J. Phys. Chem. 1995, 99, 4386-4396
4386
C(l)-C(a) Torsion Potential Function and Vibrational Assignments of trans-P-Methylstyrenefrom S1-So Supersonic Jet Fluorescence Spectra W. E. Sinclair, H. Yu, and D. Phillips Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK
R. D. Gordon' and J. M. Hollas* Department of Chemistry, University of Reading, Reading RG6 2AD, Berkshire, UK
S. Klee and G. Mellau Physikalisch-Chemisches Institut der Justus-Liebig- Universitat, Heinrich-Buff-Ring 58, 0-6300 Giessen, Germany Received: May IO, 1994; In Final Form: September 12, 1994@
-
The fluorescence excitation spectrum and the single vibronic level dispersed fluorescence spectra for the .&'Af A I A ftransition of trans-P-methylstyrene are reported. Together with information from the highresolution infrared spectrum, vibrational assignments are made in the ground and excited states. A short vinyl torsion vibrational progression has been observed in SO and fitted to a V(4) = C,Vn( 1 - cos ncp) potential with V2 = 855 cm-' and V4 = -218 cm-' which, like that for styrene, is flat-bottomed. A possible assignment for a transition involving the methyl torsional motion is made, and the results are compared with the analogous motion in propene. The role of intramolecular vibrational redistribution is also assessed with relevance to intermode coupling of the system modes to low-frequency modes of the substituent group in the excited state.
1. Introduction The structure of the trans-/3-methylstyrene (tpMS) molecule is shown in Figure 1. In this molecule there are two torsional motions of interest, those of the methyl and vinyl groups with 8 and 4 as the corresponding torsional angles marked in the figure. In this study both of these torsional motions in the ground electronic state have been investigated by single vibronic level fluorescence (SVLF) spectra in a supersonicjet; the results are compared with fluorescence studies of styrene and infrared studies of propene. The problem of the torsional potential function of styrene for the C(1)-C(a) torsion vibration (Y42) in the electronic ground state has been solved using SVLF.'., The observed vibrational levels were fitted with the potential function
I
IS
.g c Y
I
P
Pt! I
G
I
I
I
I
0
I
200
I
I
400
I
I
600
I
I
I
,
I
I
I
I
,
I
800 1000 1200 1400 1600 1i 30 A; I cm-1 -r
-
and the following parameters were determined: V, = 1070 cm-',
V, = -275 cm-'
The replacement of a ,&hydrogen with a methyl group introduces the possibility of hyperconjugation which may affect the vinyl torsional potential. The methyl torsional potential in the SO state of propene, which may be similar to that of the methyl group in @MS, has been obtained by the observation of the corresponding fundamental and associated hot bands in the far infrared by using high-resolution Fourier transform technique^.^ The torsional intervals were fitted to the potential
' Present address: Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6 Canada. * To whom correspondence should be addressed. 'Abstract published in Advance ACS Abstracts, March 1, 1995.
Figure 1. Excitation spectrum of tPMS for the A'A' X'A' electronic transition. The structure of tPMS is also shown with the a-inertial axis and the torsional coordinates defined. The insert shows the (a) observed and (b) A-type computer simulated rotational band contours for the 0; band.
in eq 1 with the following parameters: V, = 693.7 cm-',
V, = -14.0 cm -1
The way in which the methyl torsional potential changes in tPMS relative to the potential in propene in the ground state will depend on the interaction with the phenyl group. The change in potential in going to the excited state of tPMS will depend on structural changes in the region of the methyl group. The tPMS molecule belongs to the C, point group and all out-of-plane vibrations, including the vinyl torsional vibration v42. belong t o the a" symmetry species. The observation of
0022-365419512099-4386$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 13, 1995 4387
Torsion Potential of trans-P-Methylstyrene bands in the fluorescence excitation (FE) and SVLF spectra involving ~ 4 2require that A~42is even. The analysis of electronic absorption and SVLF spectra of styrene revealed a very strong Duschinskii effect? in which the vinyl torsion vibration, ~42,and the out-of-plane substituent bending vibration about C(1), v41, are strongly mixed in the A state relative to the % state. The Duschinskii effect confers intensity on such transitions as 41:42:, where n is odd, thereby providing a means of access to levels involving odd quanta of ~ 4 in 2 SO. The observation of these levels, as well as those with n even, makes the fitting of the observed levels to the potential function more satisfactory. We report here various SVLF spectra of tPMS in which we observe ~ 4 even 2 levels from a progression up to v = 6 and ~ 4 odd levels from the observation of combination bands up to v = 5 for the vinyl torsional vibration and use them to calculate the vinyl torsional potential. Many of the assignments of bands in the SVLF spectra rely on the observation of the ~ 4 fundamental in the far-infrared spectrum, which then provides confirmation of the combination bands observed involving ~ 4 and ~ 4 2 . To understand the interactions which contribute to intramolecular vibrational redistribution (IVR) requires a detailed knowledge of the energy level structure of the molecule, as has been shown for tilb bene,^ and in this work we present a detailed analysis of the vibrational modes of tPMS in the ground and excited states from fluorescence and infrared spectra. In particular the low-frequency modes are thought to contribute to this relaxation process, and this can be assessed if sufficient spectroscopic information on these modes is known. These lowfrequency modes are shown to be mixed in the excited state due to the Duschinskii effect mentioned above and represents an additional mode coupling which adds to the spectral congestion of the resolved fluorescence and may influence the dynamics of IVR. Hopkins et aL6 have indicated in their studies that the relaxed fluorescence from alkylbenzenes becomes sharper as the alkyl chain length increases and indicates an approach to complete vibrational relaxation. This suggests that a mechanism exists whereby vibrations involving torsional and bending motions of the alkyl chain can couple with the benzene-type modes. The structure of tPMS represents an extension of the substituent chain length compared to styrene with an increase in the number of low-frequency torsion and bending modes. So we examine the excess energy dependence of the SVLF spectra to assess the role of low-frequency modes in IVR.
2. Experimental Section The tPMS (99%) was obtained from Aldrich Chemical Co. Ltd. and was used without further purification. Two sets of apparatus were used to record the fluorescence spectra in this paper, one to record the fluorescence excitation spectrum and another to record single vibronic level dispersed fluorescence spectra. 2.1. Fluorescence Excitation Spectra. The continuouswave (cw) supersonic jet apparatus has been described elsewhere.' The sample vapor was produced by heating the liquid at 50 "C in a temperature-controlled oven and was then expanded through a 100 p m aperture with a backing pressure of 2.07 bar of He. Excitation of the jet-cooled molecules occurred at a nozzle-to-laser distance of 4-5 mm using the frequency-doubled output of an excimer-pumped dye laser system (Lambda Physik EMG 103MSCFL2002). Total fluorescence was collected with a n f l l lens and focused onto an EM1 XP2020Q photomultiplier tube. The output from the PMT
2
1
1
was processed by a Stanford Research Systems (SRS) dualchannel boxcar integrator. The fluorescence was then normalized with respect to fluctuations in laser intensity. 2.2. Single Vibronic Level Fluorescence Spectra. The supersonic jet apparatus used to produce a continuous jet for these spectra has been described previously.8 In this apparatus a Welsh-type arrangement of four concave mirrors in the vacuum chamber increases the efficiency of fluorescence collection. The sample was seeded into the helium backing gas (0.69 bar) by flowing it over the surface of the liquid tPMS at room temperature. The nozzle diameter was 100 pm. The frequency doubled output of a Quantel Datachrom 5000 Nd: YAG pumped dye laser system was used as the excitation source and the fluorescence dispersed using a Spex 1 m scanning spectrometer with slit widths of 150-250 pm and detected by an EM1 9789QB photomultiplier. The signal was processed by an SRS boxcar interfaced to a microcomputer. 2.3. Far-Infrared Spectrum. The infrared bands were recorded on a Bruker IFS 120 HR FT-IR spectrometer at a resolution of 0.05 cm-'. A 3 m cell was used at room temperature with a sample pressure of 1.5 mbar.
3. Vibration Wavenumbers in the SOState The tPMS molecule has 51 normal modes of vibration, but, as we are not concerned with vibrations within the CH3 group apart from the CH3 torsion which we label z and because we want to make comparisons with styrene, we treat the CH3 group like a single atom. There are then effectively 42 normal modes plus z. Table 1 gives the wavenumber and approximate descriptions of the vibrations. We adopt the mode numbering of Hollas et a1.: based on Mulliken's con~ention.~ The numbering given in brackets in the table and referred to throughout this report, is based on Wilson's conventionlo for benzene-like modes and allows a direct comparison with the styrene numbering scheme.
4. Fluorescence Excitation Spectrum of tmS
-
The pure electronic transition (0; band) of the A'A' %A' system of tPMS was detected at 34 586 cm-I (2891.3 A) in agreement with the value of 34 585 cm-' obtained by Grassian et al." using TOFMS. Part of the jet-cooled FE spectrum of tPMS is shown in Figure 1, and the wavenumbers and assignments of the excited state vibrations are given in Table 2. The observed rotational contour of the 0; band is shown in the insert of Figure 1 along with a simulated "A-type" contour using the rotational constants from an assumed geometry of tPMS in the ground and excited states. The comparison is a good one and suggests the transition dipole moment is in-plane and orientated in a direction close to the a axis of the molecule, shown in Figure 1. As expected the FE spectrum has a similar appearance to the jet-cooled spectrum of styrene12(0; at 34 760 cm-'). The greatest similarity is expected to occur where bands due to benzene ring-derived normal modes are well-known to occur for monosubstituted benzenes. on the basis of the assignments made by Hopkins et al.13 for a series of alkylbenzenes, it was determined that the frequencies of the [6b], [12], and [18a] modes are largely insensitive upon increase in the alkyl chain length and that the [6a] and [ 11 modes can change substantially. Strong vibronic activity occurs in the 0; 900 cm-' to 0; 990 cm-' region of the spectrum. The bands in this region of the FE spectrum of styrene were assigned to six fundamentals, four of which are in-plane qC-H bending vibrations.
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4388 J. Phys. Chem., Vol. 99, No. 13, 1995
Sinclair et al.
TABLE 1: Wavenumbers and Approximate Descriptions of the Vibrations of t/?MS in the Ground and Excited Electronic States ground state vibration approx vlcm-' vlcm-' excited state no.a descriptionb from IR from SVLF vlcm-' from FE 1 2 3 [2] 4 5 6 7
8 9 10[8a] 11 [8b] 12 [19a] 13 [19b] 14[14] 15 16[3] 17 [13] 18[9a] 19[15] 20 [18b] 21 [18a] 22[12] 23 24 [ 11 25 [6bl 26 27 [6a] 28 29 [9b]
a' 3080 3040 2925 2970
CH str CH str CH str CH str CH str CH str CH str CH str C=C str C-Cstr C-Cstr C-C str C-C str C-Cstr P=CH-
1671 1610 1500 1450 1370 1340 1306
1499 1482 1428 1343 1306
PC-H
X-sens (C-R str) PC-H PC-H p C-H PC-H
ring C-C str X-sens(aC-C-C)
1208 1180 1080 1050 1035 1005
969
(aC-C-C)
P -C=C def X-sens(aC-C-C)
p c=c-c
1142 1210
347
X-sen@-R)
1035 1003 944 821 62 1
959 927 795 537
405 347 210
385 283 198
a" 30 31 [5] 32 33 [17a] 34 [17b] 35 [loa] 36 [ l l ] 37[4] 38 [16b] 39[16a] 40 41 [lob] 42
y=CHyC-H y=CHyC-H yC-H yC-H 4C-C X-sens (4C-C) 4C-C yC-CHs X-sens (yC-R) +C-R
394 374 275 122 28
277 122
258 209 121 77 165
There are five comparable bands in this region of the FE spectrum of @MS at 0; 909 cm-', 0; 927 cm-', 0; 959 cm-', 0; 969 cm-', and 0; 972 cm-'. On the basis of the assignments made by Hopkins er uZ.,'~ it is clear that the modes in this region, although located in the benzene ring, are quite dependent on the alkyl chain length. Comparing the shifts and relative intensities of bands of rPMS with those of styrene and those of alkylbenzenes with chain lengths greater than two carbon atoms, the band at 0; 927 cm-' is assigned to the ring mode, [12:], and the band at the 0; 959 cm-l is assigned to the pC-H bending mode, [18aA]. The observed range of frequencies for these modes in the alkylbenzenes series is [v12] = 932 f 2 cm-' and [vlSa] = 966 f 6 cm-'. The band at 0; 969 cm-' is assigned to C-H bending mode [1$] in comparison with the same mode assigned to a band at 965 cm-' in styrene. The other pC-H bending modes [9a;] and
+
+
+
+
+
-45
41
ff
0:
49 94 155 (193) 198 (237) 243 283 329 (371) 335 376 356 385 (395) 418 424 (505) 437 466 5 14(524) 537(537) 569 770 779 795(745)
41i42i 40 42, 4 41 29; 400 28;
429 38 41;
39?424 P 29,4 1 27; 390 38i42; 28 41, 38P39: 38;
25P 29; 27 ?
::)1;42; 240
909 927 (947) 959 (959) 969 972 1135 1139 1142 (1145) 1210 (1209) 1250 1265 1282 1294 1315 1335 1345 1365 1408 1428(1428) 1452 1482(1474) 1495
+
+
? 22; 21, 19; 20;, 18:(?) ? 27;29: 16; 17; ? ? (909)'39l42: (909) 27 22A27,9 O 24'25' O P 21;27$ 17'41 P P 17,29, 12; 17 39; 13,P 17;28;
P P
'A5 = VL - V(0:) where V L is the laser wavenumber. Values in parentheses are for styrene. 0; at 34 586 cm-I.
+ +
[18b:] remain uncertain; however, the band at 0; 972 cm-' may be attributed to one of these. The band at 0; 909 cm-' is assigned as due to a coupled mode, involving mainly the 24:[ l:] mode. The band at 0; 537 cm-' is assigned to the ring mode 2$[6bA]. The observed range of frequencies for this mode in the alkylbenzenes series is [v6b] = 526 f 4 cm-'. The band at 0; -k 385 cm-' is assigned to the 27:[6a;] mode. The observed frequency for this mode in styrene is [v6a] = 395 cm-'. The 795 cm-' is assigned to the 24:[1:] mode. The band at 0; observed frequency for this mode in the styrene is [VI]= 745 cm-' and the observed range of frequencies for this mode in the alkylbenzene series is [VI]= 748 f 18 cm-'. The strongest vibronic band is at 0; 1210 cm-l and is assigned to the 17:[13:] mode, which is a vibration essentially involving a stretching motion of the C( 1)-C(a) bond. The observed frequency for this mode in styrene is [v13] = 1209 cm-'. The band at 0; 569 cm-' is assigned to the combination, 27A29;. This band is unusual in that it has significant intensity. However, there is generally, in aromatic molecules, much more extensive vibronic coupling activity in the excited electronic states than in the ground state because other excited states are nearby. The sum of these excited-state vibration wavenumbers differs by +14 cm-' from 569 cm-', but large anharmonicity in combination levels involving ~ 2 has 9 been observed in styrene. Although the spectrum is dominated by bands due to benzenetype modes, a series of bands which can only be due to the vibrations of the substituent group are observed within 520 cm-' of the origin. As has been shown in styrene: transitions involving both ~ 4 (C( 2 I)-C(a) torsion), v41 (C(1)-C(a) outof-plane bend) and two C-C ring twist vibrations are assigned. These modes are a'' vibrations and so only even overtones or combinations of these modes are allowed. In the jet-cooled spectrum of styrene,12the 42;, 41;, and 40; (v40 is a ring twist mode in styrene) transitions were observed as well as various
+
695
The main numbering scheme is according to Mulliken. Numbering given in brackets refer benzene-like modes using Wilson's convention. R, substituent group; str, stretch; a,in-plane ring angle bend, P, inplane bend; y , out-of-plane bend; 4, torsion or twist.
+
~~
+
980 962 950 810 730
yC-H
TABLE 2: Assignment of Part of the Fluorescence Excitation Spectrum of t s M S __ AP,blcm-' assignment' AVo.b/cm-' assignment
+
+
J. Phys. Chem., Vol. 99, No. 13, I995 4389
Torsion Potential of trans-P-Methylstyrene
130 155 180 205 230 255 280 305 330 355 380 405 430 455 480 505 530
AT / cm-1
-7
Figure 2. Excitation spectrum of tBMS in the low-wavenumberregion. The insert shows the hot hands observed at a lower helium backing pressure. combinations of v ~ v41, , and v42 such as 41;42:, which confirms a strong coupling between these modes. We report a similar set of transitions in the FE spectrum of tPMS with the observation of 42; and 41; at 0; -I-329 cm-' and 0; 155 cm-', respectively. Figure 2 shows the FE spectrum in the low-wavenumber region of interest, and the insert of Figure 2 shows very weak hot bands involving population of the 421 level in SO. We also observe bands due to the two ring twist modes, 198 and v39. The vibration v39 is substituent sensitive and was not observed in the FE spectrum of jet-cooled styrene. The observation of bands such as 39;, 39:42:, and 39A41: would seem to indicate an increase in coupling between the vinyl group and the benzene ring in tPMS compared to styrene. The band at 0; 198 cm-' has been assigned to 29; (v29 is the substituent in-plane bend about C(1)). This is based on a comparison with a value of 237 cm-' for ~ 2 for 9 styrene and from the SVLF spectrum which has an unusual effect similar to that observed for styrene (section 5.2). The substitution of a /3-hydrogen by a CH3 group creates an out-of-plane CH3 bending vibration (v40) and we assign the band at 0; f 243 cm-I to 40; on the basis of the SVLF spectrum and from the observation of a hot band at 0; 94 cm-' which we assign to 40i42:. The assignment of other low-frequency modes and c o n f i i a tion of benzene-type fundamentals rely on the analysis of SVLF spectra discussed in section 5 . The extent to which the substituent modes contribute to IVR will depend on the how effective the coupling mechanism is in the excited state. The jet-cooled spectrum of styrene revealed the coupling of low-frequency modes with benzene-type modes. We report a similar coupling mechanism with the observation of transitions involving v39 and also combination bands of the vibrations involving the vinyl group with benzene fundamentals such as [13;]41;, [13;]29;, and [13;]40:. The effects of such coupling will be discussed in section 10. There is no evidence for activity of the methyl torsional motion in S I .
+
+
+
5. Single Vibronic Level Fluorescence Spectra of t m S
5.1. Excitation in the 0; Band. Figure 3 shows part of the SVLF spectrum following excitation into the 0; band and the wavenumbers and assignments, compared with corresponding assignments in styrene, are given in Table 3. As for styrene,2 the spectrum is dominated by a progression in the C(1)-C(a) torsional vibration, v42, built on the 0; band. The intensity
A9 / cm-1
-v'
Figure 3. Part of the SVLF spectrum with excitation in the 0; band of tBMS.
TABLE 3: Assignment of Part of the SVLF Spectrum following Excitation into the 0; Band 0'
67(86) 150 155 210(241) 217 232 242(398) 254 300 304 326 347 405(442) 472 560 62 l(621) 688 75 1 77 1 821(776) 889 944 976 1003(999) 1031 1069 1129
0:
42; 41:42: 42: 29: 7742: 4 1Y42: 41; 42: 407427 41;42; 41042; 28: 27: 27Y42; 27y42: 25: 25Y42; 27728: 25742: 24: 24042; 23: 24042: 22: 22y42:
1155 22:42: 1166 24:28: 1208(1203) 17: 1233 1277 1306(1303) 1343 1372 1408 1440 1499(1494) 1561 1584 1624 1653 1671(1630) 1737 1827 1895 1925 2007 2035 2076 2214 2244 2290 2310 2356
17y42: 15: 14: 15042; 22727: 24Y25: 11:
22Y2.5: 15728: 9: 9:42: 17Y25: 17:25:42: 22; 9:27: 17Y22: 22Y25; 9:25: 16722: 9:25:42;
242 1 2444 2494 2585 2612 2628 2655 2673 2704 2738 2770 2835 2877 2895 2938 3015 3035 3076 3118 3168 3214 3232 3249 3292 3332 3496 3674
A s = i j ~- S(0;) where CL is the laser wavenumber. Values in parentheses are for styrene. 0; at 34 586 cm-I.
pattern characteristic of the progression in ~ 4 is2 repeated with other bands involving totally symmetric fundamentals, overtones and combinations. As mentioned in section 4, the [6b], [12], and [18a] modes are insensitive to substitution. The bands at 0; - 621 cm-', 0; - 1003 cm-l and 0; - 1035 cm-' are assigned to the [6@, [12:], and [18a:] modes, respectively. The observed frequencies for these modes in styrene are [v&] = 621 cm-', [v121 = 999 cm-' and [vlga]= 1019 cm-'. The observed ranges of frequencies for these modes in the alkylbenzenes are [v6b] = 627 f 2 cm-', [aq2] = 1010 f 2 cm-', and [vlga]= 1038 f 3 cm-l.
Sinclair et al.
4390 J. Phys. Chem., Vol. 99, No. 13, 1995
-1
00
0
.c
1
Ia
i
,
I
0
50
100 -7
1
150
ZOO
250
300
350
400
450
AT / cm-l
Figure 4. High-wavenumber region of the SVLF spectrum with excitation in the 0; band of $?MS.The insert shows the expansion of the weak bands in the 200-330 cm-' region of the spectrum.
The [l] and [6a] modes are sensitive to substitution. The The band at 0; - 405 cm-' has been assigned as 27:[6a:]. observed frequency for this mode in styrene is [ Y ~ = J 433 cm-'. The band at 0; - 821 cm-' has been assigned as 24:[1:]. The corresponding wavenumber of this vibration for styrene is 776 cm-'. The substantial shift is not unexpected as the 24:[1:] mode is sensitive to substitution. A similar observation is found in the jet-cooled alkylbenzenes,6in which the vibration wavenumber for [VI]in ethylbenzene is 778 cm-' and increases to 821 cm-' in n-propylbenzene. The band at 0; - 1208 cm-' has been assigned as 17: [13:]. The observed frequency for this mode in styrene is [ Y I ~ ] = 1203 cm-'. The observed range of frequencies for this mode in the alkylbenzenes is [v13] = 1216 f 3 cm-'. This mode is described in Table 1 as involving a stretching motion of the C(1)-C(a) bond. Due to the invariance of the frequency of this mode with the length of the alkyl chain and between styrene and tPMS, this mode is better described as a ring distortion, as assigned by Hopkins et aL6 We observe bands at 0; - 347 cm-' and 0; - 944 cm-' which have no analogues in the 0; SVLF spectrum of styrene. The replacement of a P-hydrogen with the methyl group means that the in-plane (C=C-H) bend and (C-H) stretch vibrations become the (C=C-C) bend and (C-C) stretch respectively. The 0; - 347 cm-I and 0; - 944 cm-' bands are assigned as involving the bending and stretching fundamentals,respectively, by comparison with similar modes in ~rotona1dehyde.l~The 0; - 347 cm-' band was confirmed as involving an in-plane vibration from the infrared analysis discussed in section 6. Other substituent vibrations which are active compare well with those of styrene such as v9 (C=C stretch) at 0; - 1671 cm-'. The high-wavenumber region of the 0; SVLF spectrum is very weak and congested and has been expanded in Figure 4 to reveal the weak fundamental and combination bands in this region. The bands involved in the progression in the torsional notion has been assigned up to v42 = 6. Even quanta transitions only are allowed, due to the selection rule for an out-of-plane a" vibration. There is a weak shoulder band at 0; - 150 cm-I on the high-wavenumber side of the 42: band. At first it was not clear whether this was real, but it is confirmed as the 41y42: band by the SVLF spectrum following excitation into the 41; band (section 5.2) in which the 41:42: band is intense. An intensity pattern characteristic of the progression in Y42 is built onto 41Y42: and is observable up to 41:42! with V42 odd. The
observation of this progression provides us with information on the odd levels for the torsional vibration. These can be used in fitting the levels of ~ 4 to 2 a torsional potential; however, it has to be assumed that the anharmonicity connecting Y42 and ~ 4 is 1 negligible. The fundamental Y29 is observed at 0; - 210 cm-'. This is an a' in-plane bending vibration about C(l) and has a wavenumber of 228 cm-' in styrene. We also observe 41; and 41;42; in this region and a weak shoulder band has been assigned to the combination band 40Y42:. The vibration, Y40, is an out-of-plane CH3 bend, and we obtain a wavenumber of 275 cm-I for this mode which compares with the value of 299 cm-' found for a similar vibration in ~rotonaldehyde.'~ The band at 0; - 217 cm-' is tentatively assigned as z:42:, involving the methyl torsional motion (z) and the C( 1)C(a)torsional motion, ~ 4 2 . 5.2. Excitation in the 41:, 29:, and 40: Bands. The wavenumbers and assignments of the bands in these spectra are given in Table 4. Part of the SVLF spectrum with excitation in the 41; band is shown in Figure 5a. This spectrum is dominated by a band at 0; - 150 cm-' which has been assigned as 41i42:. The appearance of this band as the strongest in the spectrum is a result of the Duschinskii mixing of the normal coordinates, Q"41 and in SI. If the ~ 4 = 2 1-0 separation can be estimated then we can obtain from the 41i42: band, a value for the V"41 = 1-0 separation. The V"42 = 1-0 separation can be obtained in the FE spectrum from the observation of bands such as 41i42: and 41; assuming Y41 to be harmonic, as in styrene. For styrene, the ~ " 4 2 = 1-0 separation was calculated in this way to be 37 cm-' which was in good agreement with that found by fitting the observed vinyl torsional progression bands to the potential in eq 1. The ~ " 4 2 = 1-0 separation for @MS is calculated in a similar way to be approximately 28 cm-'. Coupled with the 41y42: assignment we obtain a V"41 = 1-0 separation of 121 cm-'. The 41; band can then be assigned at 41; - 242 cm-'. The d'41 = 1-0 interval has been confirmed by the observation of this fundamental in the far infrared spectrum (section 6). Part of the SVLF spectrum with excitation in the 29; band is shown in Figure 5b. The strongest band in the spectrum is assigned not to 29!, as would be expected, but to 29i41: which is unusual. A similar effect was observed in styrene15in which excitation into this mode resulted in an intense 29; band but a more intense 29;41:42: band: the reason was not clear, but presumably it is a result of the Duschinskii effect. This is not unreasonable due to the extensive mixing of coordinates in such molecules. Part of the SVLF spectrum with excitation in the 40; band is shown in Figure 5c. In this spectrum the band at 299 cm-' is the most intense: it is unlikely to be an in-plane fundamental and so is attributed to the combination band 40:42: providing a fundamental vibration wavenumber for 2140 of 272 cm-'. This would suggest that there is extensive coupling between vinyl torsion and the out-of-plane CH3 bend. All these spectra show a similar intensity pattern in the lowwavenumber vibrations built on benzene-type fundamentals. 5.3. Excitation in the 28:, 38:41:, 39:42:, and 38:42: Bands. Parts of the SVLF spectra with excitation in the 28;, 38:41:, 39;42:, and 38A42: bands are shown in Figures 6a,b, 7a,b, respectively, with assignments given in Table 4. All these spectra, although dominated by one intense band, show extensive bands due to other substituent fundamentals and combinations.
J. Phys. Chem., Vol. 99, No. 13, 1995 4391
Torsion Potential of trans-P-Methylstyrene
TABLE 4: Assignment of Parts of the SVLF Spectra following Excitation into the 41;, 29:, 40:, 28:, 38:41:, Bands ~~
~
assignment
65 148 210 24 1 306 762 824 855
29; 29A42: 29A41042; 29: 29A41; 29;41:42; 25y29A41y42y 25y291 25y29A41;
968 1030 1061 1148 1210 1241 1354
24y29i41y42y 24y29: 24y29A41; 22:29;4 1742: 22y291 22y29A41: 17y29A41y42y
1416 1447 1729 1810 1872 1903
17y293 17y29A41; 9y29h42; 9y29A41y42y 9y293 9:29;41:
(Y
!
29; at 34 784 cm-l.
I
I
-v"
assignment
1167 1348 1365 1397 1424 1556 1574 1609 2007 2025
I
800
assignment A;iPlcm-'
Of 67 151 210 253 301 345 396 403
65 148 210 24 1 306 347 365 397
AB = BL - 9 where BL is the laser wavenumber. 41; at 34 751 cm-'. f38A41A at 34 921 cm-I. g 39A42; at 34 962 cm-'. * 38A42: at 35 010 cm-'.
400
AWcm-'
40:
29;
oc
0
and 38:42: ~
AWcm-l assignment AVlcm-l assignment AWcm-l assignment AWcm-'
I
39:42:,
~
1200
1600
2000
2400
Aif I cm-1
Figure 5. Parts of the SVLF spectra of @IS
with excitation in the (a) 41:, (b) 29:, and (c) 40; bands whose wavenumbers are given in Table 2.
+
The SVLF spectrum with excitation in 28; at 0; 283 cm-' in the FE spectrum ( ~ 2 8is the C=C-C bending mode) has an intense band at 28; - 347 cm-', and there is extensive coupling with other modes. This band is assigned as 28:. The decrease in wavenumber in the excited state can be explained by the decreased double-bond character in the C(a)-C@) bond. The observation of strong bands due to other substituent modes corroborates this assignment as we would expect extensive coupling with other modes from a vibration which is central to the substituent group.
40; at 34 830 cm-I.
e
28; at 34 869 cm-l.
I
I
I
400
000
1200
1600
- i
AI I cm-1
I
1
L
0
2000
I
Figure 6. Parts of the SVLF spectra of tPMS with excitation in the (a) 28; and (b) 38;41: bands whose wavenumbers are given in Table 2.
The C=C-C bending vibration wavenumber is 427 cm-' in propene16 and, although 347 cm-' differs considerably from this value, this vibration is found to be particularly sensitive to its environment, a similar mode in ~rotonaldehyde'~ being assigned at 542 cm-' in the infrared spectrum. The SVLF spectrum with excitation in 38A41; at 0; 335 cm-' in the FE spectrum is dominated by a strong peak at 38;41: - 515 cm-', and this is assigned to the 38:41: transition. Although there is strong coupling, analysis of the FE spectrum confirms that the 0; 335 cm-' band is a combination between one quantum each of Y41 and Y38 (77 f 256) cm-'. The band at
+
+
4392 J. Phys. Chem., Vol. 99, No. 13, 1995
Sinclair et al.
b
I
I
I
,
,
,
,
,
,
I
,
,
(C)
I
0
400
800
1200
1600
2
2000
I
+ l AF I cm-1
0
400
800
1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200
-z
A71 cm-1
Figure 7. Parts of the SVLF spectra of tPMS with excitation in the (a) 39A42; and (b) 38;42: bands whose wavenumbers are given in Table 2.
Figure 8. Parts of the SVLF spectra of @MS with excitation in the (a) 27:, (b) 25A, and (c) 27A29; bands whose wavenumbers are given in Table 2.
38;41; - 515 cm-' in the SVLF spectra confirms this assignment and the fundamental vibration for v38 in SO can be calculated to have a vibration wavenumber of 394 cm-' (515122 cm-'). This mode couples with ~ 4 2in a band at 38A41; - 420 cm-' and, due to the Duschinskii effect, the SVLF spectrum is dominated by other bands due to substituent modes. The SVLF spectrum with excitation in 39;42; at 0; 376 cm-' is dominated by strong bands at 39A42; - 401 cm-' and 39h42; - 580 cm-'. These have been assigned to 39:42: and 38:39:41:42:, respectively. The observation of a strong 39i42; band supports the excited-state assignment. The observation of 39:42: provides a vibration wavenumber for v39 (C-C ring twist mode) of 374 cm-' in SO. The SVLF spectrum with excitation in 38A42; at 0; 424 cm-' is similar to that with excitation in 39A42:. However, the band at 38i42; - 401 cm-' is not as strong. The band at 38A42; - 580 cm-' remains dominant and is assigned to 38:41:42:. With these low-wavenumber fundamentals assigned, many bands that occur in the SVLF spectra can be explained by various overtones and combinations of these fundamentals. All these spectra show a similar pattem of bands built on benzenetype fundamentals. 5.4. Excitation in the 25:, 27:, and 27:29: Bands. The SVLF spectra with excitation into 27;[6a;], 25;[6b;], and 27A29; are shown in Figure 8 with wavenumbers and assignments given in Table 5. As mentioned in section 4, the vibrations v6a and v6b are insensitive to substitution, and their wavenumbers in the ground and excited state are expected to remain fairly constant over a series of substituted benzenes. The SVLF spectra with excitation into 25; and 27; are dominated by strong peaks at 25; - 621 cm-' and 27; 405 cm-', respectively, which confirms their 25: and 27; assignments. The resolved fluorescence from 2$[6b;] indicates a possible buildup of broad fluorescence which is an indication of relaxed fluorescence, to be discussed in section 10. The SVLF spectrum after exciting into the 27A29; band at 0; 569 cm-' in the FE spectrum is unusual in this respect as it exhibits a fairly flat background with a strong band at 0; - 613 cm-'. This vibration wavenumber is also observed in the infrared spectrum (section 6). We assign the 0; 569 cm-' band as the combination between two in-plane funda-
mentals each of lower wavenumber than ~ 2 and 5 which do not exhibit relaxed fluorescence: the opposite would be expected if the 569 cm-' band were due to a fundamental. The best candidates for this combination are v27 (405 cm-') and "29 (210 cm-'). All these spectra show a similar intensity pattem of bands built on benzene-type fundamentals. 5.5. Excitation in the 24:[1:], 22:[12:], 21:[18a:], 17: [13:], and 0; 908 cm-' Bands. The SVLF spectra with excitation into these bands are shown in Figure 9 with wavenumbers and assignments from the 24;[ l;] and 17;[ 13;] spectra given in Table 6. As mentioned in section 4, the [v13] and [ V I ] vibrations are insensitive to substitution and their vibration wavenumbers are expected to remain fairly constant over a series of substituted benzenes. The SVLF spectrum with excitation into 24; has a strong 24; band at 24; - 821 cm-' confirming the assignments by comparison with fluorescence spectra of a series of jet-cooled alkyl substituted benzenes? The SVLF spectrum with excitation into 20; and 22; have maxima in a region where C-H bending and ring breathing vibrations are known to occur, confirming the excited-state assignments. The SVLF spectrum with excitation into 17; has a strong band at 17; - 1208 cm-' and this has been assigned as 17:. The SVLF spectra exhibit a build up of broad fluorescence at increasing excess excitation energy which is tentatively identified in the SVLF spectrum of the 25; band at 0; 537 cm-' in the FE spectrum (section 5.4) and is more prominent in the SVLF spectrum with excitation in 17;. This is discussed in section 10. As discussed in section 4, we observe a strong band at 0; f 908 cm-' in the FE spectrum. The SVLF spectrum with excitation into this band has a strong similarity to the SVLF spectrum with excitation into 24;[1:] with a strong band at 821 cm-', suggesting a coupled mode with a significant contribution from [VI].
+
+
+
+
+
+
6. Analysis of the Infrared Spectrum
The infrared spectrum of tPMS was recorded from 40 to 3200 cm-' in the gas phase at room temperature with sufficient resolution to observe rotational band contours. The observed bands at this resolution are of two types: (1) C-type bands due to a" vibrations and showing an intense sharp central Q-branch region and weaker, broader P- and
J. Phys. Chem., Vol. 99, No. 13, 1995 4393
Torsion Potential of trans-@-Methylstyrene
TABLE 5: Assignment of Parts of the SVLF Spectra following Excitation into the 27;, 25;, 27;29:, 24;, and 17; Bands AWcm-' assignment AWcm-l assignment AWcm-l assignment AVIcm-' assignment 27;
Ob 67 347 405 469 811 1024 1088 1221 1285 1405 1469 1611 2068
27; 27;42: 27l28,
P
27,42: 27t 27i 25b27i 25 27,42: 24b27; 24b27l42: 22b27,t 22,27;42; b 17y273 9:27;42:
@ 62 1 685 770 1024 1088 1237 1302 1437 1501 1621 1647 1687
25; 25; 25 1 25,42: 25 ;426 25i27b 25'27'42; 25) 25$42; 24b25 24 25,42: b 22b25 21b251 22,25,42;
1825 1857 1891 2238 2283
2729;
24;
od 405 615 677 1021 1235 1299 1435 1499 1617 1681 1820 1884
27'29; P 27129; 27129, 27 29 42; I t 27829t 25 27 29, 25b27 l 29l 42; 24b27 l 29,l 24b27,29;42: l 22i27 29 22 27 29 42; 17A27 l29 l b t l 17,27,29,42i
P
2234 227 1
22:25;27i29; 15 27,28,29
Of 405 620 823 1003 1210 2425
ii
2347 2441 2865 291 1 3294
f
A6 = 6~ - 6 where 6~ is the laser wavenumber. 27; at 34 971 cm-l. 17; at 35 790 cm-'.
R-branch wings. The observed C-type bands are shown in Figure 10. These bands arise from out-of-plane vibrations in which the dipole moment change is parallel to the axis of largest moment of inertia, which is perpendicular to the molecular plane of symmetry. (2) Hybrid bands due to a' vibrations. These may contain A- and B-type components, but those shown in Figure 11 appear to be dominated by the A-type component showing a sharp Q-branch region and broad, intense P- and R-branch wings. Two fundamentals in the far infrared spectrum were observed weakly at 122 and 347 cm-' and are shown in Figure 10a and Figure l l a , respectively. The sharp peaks overlaying these bands are due to water vapor, but the general shape of the underlying rotational contours can be observed sufficiently clearly to classify them as C-type and AE? hybrid bands, respectively. As discussed in section 5.2, and 122 cm-' band represents ~ 4 1the , out-of-plane substituent bend about C( l), and so the rovibrational structure should follow C-type selection rules. This is a very important assignment as it confirms the assignment of the shoulder at 0; - 150 cm-' in the 0; SVLF spectrum in Figure 4 as 41y42:. The 347 cm-' band discussed in section 5.1 is assigned to v28, the in-plane C=C-C bend, consistent with the AE? hybrid selection rules with the A-type component dominant. Two bands in the infrared spectrum at 496 and 613 cm-' are shown in Figure 10b and 1lb, respectively. The assignment of the 496 cm-' band which is apparently C-type, is not clear. It is possible that it is an a" fundamental but we have already made possible assignments for v38 and v39 which are expected to have wavenumbers below 400 cm-'. The 613 cm-l band contour is distorted but we assign it as a hybrid band, with a dominant A-type component because this interval has also been observed strongly in the SVLF spectrum with excitation in the 0; 569 cm-' band in the FE spectrum discussed in section 5.4. The only possible assignment for this band in the infrared spectrum is a combination between in-plane vibrations v27 and
+
25; at 35 123 cm-'.
e
24; 24,42! 24;42$ 24A25 24 1 24'25:
67 155 62 1 821 1444 1646 1821 2030 2495 2650
,
248 22 24; b 17,24; 9:24l 17:2:125: 17;
27A29; at 35 155 cm-'.
17; 17?27i l7O23 17i24b 17 22, P 170 17;
e
24; at 34 869 cm-I.
~ 2 9 showing ,
that these modes are significantly coupled in the ground state as well as in the excited state. The assignments and vibration wavenumbers of these and other fundamentals observed in the infrared spectrum are given in Table 1.
7. C(1)-C(a) Torsional Potential in SO As in styrene we expect the C(1)-C(a) torsional potential function V(4) to be of the general form given in eq 1 and to be dominated by the V2 and V4 terms, with a minimum energy at 4 = 0 corresponding to the planar configuration and a maximum energy at 4 = nl2. The 4 = 0 configuration corresponds to maximum n-electron conjugation and maximum steric hindrance whereas 4 = nI2 corresponds to zero conjugation and minimum steric hindrance. For styrene the potential was found to be
V(@)/cm-' = '/J( 1070 f 8)( 1 - cos 24) (275 f 1)(1 - COS 44)] (2) The molecule is planar, but the large and negative V4 term which was required to fit the observed levels resulted in a flatbottomed potential. There is competition between conjugation which favors planarity and steric effects which favour nonplanarity and the flat-bottomed potential shows that conjugation and planarity are favored only marginally over steric effects. From the SVLF spectra of t@MS we have obtained the first six levels of the C(1)-C(a) torsional vibration v42 in SO. Like those of styrene they diverge more rapidly in the lower levels, requiring the need for the large negative V4 term in addition to the dominant V2 term. To fit the observed levels to a potential function, the internal rotation constant F is required. However, F changes with the torsional angle due to the unsymmetrical nature of the vinyl group and so the following form derived by Lewis et al." was
'
Sinclair et al.
4394 J. Phys. Chem., Vol. 99, No. 13, 1995 100
-- ~
I
85 100
-
I V
90 95
80
140
120
70 470
160
100
510
490
530
V 0
1
1
I
e
I
m
.-
50
E YI
e I b
o
830
C
I
I
I
I
850
870
890
710
100 r
-58
780
730
750
I
1
770
790
100 r
BOO
820
840
-
0
920
940
980
980
1000
W a v e n u m b e r I cm-1
0
800
1600 2400 3200 4000 4800 5
Figure 10. Infrared rotational band contours of (a) v41, (b) 496 cm-I, , (e)v32. The spectrum in (a) is contaminated (c) v3-i and v35. (d) ~ 3 4 and with that of water vapor whose absorption results in the sharp lines superimposed on the broad band contour.
I
AT I cm-1
- f
Figure 9. SVLF spectra of fPMS with excitation in the (a) 24:, (b) 0; + 908 cm-I, (c) 22;, (d) 21:, and (e) 17; bands whose wavenumbers are given in Table 2. TABLE 6: Observed and Calculated Term Values for the Vinyl Torsional Vibration Y42 in the SOState of tmS v42 obslcm-I calc"/cm-I 0 1 2 3 4 5 6 a
0 28 67 110 155 204 252
With V2 = 855 cm-I,
V4
= -218
-
94
bQ L
92 0
0 28 67 109 155 203 252
e
2-.
BO
335
352
380
'$95
1028
1037
1045
50 1485
cm-I.
used: '?020
where i = 0, 2, 4, .... The planar geometry of t@MS was obtained by using estimated bond lengths and angles from those of styrene' and propene.16 After allowing for a change in the angle C(1)C(a)C@) from 129" in the planar to 127" in the perpendicular configuration and using the method of fiber,'* we have obtained F(4)lcm-' = 1.249
+ 0.095 cos 24
(4)
The form of the potential function which best fits the experimental data is
u 807 618 830
343
1485
1505
1515
W a v e n u m b e r / cm-1 Figure 11. Infrared rotational band contours of (a) v2g. (b) v27 v29 (c) v21. and (d) v11. The spectrum in (a) is contaminated with that of water vapor whose absorption results in the sharp lines superimposed on the broad band contour.
+
The observed and calculated levels are given in Table 6 and the potential function for eq 5 is plotted in Figure 12 for ,$ = -45" to +45". Because 4lV41 > V2 the potential shows a very slight maximum at ,$ = 0. Strictly this would indicate that t p M S is quasiplanar but, within the uncertainties in eq 5 , this is not significant. There is an 855 cm-1 barrier at ,$ = 90".
8. C(l)-C(a) Torsional Potential in S1 V(qb>/cm-' = '/,[(855 f 88)(1 - cos 24) (218 f 35)(1 - COS 4 4 ) ] (5)
In the excited state, v42 can no longer be described as a purely torsional motion of the vinyl group due to the Duschinskii
Torsion Potential of trans-/3-Methylstyrene
On the other hand, it is consistent with other work in that Bemstein et aL20 found no methyl torsional structure in a-methylstyrene, and Wallace et al.*l found no methyl torsional structure in 2- or 3-methylindole (analogous to P- and amethylstyrene), although methyl torsion was found in other methylindoles, analogous to methyl torsion found in 2-, 3-, and 4-methylstyrene~.~~-~~
1
340
290
11
-
2401
,E"
190
5-1
I
I
-I\
40]
-10
J. Phys. Chem., Vol. 99, No. 13, 1995 4395
, -45
10. IVR
=A1 \ 1
I
I
I
I
I
-30
-15
0
15
30
45
Angle (pldeg
Figure 12. C(1)-C(a) torsional potential for rj3MS in the ground electronic state.
mixing of the ground state normal coordinates Q"42 and Q"41 in the excited state. It would seem that because v'41 (77 cm-l) is much less than v'42 (122 cm-'), it contains more torsional character than ~ 4 2 . However, the harmonic nature of both these vibrations in the excited state (Table 2) indicates a vinyl group which is coplanar with the benzene ring and a torsional potential in which V2 is much more dominant than in the ground state.
9. Methyl Torsional Potential in SO In the high-wavenumber region of the 0; SVLF spectrum in Figure 2, only the very weak band at 0; - 217 cm-' had no obvious assignment and so was attributed tentatively to the methyl torsion fundamental v, in combination with ~ 4 2 . The calculated methyl torsion fundamental is then 189 cm-'. When only the v = 1-0 separation is known for a torsional vibration the barrier height can be estimated by the method of Fateley et aZ.19 The rotating methyl group has a 3-fold barrier and, if we reduce the potential function in eq 1 to a single value of n = 3, V3 is given by
V, = v219F
(6)
where v is the v = 1-0 separation and F is the intemal rotation constant for methyl torsion which was estimated to be 5.88 cm-'. It follows from eq 6 that V3, the torsional barrier height, is 675 cm-'. For propene, in its ground electronic state, the potential for methyl torsion was found to be3 V(4)/cm-' = '/,[693(1 - cos 34) - 14(1 - cos 64)]
(7)
The fact that the tentative value of V3 = 675 cm-' for tPMS is similar to that for propene, supports the zp427 assignment. It also suggests that the phenyl group hardly affects the methyl torsional potential in SO. No evidence of any transitions due to methyl torsion in S1 was found in the FE spectrum. The apparent conclusion is that methyl torsion is unaffected by electronic excitation. This is surprising in that since the effects of excitation would be expected to extended to the C(a)CCp) bond, the methyl torsional potential should be affected.
The observation of a buildup of a broad rising background of fluorescence in the SVLF spectra at increasing excess energy of excitation, shown in Figure 9, is similar to that in the jetcooled SVLF spectra of styrene12 which has been attributed to IVR. In a qualitative sense, it reflects the degree of IVR from the optically excited level to quasidegenerate rovibronic states that are not directly accessible from the ground state. We can begin to see the slight broad background fluorescence in the SVLF spectrum with excitation in the 25; band. This suggests that some vibrational energy redistribution is occurring at this excess energy. These additional transitions that result from the relaxed fluorescence increase in intensity with excess energy of excitation, and the result is a fluorescence spectrum which becomes increasingly congested, indicating the rate of IVR increasing with excess energy. The onset of IVR for stilbene5 occurs on a picosecond time scale at an excess energy of about 900 cm-', whereas, for styrene,12an approximate time for IVR has been estimated at 5 ns with a similar excess energy. The difference in rates is due to the significantly larger density of states in stilbene. However, the much lower rates estimated for styrene indicate that isomerization plays a minor role and that other mode coupling mechanisms exist. It was found in styrene that the effective density of states is similar to that for stilbene because, although the latter has more atoms, it has higher symmetry. The density of states in styrene and tPMS should be closely parallel. As found with styrene the FE spectrum of tPMS exhibits sharp structure with no onset of broadening at excess energies up to 1800 cm-'. The SVLF spectrum obtained even with excitation at an excess energy of 1210 cm-' (Figure 9e) has many sharp emission bands. The rate of IVR in t6MS would therefore seem not to be determined to any great extent by an isomerization process. A direct comparison of the SVLF spectra of tPMS with those of styrene for exciting into equivalent bands indicates that the degree of IVR is greater in t/3MS. For example the build up of broad resolved fluorescence recorded with excitation in the 24; band of styrene is significantly less than that from the 24; band of tPMS (Figure 9a). The relaxed fluorescence from the benzene ring modes therefore would seem to arise from a flow of vibrational energy to low-frequency modes (bath modes), as described by Hopkins et aL6 in the alkyl-substituted benzene study. However, there was too little spectroscopic information on the low-frequency modes of alkyl chains coupled to benzene rings to permit a deconvolution of the relaxed fluorescence. The contribution of low-frequency modes to the extent of IVR relies on some degree of coupling between vibrations of the substituent group and those of the benzene ring. In tPMS we have observed vibrations of the substituent group having some degree of coupling with the benzene ring vibrations in the excited state. For example, this occurs with the substituentsensitive ring twisting vibration v38 as well as combination bands of benzene-type fundamentals and vibrations of the vinyl group in the excited state and this may provide a mechanism for coupling in the excited state resulting in IVR.
Sinclair et al.
4396 J. Phys. Chem., Vol. 99, No. 13, 1995 11. Conclusion We have shown that the vinyl group in tPMS is coplanar or very nearly coplanar with the benzene ring. Such a structure, with a flat-bottomed or almost flat-bottomed torsional potential, is expected as tPMS is a sterically unhindered derivative of styrene. Grassian et al." have observed in the TOFMS spectrum only one very intense band, the 0; band, and this is free of any low-wavenumber vibrational progression. They attribute this to the molecule possessing a similar conformation in both ground and excited states, i.e., essentially planar. Our studies agree with this general conclusion but show that the excited state is much more rigidly planar than the ground state. This is presumably due to the increased ~t character in the C( 1)C(a) bond. There is very little observed activity of the methyl torsional vibration in SOand none in SI. A tentative assignment of one band in the 0; SVLF spectrum suggests a similar 3-fold methyl torsional potential to that in propene which in turn indicates that the phenyl group has little effect on the potential in SO. Although some change in the methyl torsional potential function might be expected in the excited state because of the changes in the C(a)=C@) bond that take place, the observation that methyl torsional activity is at the most very weak does not support this, and we conclude that the excited- and groundstate potential functions are nearly identical. A vibrational coupling mechanism has been found between the modes of the benzene ring and those of the substituent, and this may contribute to the IVR process via low-frequency bending and torsional motions.
Acknowledgment. We thank the Science and Engineering Research Council for the financial support for W.E.S. and the Laser Support Facility at the Rutherford Appleton Laboratory for the loan of the Quantel laser system. We also express our thanks to Professor Manfred Winnewisser for the use of the Bruker FT-IR spectrometer. Note Added in Proof. Haas et al. (personal communication) have carried out ab initio calculations of SO vibration wave-
numbers which strongly indicate that the infrared bands that we observe at 496 and 613 cm-' are a" and a' fundamentals, respectively.
References and Notes (1) Hollas, J. M.; Ridley, T. Chem. Phys. Lett. 1980, 75, 94. (2) Hollas, J. M.; Musa, H.; Ridley, T.; Turner, P. H.; Weisenberger, K. H.; Fawcett, V. J. Mol. Spectrosc. 1982, 94, 437. (3) Durig, J. R.; Guiris, G. A,; Bell, S. J. Phys. Chem. 1989, 93, 3487. (4) Hollas, J. M.; Khalipour, E.; Thakur, S. N. J. Mol. Spectrosc. 1978, 73, 240. (5) Chiang, W.-Y.; Laane, J. J. Chem. Phys. 1994, 100, 8755. (6) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1980, 72, 5049. (7) Auty, A. R.; Jones, A. C.; Phillips, D. Chem. Phys. Lett. 1984, 112, 549. (8) Hollas, J. M.; bin Hussein, M. Z. Chem. Phys. Lett. 1989,154, 14. Bacon, A. R.; Hollas, J. M. Faraday Discuss. Chem. Soc. 1988, 86, 129. (9) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1997. (10) Wilson, E. B. Phys. Rev. 1934, 45, 706. (11) Grassian, V. H.; Bemstein, E. R.; Secor, H. V.; Seeman, J. I. J. Phys. Chem. 1989, 93, 3470. (12) Syage, J. A.; Aladel, F.; Zewail, A. H. Chem. Phys. Lett. 1983, 103, 15. (1 3) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1980, 72, 5039. (14) Oelichmann, H. J.; Bougeard, D.; Schrader, B. J . Mol. Struct. 1981, 77, 179. (15) Taday, P. F. Ph.D. Thesis, University of Reading, 1990. (16) Lide, D. R.; Christensen, D. J. Chem. Phys. 1961, 35, 1374. (17) Lewis, J. D.; Malloy, T. B.; Chao, T. H.; Laane, J. J . Mol. Struct. 1972, 12, 427. (18) Pitzer, K. S. J. Chem. Phys. 1946, 14, 239. (19) Fateley, W. G.; Harris, R. K.; Miller, F. A.; Witkowski, R. E. Spectrochim. Acta 1965, 21, 231. (20) Grassian, V. H.; Bemstein, E. R.; Secor, H. V.; Seeman, J. I. J. Phys. Chem. 1980, 93, 3470. (21) Bickel, G. A..; Leach, G. W.; Demmer, D. R.; Hager, J. W.; Wallace, S. C. J. Chem. Phys. 1988, 88, 1. (22) Hollas, J. M.; Taday, P. F. J . Chem. Soc., Faraday Trans. 1991, 87, 3335. (23) Hollas, J. M.; Taday, P. F. J . Chem. Soc., Faraday Trans. 1991, 87, 3585. (24) Hollas, J. M.; Taday, P. F. J. Chem. Soc., Faraday Trans. 1990, 86, 217.
JP941135S
