basis set, we calculated the planar form to be slightly more stable by 0.22 kJ/mol. The difference of -0.2 kJ/mol in the two 4-21G calculation^^^^^^ arises because of the aforementioned differences in orbital exponents and coefficients. We note that the energy ordering of the two conformers is now reversed, but such an energy difference lies within the uncertainty limits of these calculations. Inequivalent C H bonds in methyl groups have been studied extensively in recent years, both experimentally and theoretically. Ermler and Mulliken30used a large contracted basis set to examine hyperconjugation in toluene. However, only a standard nonoptimized geometry was used, and only the orthogonal conformer was considered. The quasi-a methyl MO was shown to interact strongly with the three occupied ring a MO’s, resulting principally in an antibonding interaction. In our calculation, the antibonding interaction of the quasi-a methyl M O with the ring M O s is also clearly evident. However, detailed analysis of all the nonbonded
+ +
+
+
+
+
(27) Y. Mizugai and M. Katayama, J. Am. Chem. Soc., 102,6424 (1980). (28) Y. Mizugai, M. Katayama, and N . Nakagawa, J. Am. Chem. SOC., 103, 5061 (1981). (29) K. M. Gough and B. R. Henry, J . Am. Chem. Soc., in press. (30) W. C. Ermler and R. S. Mulliken, J . Am Chem. Soc., 100, 1647 (1978).
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1301 interactions between the methyl hydrogens and the remaining atoms (Table IV) shows that the u MO’s also make a significant contribution to this interaction. The antibonding interactions are at a maximum for a methyl C H bond perpendicular to the ring plane and account for the predicted bond length variation. The barrier to internal rotation in o-xylene is quite large (6.3-9.2 k J / m ~ l ) . ~ lBoth methyl groups are in a planar configuration, which may or may not be geared.31,32 In either case, only two peaks are expected with intensities in the ratio 2:l and with the less intense peak at the higher frequency. This is exactly what is found in the spectra at Av = 3 and 4 (Figures 1 and 3). The ratio of the two peak areas is 1.8:1.0, and the frequency difference corresponds to a change in bond length of 0.003 8, (Table I). The methyl regions of the overtone spectra of toluene and of m- and p-xylene are essentially identical, apart from the increased relative intensity in the spectra of the disubstituted molecules (Figure 1-4). There are three major peaks at Av = 3 and 4. At Av = 3, additional lower intensity peaks can be seen. These additional peaks can be assigned to combinations involving two quanta of CH stretching and two quanta of C H bending. These combinations are also present at Av = 2 and in the fundamental spectra.33 Of the major peaks, the positions of those at the highest and lowest frequencies are almost identical with those of o-xylene (Table I). This correspondence suggests that these peaks arise from the two types of inequivalent hydrogens in the same planar conforma tion. The central methyl peak could arise from a resonance interaction with lower frequency normal modes. However, such combination peaks generally “tune in” and “tune out” of resonance for different overtones because of different anharmonicities for the interacting modes. As a result, the energy splitting between the combination and the pure local-mode overtone and their relative intensities change as a function of Au. Neither of these effects is observed in the spectra of toluene, m-xylene, and p-xylene. In fact, in toluene, the central methyl peak can be fitted extremely well to eq 1 (Table 111). Moreover, in all three molecules, the central peaks do not vary very much in relative intensity. Another possible origin of this central peak could be the presence of a second minimum-energy conformation. However, this explanation is also unlikely. Since there is only one peak, in such a conformation all three methyl C H bonds would have to be equivalent. The barrier to internal rotation in toluene is very low. In a recent of the gas-phase fundamental IR and Raman spectra of C6D,CHD2, two methyl CH-stretching bands were observed. The most intense band was assigned to transitions originating from rotational levels with energies above the rotational barrier. In C6D5CHD2,this barrier was calculated as a V, V4 V, potential, the first term being the largest. The small lower frequency peak was assigned to transitions involving the lower rotational levels and associated with the C H oscillator in an out-of-plane position. The splitting between these two peaks is 14 crn-’. A similar situation arises for N02CHD2.34,35In toluene, m-xylene, and p-xylene, the separation between the lowest frequency methyl peak and the central peak is close to 42 cm-I for Av = 3 and to 56 cm-’ for Av = 4 (Table I), i.e., just 3 and 4 times the observed fundamental difference. In earlier work, McKean and his co-workers used the term “average frequency” in referring to the central peak in CHD2N025 and in the series CHD2BF2,CHD2BC12,and (CHD2B0)3.36 It is particularly interesting that, in these latter molecules, it is the average frequency peak that dominates the Raman spectrum in
+
+
(31) H. D. Rudolph, K. Walzer, and I. Krutzik, J. Mol. Spectrosc., 47, 314 (1973). (32) J. Haupt and W. Muller-Warmuth, 2.Nazurforsch., A , 23A, 208 (1968). (33) A. B. Dempster, D. B. Powell, and N. Sheppard, Spectrochim. Acto, Part A , 31A, 245 (1975). (34) D. Cavagnat and J. Lascombe, J . Mol. Spectrosc., 92, 141 (1982). (35) D. C. McKean and R. A. Watt, J . Mol. Spectrosc., 61, 184 (1976). (36) D. C. McKean, H. J. Becher, and F. Bramsiepe, Spectrochim. Acta, Part A , 33A, 951 (1977).
J . Phys. Chem. 1984, 88, 1302-1307
1302
all three cases, whereas in the IR spectrum the dominant peaks correspond to the C H oscillator at positions associated with conformational energy minima.36 Cavagnat and L a ~ c o m b edeveloped ~~ a quantum description which explicitly introduced the frequency dependence of the CH-stretching mode of CHD2C6D,on the dihedral angle between the CH bond and the ring plane. Their treatment predicted that the Raman spectrum should be dominated by transitions involving states where the rotation of the CHDz group is free. Such transitions occur with a frequency corresponding to the average for the C H vibration during rotation. The treatment also leads to the existence of such a peak in the IR spectrum thus accounting for the observations of McKean et a1.35336 Because of the correlation between B and rcH5,8,14 and since rCH is a function of the dihedral angle 8, one might anticipate that a distribution of frequencies over all possible rCHshould be observed for the freely rotating methyl group. However, the central peak which we have assigned to the freely rotating methyl group is relatively narrow. This situation could well be analogous to that which occurs in N M R couplings37 and ESR hyperfine s p l i t t i n g ~also , ~ ~involving freely rotating methyl groups. In both cases, a quantum-mechanical treatment demonstrates that the ensemble average (sinz %) must be computed before the value of the observable can be predicted. This latter result will correspond to the average over the distribution and not to the whole range of values in the entire distribution. This value of the observable can also be approximated by the classical average. The methyl C H bond lengths from the optimized geometries13 follow the (37) W. J. E. Parr and T. Schaefer, Acc. Chem. Res., 13, 400 (1980). (38) P. B. Ayscough, M. C. BriceTand R. E. D. McClung, Mol. Phys., 20, 41 (1971).
expected sin2 % dependence. The classically averaged value of the bond length is 1.0944A, which is virtually identical with the value calculated on the basis of the overtone frequency shifts (1.0946 A). Thus, it appears that, for toluene, m-xylene, and p-xylene, the most reasonable assignment of the central peak in the methyl region is to transitions originating from rotational levels with energies above the rotational barrier.
Summary The overtone spectra of toluene and the xylenes in the gas phase are found to ,provide direct experimental evidence of subtle asymmetry in the ring, with variations in rcH(aryl) of up to 0.003 A. The two inequivalent CH bonds in the methyl group of oxylene lead to well-resolved spectral peaks which are useful in assigning the more complex toluene and m- and p-xylene spectra. A possible interpretation of the methyl regions of these latter spectra involves "free-rotor'' states. Nonbonded interactions, as determined from ab initio 4-21G calculations, account for the observed variations in methyl C H bond length. Both the aryl and methyl C y bond lengths determined from overtone frequency shifts are in better agreement with 4-21 calculation^^^ than those from any other technique.
Acknowledgment. We are grateful to Professor T. Schaefer and to Drs. W. Siebrand and T. Wildman for helpful discussions. We are indebted to Dr. J. D. Goddard for a modified version of the GAUSSIAN80 program. K.M.G. is grateful to the University of Manitoba for a graduate fellowship. Lastly, we are grateful to the National Sciences and Engineering Research Council for financial support. Registry No. Toluene, 108-88-3; o-xylene, 95-47-6; m-xylene, 10838-3; p-xylene, 106-42-3.
Optimization of Conditions for Photochemical Water Cleavage. Aqueous Pt/TiO, (Anatase) Dispersions under Ultraviolet Light J. Kiwi* and M. Gratzel Institut de Chimie Physique, Ecole Polytechnique F2dZrale. CH- 1015 Lausanne, Switzerland (Received: May 26, 1983; In Final Form: August 22, 1983)
This paper describes the preparation and optimization of a water-cleavage catalyst consisting of Pt/TiOz (P-25 Degussa). Two different methods were used to prepare the catalyst: impregnation followed by reduction (calcination in certain cases) and exchange. HzPtC16and Pt(NH,),(OH), were used as base materials. The efficiency of the catalyst was tested in relation to its ability to mediate Hz evolution for water under UV light. The influence of pH, temperature, concentration of Pt, as well as preparation temperature and time of reduction are described. Characterization by electron microscopy (EM), surface area measurements (BET), diffuse reflectance spectroscopy (DRS), hydrodynamic particle radius Rh,and electrophoresis is carried out. It is shown that metal islands with a particle size below 8 A deposited onto TiOz agglomerates of 3000 < Rh < 6000 A are the most active catalytic species. The fate of oxygen produced by conduction band holes is also investigated.
Introduction The problem of H z evolution in sacrificial' as well as cyclic photoinduced processesZis a topic of relevant research in several laboratories working in the area of energy-conversion processes. ~~~~
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Lately, a need has arisen to improve deposition techniques of small metal islands of platinum over the existing procedure^.^,^ Pt5,6 deposited on a photosensitive support like TiOz has been shown to be a suitable material to use in photocleavage processes. TiOa has also recently received attention as a noncoventional support
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(1) (a) B. Koryakin, S. Dzhabiev, and E. Shilov, Dokl. Akad. Nauk. SSSR, 233,359 (1977); (b) J.-M. Lehn and P. Sauvage, Nouu. J . Chim., 1, 449 (1977); (c) K. Kalyanasundaram, J. Kiwi, and M. Gratzel, Helu. Chim. Acta, 61, 2720 (1978); (d) J. Kiwi and M. Gratzel, J . Am. Chem. SOC.,103, 2939 (1981). (2) (a) J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca, and M. Gratzel, Angew. Chem., In?. Ed. Engl., 19, 647 (1980); (b) J.-M. Lehn, P. Sauvage, R. Ziessel, and L. Hilaire, Isr. J . Chem., 22, 168 (1982); (c) E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, and M. Gratzel, J. Am. Chem. SOC.,103,6324 (1981); (d) J. Kiwi, K. Kalyanasundaram, and M. Gratzel, Struct. Bonding (Berlin), 49, 37 (1982); (e) J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca, and M. Gratzel in 'Photogeneration of H2", Academic Press, London, 1982, p 119.
0022-3654/84/2088-1302$01.50/0
(3) (a) J. Turkevich, K. Aika, L. Ban, J. Okura, and S. Namba, J . Res. Inst. Catal., Hokkaido Uniu., 24, 54 (1976); (b) J. Kiwi and M. Gratzel, Nature (London), 281 657 (1979); (c) E. Borgarello, K. Kalyanasundaram, Y . Okuno, and M. Gratzel, Helu. Chim. Acta, 64, 1937 (1981); (d) J.-M. Lehn, J. Sauvage, and R. Ziessel, Nouu. J . Chim., 5, 291 (1981); (e) P. Keller and A. Moradpour, J . Am. Chem. SOC.,102, 7193 (1980); (f) A. Mills and G. Porter, J. Chem. Soc., Faraday Trans. 1,78,3659 (1982); (g) A. Harriman and G. Porter, J . Chem. SOC.,Faraday Trans. 2, 78, 1937 (1982). (4) H. Miles and A. Thomason, J . Electrochem. SOC.,123, 1459 (1976). (5) A. Heller, "Semiconductor Liquid Junction Solar Cells", The Electrochemical Society, Princeton, NJ, 1977, pp 195-208. (6) H. Gerischer, Pure Appl. Chem., 52, 2649 (1980).
0 1984 American Chemical Society
