J. Phys. Chem. 1985,89, 1087-1094 The idea is to try to characterize the spectra in terms of ensembles of Hamiltonians with simple statistical properties of their matrix elements. The statistical properties of the resulting eigenvalues and eigenvectors are then analyzed and compared with the experiment. This approach had remarkable success in nuclear physics. The statistical properties of interest are connected with properties of “n level distribution functions” such as distributions of level spacings, fluctuations in spectra, etc. Several attempts were made recently to apply these ideas to molecular absorption spectra.6Mg An interesting open question is whether, by making some simple assumptions regarding the distribution of molecular frequencies, anharmonicities, etc., it will be possible to predict and characterize fluorescence spectra in a statistical way. The present summary shows that intramolecular relaxation and dephasing, as reflected in fluorescence line shapes, are subjective (64) Brody, T. A.; Flores, J.; French, J. B.; Mello, P. A.; Pandey, A.; Wong, S . S . M. Reu. Mod. Phys. 1981, 53, 385 and other references cited therein. (65) Chaiken, J.; Gumick, M.; McDonald, J. M. J . Chem. Phys. 1981,74, 117; 1981, 74, 123. (66) Abramson, E.; Field, R. W.; Imre, D.; Innes, K. K.; Kinsey, J. L. J . Chem. Phys. 1984,80,2298. (67) Buch, V.; Gerber, R. B.; Ratner, M. A. J . Chem. Phys. 1982, 76, 5397. (68) Haller, E.; Koppel, H.; Cederbaum, L. S. Chem. Phys. Lett. 1983, 101, 215. (69) Mukamel, S.; Pandey, A.; Sue, J. Chem. Phys. Lett. 1984,105,134.
1087
quantities which depend on our level of theoretical description. In methods i and ii the spectrum is static and carries no dynamical information. Method iv interprets the spectra as pure dephasing. In method v the key quantities are T I and T2,relaxation processes, and in method vi the information is purely statistical. All these approaches are useful and there is no contradiction among the various interpretations. The right approach should be adopted depending on the level of information contained in the experiment which depends on molecular size, degree of excitation, experimental resolution, etc. There is no point in interpreting a simple Lorentzian line in terms of millions of unresolved eigenstates. A question that is often raised is whether a particular intramolecular line shape arises from dephasing ( T2)or relaxation of population ( T I )processes. The present analysis shows that this question has no significance unless we specify our level of de~cription.’~The same line stape can be interpreted in a variety of ways and the convenience and the information content should dictate which approach to choose.
Acknowledgment. This research was supported by the National Science Foundation. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. (70) Mukamel, S. Chem. Phys. 1978,31,327, Chem. Phys. Lett. 1979,60, 310.
ARTICLES Vibrational Overtone Spectra of Methyl-Substituted Silanes R. A. Bernheim,* F. W.Lampe, J. F. O’Keefe, and J. R. Qualey I11 Department of Chemistry? and Department of Physics,$ The Pennsylvania State University, University Park, Pennsylvania I6802 (Received: July 5, 1984; In Final Form: October 2, 1984)
Absorption spectra between 12 800 and 18 200 cm-l have been recorded for gaseous CH3SiH3, (CH3&3iH2, (CH3)3SiH, and (CH3),Si by using intracavity photoacoustic detection with CW dye lasers at 1-cm-’ resolution. Transitions corresponding to the AvCH = 5,6,7 overtones of the C-H stretch and AvsiH = 8 and 9 overtones of the Si-H stretch can be assigned. These can be described appropriately in a local-mode representation with spectroscopic constants consisting of an effective harmonic frequency and a diagonal anharmonic constant. The spectra consist of additional features, many of which can be assigned as combinations with the local-mode oscillator. Comparison of the C-H and Si-H stretching overtones are made with those observed id CHI and SiH4.
Introduction High-energy molecular vibrational states have received considerable recent attention, partly because of their potential role in photochemical reactions that possibly could be driven in a mode-specific direction.14 Vibrational excitation in the visible spectral region can be detected in liquids with thermal lensing techniques’ and in gases with photoacoustic methods5 Previous photoacoustic studies of the Si-H stretch in our laboratory have revealed several interesting results.&* First, the Si-H stretch is Address correspondence to this author at the Department of Chemistry, 152 Davey Laboratory, The Pennsylvahia State University, University Park, PA 16802. 152 Davey Laboratory. 104 Davey Laboratory.
*
0022-3654/85/2089-lO87$01 .50/0
very well described as a local-mode oscillator in SiH4, SiD3H, SiHC13,and SiH2C12. The transition energies show a remarkable (1) R. L. Swofford, M. E. Long, and A. C. Albrecht, J . Chem. Phys., 65, 179 (1976). (2) B. R. Henry, Acc. Chem. Res., 10, 207 (1977). (3) K. V. Reddy, R. G. Bray, and M. J. Berry in “Advances in Laser Chemistry”, A. H. Zewail, Ed.,Springer-Verlag, Berlin, 1978, p 48. (4) M. L. Sage and J. Jortner, Adu. Chem. Phys., 47, 293 (1981). (5) Y. H. Pao, “Optoacoustic Spectroscopy and Detection”, Academic Press, New York, 1977. (6) R. A. Bernheim, F. W. Lampe, J. F. O’Keefe, and J. R. Qualey 111, Chem. Phys. Lett., 100, 45 (1983). (7) R. A. Bernheim, F. W. Lampe, J. F. O’Keefe, and J. R. Qualey 111, J . Mol. Specrrosc., 104, 194 (1984). (8) R. A. Bernheim, F. W. Lampe, J. F. OKeefe, and J. R. Qualey 111, J . Chem. Phys., 80, 5906 (1984).
0 1985 American Chemical Society
1088 The Journal of Physical Chemistry, Vol. 89, No. 7, 1F
fit to a Birge-Sponer type relation9 of the form
AE = uiwj
+ vi'xij
where AE is the energy of the excited state relative to the energy of the ground vibrational state, wi is an effective harmonic frequency, and Xii is a diagonal anharmonic constant. Extrapolating to the diatomic molecule limit where the Birge-Sponer relation is usually applied, wi would correspond to we - w g e and X , would correspond to -wse.The linear fit for the above molecules extends up to AuS,H = 9 a t energies near 18 000 cm-'. This behavior is perhaps not so surprising because of the large mass difference between H and Si which is an indicator of how well the local-mode model is expected to work. In addition, no evidence was found for homogeneous broadening at the 1-cm-' resolution of the experiments which were carried out at gas pressures near 1 atm. This is in contrast to the behavior of the C-H stretch in analogous compounds under the same experimental conditions.6 The bandwidths for the Si-H stretch in all cases are consistent with the rotational distribution in the ground vibrational state. For the lighter silanes SiH4and SiD3H, resolvable rotational structure has been observed and in the case of SiD3H can be assigned.',* In the present work these findings are extended to the methyl-substituted silanes where both the C-H and Si-H stretches can be observed in the same molecule. The original motivation for including the methylsilanes in a study of the Si-H stretching overtones was to learn more about the effects of substituents and different vibrational degrees of freedom upon the spectroscopic constants and vibrational energy transfer out of the Si-H stretching local mode. Not surprisingly, the spectra are considerably more complex than the pentatomic hydrides reflecting the various inequivalent sites for the C-H hydrogen and the Qccurrence of more combinations. Nevertheless, in many cases both the C-H and Si-H stretching overtones can be assigned and values of w, and XI,obtained. However, the analysis must be modified to include the effects of local mode-local mode and local modenormal mode coupling for the numerous combinations. For a polyatomic molecule the local-mode representation is usually expressed aslo
where the w,, are harmonic coupling terms which are usually small and often neglected. Besides the diagonal XI, local-mode anharmonicities, off-diagonal Xi, local mode-local mode and local mode-normal mode anharmonicities are sometimes necessary to assign the spectra. In addition, it has been found that the overtone transitions for sterically nonequivalent M-X local-mode oscillators are often In the present work the overtone spectra are complicated by all of these effects. However, it is possible to obtain the local-mode frequencies wi and diagonal anharmonicities Xi, for the Si-H and C-H stretching vibrations. A tentative assignment for a number of the combinations can also be made. Finally, the surprisingly narrow bandwidths of the Si-H and C-H stretching vibrations are commented upon. Experimental Section The spectra were recorded by using a nonresonant photoacoustic cell located within the cavity of a C R 490 C W dye laser. Radiation in the spectral region 560-705 nm was produced by pumping rhodamine 560,610, and DCM with 7 W (all lines) from an argon ion laser, and the spectral region 690-780 and 797-830 nm was investigated by using LD700 pumped by 3.5 W (all lines red) from a krypton ion laser. Intracavity powers of 10-15 W were obtained with 1-cm-I laser linewidths. The photoacoustic (9) R. T. Birge and H. Sponer, Phys. Rev., 28, 259 (1926). (IO) R. J. Hayward and B. R. Henry, J . Mol. Specfrosc.,57, 221 (1975). ( 1 1) W. R. A. Greenlay and B. R. Henry, J . Chem. Phys., 69.82 (1978). (12) J. S. W o n g and C. B. Moore, J . Chem. Phys., 77, 603 (1982). (13) H. L. Fang and R. L. Swofford, J . Chem. Phys., 73, 2607 (1980).
Bernheim et al. signal was detected by a Knowles BT 1759 electret microphone cemented to a hole in the side of the 7 mm i.d. by 15 cm long glass cell. The laser was interrupted at 1350 Hz, and the microphone signal output was monitored with a lock-in amplifier and recorded. Wavelength calibration was achieved by simultaneously recording the optogalvanically detected spectrum of neon and fringes produced by a Fabry-Perot etalon. Calibration accuracy was estimated at 0.2 cm-'. Fundamental and lower overtone spectra were recorded on Perkin-Elmer 580 and Cary 17 spectrophotometers, respectively. The compounds studied, methylsilane, dimethylsilane, trimethylsilane, and tetramethylsilane, were obtained from PCR Research, distilled into a vacuum system, and subjected to several freezepumpthaw cycles at liquid-nitrogen temperature to remove volatile impurities. The cells were filled to a pressure of 100 torr. Spectra were recorded between 4000 and 8500 cm-' and between 12 800 and 18 200 cm-'. The latter region includes AuCH = 5 and 6 and terminates at AvcH = 7. It is not certain whether all of the main features of the AvcH = 7 overtones were observed. The 4 = 8 and 9 overtones could be assigned in all cases except trimethylsilane where AuSiH = 9 is obscured. At the low resolution (1 cm-I) of the experiments, rotational structure was not resolved. The photoacoustically detected band positions are recorded in Table I. The lower overtones are presented in other tables below. While the intensity of the Si-H stretching absorption is definitely greater than that for the C-H stretch in the fundamental and first overtone regions, the second C-H stretching overtones could be observed, but the second Si-H stretching overtones were too weak. In each case, the fourth C-H overtone and sixth Si-H overtone are overlapped, with the observed band dominated by the C-H overtone. The features that were most interesting for the present work are those that are related to the C-H and Si-H stretching overtones. In order to extract the spectroscopic constants the overtones have to first be assigned. A very helpful aid to the assignment is the related overtone spectra of CH4 and SiH4. If, indeed, the upper overtones of the C-H stretch and Si-H stretch have a large degree of local-mode character, the spectroscopic constants wi and Xti will show similarity to their counterparts in CH4 and SiH4.
Results and Discussion There have been a number of previous studies of the fundamentals and low overtones of the four methylsilanes using Raman and IR absorption spectroscopy.'c26 There have also been three previous absorption studies of tetramethylsilane (Me4Si) which extended into the visible and near-IR region. The liquid-phase study of Burberry and Albrecht included the AuCH = 2-6 transitions of the C-H stretch which was interpreted in terms of the local-mode model.27 They also reported bands which were assigned to local mode-local mode (LM-LM) combinations. Perry (14) D. C. McKean, I. Torto, and A. R. Morrisson, J . Phys. Chem., 86, 307 (1982). (15) R. E. Wilde, J . Mol. Specrrosc., 8, 427 (1962). (16) I. F. Kovalev, Opt. Specfrosc. (Engl. Transl.), 8, 166 (1960). (17) E. A. V. Ebsworth, M. Onyszchuk, and N. Sheppard, J . Chem. Soc., 4, 1453 (1958). (18) D. F. Ball, P. L. Goggin, D. C. McKean, and L. A. Woodward, Spectrochim. Acta, 16, 1358 (1960). (19) V. H. J. Spangenberg and M. Pfeiffer, 2.Phys. Chem., 232, 343 (1966). (20) S. C. Graham, Specfrochim.Acta, Part A , 26, 345 (1970). (21) K. Shimizu and H. Murata, J . Mol. Spectrosc. 5, 44 (1960). (22) E. N. Tikhomirova, I. F. Kovalev, M. G. Voronkov, and E. Y. Lukevits, Opt. Specfrosc. (Engl. Trawl.), 27, 334 (1969). (23) S. Sportouchi, C. Lacoste; and R. GaufrEs, J . Mol. Srruct., 9, 119 ( 1 97 1). (24) A. M. F'yndyk, M. R. Aliev, and V. T. Aleksanyan, Opt. Spectrosc. (Engl. Transl.), 36, 393 (1974). (25) H. Burger and S. Biedermann, Specfrochim. Acfa,Part A, 28,2283 (1972). (26) C. W. Young, J. S. Koehler, and D. S. McKinney, J . Am. Chem. Soc., 69, 1410 (1947). (27) M. S. Burberry and A. C. Albrecht, J . Chem. Phys., 71,4631 (1979).
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1089
Spectra of Methyl-Substituted Silanes
TABLE I: Vibrational Frequencies, Intensities, and Off-Diagonal Anharmonicities for the Methylsilanes Measured by Photoacoustic Spectroscopy' CH3SiH3 V
13026 13 180 13 264 13 358 13 539 13580 14 069 14400 14862 14898 15 442 15 554 15689 15929 16051 16436 16 739 16911 17082 17 146 17287 17593 17 844 17 965 18 159
xi
W W
W W
vs S W
vw vw vw W W W
vs W W W
vw vw vw vw vw vw vw vs
-4 -3 -2
(CH3)2SiH2
12963 13 135 13 195 13 327 13484 13516 13 552 13 704 13 788 13 946 14033 14 125 14 193 14324 14 387 14792 14860 15 196 15 259 15 621 15836 15868 16006 16 301 16476 16669 16872 17010 17096 17 237 17 926 18077 18 132
(CH&SiH
xi,
y
vw W W
W VS
vs vs W W W W W W W
vw W W W W W
W
VS
vw
vw vw vw
vw vw
vw vw vw VS S
7YCH 7uCH
(CH314Si
xi,
V
12898 vw 12957 vw 13104 w 13184 w 13272 w 13310 w 13422 s 13470 vs 13536 s 13613 w 13674 w 13745 vw 13982 w 14014 w 14080 w 14133 w 14218 w 14285 w 14346 vw 14621 vw 14774 vw 14838 vw 14921 vw 15205 vw 15376 vw 15605 vw 15668 vw 15759 s 15814 vs 15862 vs 15997 w 16058 w 16347 w 16405 w 16460 w 16979 vw 17069 vw 17222 vw 17870 vw 17996 vs 18072 vs
-6 -2 +7 -3 -3
-2
0
-5 -2 +10
-2 -10
xi,
y
13 045 13111 13 191 13281 13431 13 501 13601 13 665 13 940 14079 14 147 14218 14 287 14361 14626 14730 14812 14884 15 181 15 534 15 584 15768 15 817 15 954 16001 16 364 16 408 16457 16932 17020 17 167 17 200 17318 17 697 17821 17 887 18 009
vw vw vw W VS S
-2
W W W
-1 -1
W W W W
-3
vw vw W W
-10
W
vw vw vw vs vs W W
-2 -1
vw W
vw
+2 -1
W
vw vw vw vw vw vw vw vs
-3 -5
'Values in cm-l. Methylsilane belongs to point group C3, and has 18 modes of et al. reported several features of the gas-phase spectrum of vibration. The C-H stretching fundamentals are15 tetramethylsilane in the 16 150-16 580-cm-' range.2s They tentatively assign a band at 16412 cm-' to the LM-LM comv,(E) = 2983 cm-l bination state ISCH,lCH) and ascribe other bands to local modenormal mode (LM-NM) combinations. Henry et al. have obvI(A1)= 2927 cm-l served the bCH = 2 to bcH = 5 overtone spectra in the gas phase and AvCH = 2 to AuCH = 6 in the liquid phase.29 They also and the Si-H stretching fundamentals are15 comment upon the very narrow bandwidths which here we have found to be characteristic of all of the methylsilanes. The high v2(A,)= 2169 cm-' vibrational overtones and combinations of the carbon-containing compounds analagous to the methylsilanes have been studied, vs(E) = 2165.69 cm-I including ethane,30 propane,I2 isobutane,12 and n e ~ p e n t a n e . ~ ~ - ~ ~ In the present work the discussion is centered around the Si-H The nearly equal values for v2 and vs are an indication of the and C-H stretching vibrations which are well described in terms appropriateness of the local-mode description for the Si-H stretch. of local-mode oscillators. Before we turn toward a consideration The larger discrepancy for the C-H stretching modes indicates of these individual features, the symmetry of the normal modes that it is less well described by the local-mode model. This general will be summarized as well as those frequencies which relate to observation holds for (CH3),SiH2 and (CH3)3SiHas well. the Si-H and C-H stretching local modes. Dimethylsilane belongs to point group C , and has 27 modes of vibration. The C-H stretching fundamentals are1s (28) J. W. Perry, D. J. Moll, A. H. Zewail, and A. Kupperman, to be submitted for publication. (29) B. R. Henry, M. A. Mohammadi, I. Hanazaki, and R. Nakagaki, J . Phys. Chem., 87, 4827 (1983). (30) B. R. Henry and W. R. A. Greenlay, J . Chem. Phys., 72, 5516 (1980).
u,(A,) = v10(A2)= u15(B1)= v22(B2)= 2967 cm-I
v2(Al) = vI6(B1)= 2912 cm-' and the Si-H stretching fundamentals areI8
\ - - - - I -
(31) B. R. Henry, A. W. Tarr, O. S. Mortensen, W. F. Murphy, and D. A. C. Compton, J. Chem. Phys., 79, 2583 (1983). (32) B. R. Henry and M. A. Mohammadi, Chem. Phys. Lert., 75, 99 (1980).
v3(A1)= 2145 cm-I
~23(B2)= 2142 cm-'
1090 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Bernheim et al.
TABLE II: Flllld.mental Frequencies of Lower Frequency Vibrational Modes Used for the Vibrational Assignments of Table I for the Methvlsiinesf
CH3SiH3"
(CH3)2SiHt
~,5(A2)~183 ~12(E) 545 vll(E) 871 Y ~ ( A , ) 1264 vg(E) 1403
YZ~(BZ)' -175 u ~ ~ ( B Z ) 467 u ~ ~ ( B I ) 643 u,(AI) 838 ~ 5 ( A l ) 1260 ~ 4 ( A l ) 1340
(CH&SiHb ~lz(A2)' -175 vs(A1) 216 ~7(Al) 625 ~zo(E) 856 ~g(A1) 1267 ~4(Al) 1467
(CH3),Sic
~4(Az)r 177.5 ~19(F2) 239 ~is(F2) 696 ~17(Fz) 869 Y I ~ ( F ~1254 ) Y I ~ ( F ~1430 )
Reference 15 unless otherwise specified. Reference 18 unless otherwise specified. Reference 26 unless otherwise specified. dReference 44, CH3 torsional mode. 'Estimates of CH3 torsional frequency based on results reported for CH3SiH3(ref 44) and (CH3),Si (ref 49). fReference 49, CH3 torsional mode measured for crystalline (CH3)4Si. Values in em-'.
/i
CH,SiH,
AvcH=6
1
f
t
I
I5750
/ L
1
I7800
,
I5850
.
L
l
,
I
I
I
1
,
I
/
I6050 cm-I
-
- 1 11
7900
,
I5950
1803C
L'_
181~&&0~m-~
Figure 2. Photoacoustically detected absorbance of high C-H overtone spectra observed for methylsilane at 100 torr and room temperature.
l
l
l
1
2
3
l
l
4
i
5
i
6
l
7
8
9
~
Figure 1. Birge-Sponer type plot of the band origins of the Si-H stretching local modes vs. vibrational quantum number for the methylsilanes and a comparison with SiH,.
Trimethylsilane belongs to point group C3, and has 36 modes of vibration. The C-H stretching fundamentals are1* vl(Al) = ug(A2) = ~13(E)= vI4(E) = 2964 cm-' v2(A1)= vl,(E) = 2900 cm-'
and the Si-H stretching fundamental isI4 u3(AI)= 2128.75 cm-' Tetramethylsilane (Me4Si) belongs to point group Td and has 45 modes of vibration. The C-H stretching fundamentals are27 q ( E ) = uI3(F2)= 2957 cm-' ~14(F2)= vl(Al) = 2900 cm-'
In addition to the above stretching fundamentals a number of the other normal-mode fundamentals were used in arriving at the tentative assignments of the combinations shown in Table I. The frequencies used for this purpose are given in Table 11. Equation 2 was used to find the off-diagonal anharmonicity or coupling required for the assignment. These are listed in Table I for the various combinations. Further verification of the overtone assignments was obtained by measuring the first and second Si-H and C-H stretching overtones and using that data in the BirgeSponer type fit of the local-mode overtones as discussed below. Si-H Stretch. In comparison to the C-H overtone and combination features, the Si-H stretching overtones were weak and, in some cases, obscured by the C-H features. Nevertheless, the AuSjH= 8 and 9 overtones in methyl- and dimethylsilanes were
observed, but only the AvSiH = 8 overtone could be assigned for trimethylsilane. These overtone band origins are listed in Table 111 together with the fundamentals and first o ~ e r t ~ n e s for ~~J~,~~,~~ the Si-H stretch. Included for comparison are the same data for7,8*34-40 SiH4 and SiD3H. A fit to the Birge-Sponer relation, eq 1, was performed for each molecule as shown in Figure 1, and the resulting effective harmonic frequencies wi and anharmonicities Xii are listed in Table IV. For each molecule the fundamental and first overtone of the Si-H stretch were included in the fits, the overall quality of which are indicated in the linear plots in Figure 1 and the error limits in Table IV. It can be seen that the high Si-H overtone behavior is similar for all of the species. Increasing methyl substitution results in a decrease in the local-mode Si-H stretching frequency wi and an increase in the negative diagonal anharmonicity Xi? These values can be used to find an upper limit of M,,, = w?/4Xii for the Si-H bond energy. Approximate bond energies Docan be found by using an empirical scaling factor of Do= 0.87 A&,, for Si-H bonds41 from which a trend of decreasing Si-H bond energy with increasing methyl substitution is found as shown in Table IV. This trend has been observed and its validity discussed previously in studies of isolated bond frequencies obtained for these compounds from measurements on isotopically substituted speci e ~ . ~These ~ - values ~ ~ are also listed in Table IV, where the agreement is quite reasonable. There is a slight uncertainty in the present work concerning the diagonal anharmonicity and bond energy for (CH3)3SiHwhere the fundamental and first overtone of McKean et al.I4 were used to identify the only Si-H overtone (33) J. F. OKeefe, Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1983. (34) A. Owyoung, P. Esherick, A. G. Robiette, and R. S . McDowell, J .
Mol. Spectrosc., 86, 209 (1981). (35) G. Pierre, personal communication. (36) L. Halonen and M.S.Child, Mol. Phys., 46, 239 (1982). (37) H. W. Kattenberg and A. Oskam, J . Mol. Spectrosc., 49, 52 (1974). (38) R. Bregier and P. Lepage, J. Mol. Spectrosc., 45,450 (1973). (39) C. Frommer, R. W. Lovejoy, R. S. Sams, and W. B. Olson, J . Mol. Specrrosc., 89, 261 (1981). (40) D. R. J. Boyd, J . Chem. Phys., 23,922 (1955). (41) C. L. Beckel, M. Shafi, and R. Engelke, J . Mol. Spectrosc., 40,519 (1971) (42) D. C. McKean, J. Mol. Struct., 113, 251 (1984).
Spectra of Methyl-Substituted Silanes
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1091
TABLE III: Observed Band Origins for the Si-H Stretching Fundamentals and Overtones for Silane and the Methylsilanesk
SiH4 2 186.873" 2 189.1901" 4308.38b*c 4 309.36b9C
b i H
sYm antisym
1
2 3 4 5 6 7 8 9
12 121.2d 13914.4d 15 625.4d 17 266.6d
SiD3H 2 187.2O7Oe 4 307.09/
13 897.88 15615.18 17 265.98
CH3SiH3 2169h 2165.69h 4258
(CH3)ZSiHZ 2 145' 2 142' 4225
15442 17082
(CH3),SiH 21288.74' 4189.2'
15259 16872
15205
OReference 34. bAssignments to 2vl and v1 + v3 from ref 37 and 7. CValuesfrom ref 35. dReference 8. CReference 39 for ?3iD3H. ZReference 40 for %iD,H. 8Reference 7. "Reference 15. 'Reference 18. 'Reference 14. kAll values in cm-I. TABLE IV: Vibrational Constants for Si-H Stretching Local Modes of the Methylsilanes with a Comparison with SiH, and SiDJH"
molecule SiD3Hb SiH( CH3SiH3 (CH&3H2 (CHJ3SiH
@i
2220.77 i 0.08 2222 1 2198 7 2179 4 2167.2 i 9
* *
xi,
i04Mm, 3.67 3.67 3.61 3.51 3.11
*** 0.1 0.2 1 * 0.6 2
-33.6 -33.6 -33.5 -33.8 -37.7 &
10~0, 3.19, 3.18d 3.19, 3.18d 3.14, 3.08d 3.06, 3.03' 2.71, 2.98d
I
"Values in cm-l and errors are 95% confidence intervals. Morse potential bond energies Mmx and empirically scaled values of Do are also given. Reference 7. cReference 8. dDetermined from isolated Si-H stretching frequencies from the fundamental and first overtone by using partially deuterated molecules in some cases (ref 14).
(CH3I3 SIH Av,, = 6
I
1
,
1
,
I
15650 I57W 15750 15800 15850 15900 i5950 16ooo 16050 16100cm-I
(CH&SIH~ AV,
1
=5
DVCH = 7
Figure 4. Photoacoustically detected absorbance of high C-H overtone spectra observed for trimethylsilane at 100 torr and room temperature.
A
ICH31,Si AV,
=5
I
km
I
13300
(CH, A,v, ),Si = 6
13350 134
%1
Figure 3. Photoacoustically detected absorbance of high C-H overtone spectra observed for dimethylsilane at 100 torr and room temperature.
found for this molecule in the present study (8Ysi~at 15 205 cm-' in Table I). A serious obstacle to the identification of the Si-H stretching overtones was the relatively strong intensities of the C-H vibrational features. T h e C-H stretching overtones a r e 30-50 times more intense than the nearest Si-H overtone in each case. A contributing factor t o this relative intensity is the fact that the C-H overtone transition is to a state that is two vibrational levels lower than that for the nearest Si-H overtone (e.g., 5vCH vs. 7vsw, 6vCH vs. 8Y&, etc.). T h e b S i H = 7 overtones were not observed at all because they were obscured by the AvCH = 5 overtones. In the case methylsilane, the AvS~H= 7 overtone may be responsible for a blue tail on the AvCH = 5 overtone at 13 750 cm-', but the
15650
15/00
15750
15800
AV,,
15850 159oc
15950
16000 16050 cm-1
=7
17700 17800 17900 I8000 18100 cm-' Figure 5. Photoacoustically detected absorbance of high C-H overtone spectra observed for tetramethylsilaneat 100 torr and room temperature.
1092 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Bernheim et al.
C-H Stretching Fundamentals a d Overtows for Methane and the Methylsllplresk CH4 CH,SiH, (CHASiH2 (CH,),SiH
TABLE V Band Origins for the
Avcn
5
2915.43‘ 3 019.49b 5 86lC 6 004.9679d 9 045.92p 11 2 2 6 13 7 5 6
13539
6
16 1 6 6
15929
7
18426
18159
8 9
20 6 0 6 22 6 6 6
1
2
3 4
2981.578 2928.848 5755.4 5784.5 8499.3
2912* 2 967h 5 760.6
2 900h 2 964h 5 751.3
2900‘ 2957’ 5742’
8 462.2
8 447.7
8432j 11025’ 13508/
13 484 13516 15868
13422 13 470 15759 15 814 15 862 17 996 18 072
18 077 18 132
(CHASi
5 742.0 8 422.8
13 430.7 15 768 15817 18009
“Reference 50. bReference51. CReference52. dReference 53. CReference54. ’Reference 55. EReference 15. Reference 18. ‘Reference 26. ’Reference 29. kAll values in cm-l. and all data are from this work except where noted. uncertainty in spectral position precludes its definite assignment. Nevertheless, sufficient assignments were made to demonstrate the local-mode behavior of the upper overtones of the Si-H stretch. C-H Stretch. Portions of the photoacoustically detected absorption spectra recorded in the vicinities of the AUCH= 5,6, and 7 overtones of each of the four methylsilanes are shown in Figures 2-5. The principal peaks in each overtone region are indicated by arrows and are listed in Table I along with the positions of the numerous other less intense spectral features observed in the 12800-18 200-cm-’ region. The values listed in Table I are the average of at least three measurements. The precision of measurement for the strong overtone features is estimated to be about f l cm-I except for AuCH = 5 and 6 of dimethylsilane, where it is f 2 cm-’. An error of at least f5 cm-l is estimated for the other, less intense features in Table I. In order to understand the multiple, and sometimes overlapping, band structures in the immediate C-H overtone regions for each molecule, it is important to realize that there are a number of possible sources for these features. Besides the contributions of combinations and degeneracy lifting perturbations, the multiple-band structure can arise from sterically inequivalent C-H bonds in the methyl groups or from hot bands arising from the torsional levels of the hindered methyl group rotor. The groups of strong overtone features in Figures 2-5 generally follow the Birge-Sponer relationship, and a reasonable assignment will be given below. However, a complete understanding of the individual bands that make up Figures 2-5 as well as the other features listed in Table I is not possible at the present time, and the assignments given must be understood to be only tentative. Of the four methylsilane molecules in this study, only two present the possibility of containing two sterically distinguishable types of C-H bonds in each methyl group: (CH3)2SiH2and (CH3)3SiH. For the other two, CH3SiH3and (CH3),Si, the methyl groups are bonded to a threefold or axially symmetric site in the molecule. One observation that is evident at the outset is that for some of the overtone regions at least three distinct strong bands can be observed. These happen to occur only for (CH3)2SiH2and (CH3)3SiH,molecules which can exhibit sterically distinguishable C-H bonds in the methyl groups, while the C-H overtone regions of CH3SiH3 and (CH3),Si apparently never contain more than two strong bands. The observation of separate C-H local-mode vibrations for sterically inequivalent C-H bonds in methyl groups has been reported in propane and isobutane,’* toluene, and o - ~ y l e n e . If ~~ free rotation of a methyl group in a nonaxial environment occurs, the inequivalent C-H local modes will be expected to occur at some “average” frequency. Such is observed to be the case in toluene,43 where the bands due to the inequivalent C-H bonds of the hindered methyl rotor fall on either side of that due to the (43) K. M. Gough and B. R. Henry, J. Phys. Chem., 88, 1298 (1984). (44) D. R. Lide and D. K. Coles, Phys. Reo., 80, 911 (1950).
“free” methyl rotor. If the methyl group were bound to an axially symmetric site, observations of separate bands could be due to absorption from different levels of the hindered rotor, i.e., “hot bands”, but not to sterically different C-H bonds. The carbon compounds analogous to di- and trimethylsilanes, propane and isobutane, each contain three sets of spatially equivalent C-H oscillators. Previous studies have shown that the C-H stretching overtones of these two molecules each consist of three separate components, one for each type of C-H oscillator, with the separations between the components being on the order of 100 cm-’ and with intensity ratios of the components being roughly the same as the ratios of the numbers of each type of oscillator p r e ~ e n t . ~On ~ , the ~ ~ basis of the number of sets of equivalent oscillators in each of the methylsilanes studied here, and on the basis of the previously reported results for the methyl-substituted methanes, it would be expected that the visible and near-IR C-H stretching overtones of methylsilane and Me4Si would each consist of a single band while those of di- and trimethylsilanes would each consist of two bands with a roughly 2:l intensity ratio. Reference to Figures 2-5 shows, however, that only the hCH = 6 overtone feature of methylsilane and the AuCH = 7 overtone features of the di, tri-, and tetramethylsilanes appear to conform to these expectations (no conclusion can be stated concerning AUCH = 7 of methylsilane because the recording of that band is incomplete due to the upper limit of the rhodamine 560 dye gain curve at 18 200 cm-I). For each of the four methylsilanes, the AuCH = 5 overtone feature exhibits an additional peak compared to what would be expected on the basii of the number of sets of equivalent oscillators present in the molecule. In each case, this extra peak can be accounted for in terms of a LM-NM combination involving the hVcH= 4 C-H stretching overtone and two quanta of a symmetric CH3distortion where the combination borrows intensity via Fermi resonance with the AuCH = 5 overtone. A tentative assignment is listed in Table I for each of these cases. The AuCH = 6 overtone feature of dimethylsilane consists of a single, broad, irregularly shaped peak, instead of resolved bands. The bCH = 6 overtone features of trimethylsilane and Me,Si each contain an additional band compared to the number expected on the basis of inequivalent C-H oscillators. It is possible that the anomalous appearance of the AUCH= 6 overtone feature of each of these three methylsilanes is due to a LM-NM combination in Fermi resonance with the overtone itself. No specific LM-NM or LM-LM assignments are suggested here, however, because in each case none could be found which was consistent with the other spectra observed for the different molecules. The barriers to methyl group internal rotation have been det e r m i ~ ~ e d for ~ ” ~all of the methylsilanes and have values between (45) (46) (47) (48)
R. W. Kilb and L. Pierce, J. Chem. Phys., 27, 108 (1957). D. Kivelson, J. Chem. Phys., 22, 1733 (1954). L. Pierce, J . Chem. Phys., 34, 498 (1961). L. Pierce and D. H. Petersen, J . Chem. Phys., 33, 907 (1960).
Spectra of Methyl-Substituted Silanes
300P
The Journal of Physical Chemistry, Vol. 89, No. 7, I985 1093
2900
\;t
) 2 5 0 0 ' ;
:
:
:
:
6
V
:
;
9
2 5 o o ' I
:
:
b
S
b
:
b
9
V
Figure 6. Birge-Sponer type plots of the band origins of the C-H stretching overtone vs. vibrational quantum number for methylsilane.
3000
Figure 8. Birge-Sponer type plots of the band origins of the C-H stretching overtone vs. vibrational quantum number for trimethylsilane.
3000
b
A
2900
2800 AE/v c in-'
2700
2600
) 2500
1
2
3
4
5
6
7
8
9
2 5 0 o L I
V
:
:
h
:
k
:
b
9
V
Figure 7. Birge-Sponer type plots of the band origins of the C-H stretching overtone vs. vibrational quantum number for dimethylsilane.
Figure 9. Birge-Sponer type plots of the band origins of the C-H stretching overtone vs. vibrational quantum number for tetramethylsilane.
1.5 and 2.0 kcal/mol. A number of the torsional vibrations are
relation. The objective was to estimate values for the effective harmonic frequency w , and the anharmonicity Xii for the C-H stretch of these molecules. The C-H fundamental and lower overtone data from.sources in the literature15J8*26-29~s55 are given
also given in Table 11. Since the present experiments were carried
out at room temperature, these levels will be expected to carry some population. While the assignment of some of the extra peaks to hot bands is possible, their presence should also be expected in the higher (e.g., Au = 7) overtones, but they appear to be absent. For each of the four methylsilanes, an attempt was made to fit the AvCH = 5-7 overtone peak positions to the Birge-Sponer (49) J. R. Durig, S.M. Craven, and J. Bragin, J. Chem. Phys., 52, 2046 (1970).
(50) R. S. McDowell, C. W.Patterson, and A. Owyoung, J. Chem. Phys. 72, 1071 (1980). (51) J. Herranz and B. P. Stoicheff, J . Mol. Spectrosc., 10, 448 (1963). (52) G. Herzberg, 'Infrared and Raman Spectra", Van Nostrand, New
York. 1945. (53) K.FOX,G. W.Halsey, and D. E. Jennings, J . Mol. Specrrosc., 83, 213 (1980).
1094 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 TABLE VI: Vibrational Constants for C-H Stretching Local Modes of the Methvlsilanes with a Comparison with C€L" molecule wi xii lO-"AE,, CH3SiH3 3009 f 2 -59.2 f 0.2 3.82 -59.7 f 0.4 3.77 (CH3)2SiH2 2992 f 2 -59.8 f 2 3.75 (CH&SiH 2994 f 12 (CH3)4Si 2992 f 9 -60.3 f 2 3.72 2982b -56.3' 2974.W -58.7C CH4 303Sd -57.gd aValues in cm-I and errors are 95% confidence intervals. bReference 29. 'Reference 27. dData from ref 49-54. (CHJ12 SiHp
Av,,,:~
I
15200
15;oo
I ~ O cm-' O
Figure 10. Photoacoustically detected absorbance of the AusiH = 8 overtone of dimethylsilane at 100 torr and room temperature.
in Table V together with a summary of the C-H stretching overtones measured in this work. The four Birge-Sponer plots are given separately in Figures 6-9. Each of the strong bands shown in Figures 2-5 are given as data points as are the fundamentals and lower overtones. Also given in Table V and on each plot are the data for CH4 which are useful for comparing the diagonal anharmonicities. In choosing which of the strong overtone bands should be used in the Birge-Sponer overtone fits, we used the first and second overtones as a guide. None of the fundamentals fell on the fit to the overtones. In most cases there was a reasonable fit of the overtone bands to the Birge-Sponer plot, but an unambiguous assignment of each band cannot be made in every case as can be seen from Figures 6-9. The harmonic frequency, diagonal anharmonicity, and maximum bond energy AEmaX obtained from these data are given in Table VI. A slight decrease in bond energy with increasing methyl substitution is evident. An attempt to scale the AE,,, values to Dovalues was not made in this case. Bandwidths. Narrow bandwidths and resolvable rotational band structure have been observed in the Si-H local-mode stretching overtones of a number of pentatomic silanes.&* In the present work both the C-H and Si-H high overtones are also found to have narrow bandwidths but without indication of distinctive branches of rotational structure. The visible Si-H overtones are very much weaker than the visible C-H overtones at the sample pressures used. A typical observation is shown in Figure 10 for A u ~ H= 8 for (CH3)*SiH2 where the signal-to-noise ratio is several orders of magnitude weaker than the C-H overtones shown in Figure 3 for the same sample. However, for each of the three methylsilanes having a Si-H bond, the feature assigned to AusH = 8 appeared as a distinct (54) G. Pierre, J . 4 . Hilico, C. DeBergh, and J.-P. Maillard, J . Mol. Specrrosc., 82, 379 (1980). (55) L. P. Giver, J . Quant. Specrrosc. Radia?. Transfer, 19, 31 1 (1978). (56) J. R. Qualey 111, Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1984.
Bernheim et al. peak. The features assigned as A u ~ ~=H9 overtones for methyland dimethylsilanes occurred as shoulders on the long-wavelength sides of more intense features, which are most likely due to LMN M combinations involving the C-H stretching overtones. The I1uSi-H. = 7 overtones are obscured in all cases. Whle the upper Si-H overtones did not show rotational branch features, they did exhibit bandwidths comparable to those observed for the Si-H stretching fundamentals a t lower pressures where rotational features are evident. The widths of the fundamentals are -50 cm-' for methyl- and dimethylsilanes at 10-20 torr. A fit of a Lorentzian or other line-shape functions to the Si-H overtones was not attempted, but it can be said that homogeneous broadening effects are not significantly larger than the rotational contributions to the bandwidths. The intense C-H bandshapes can be fitted by Lorentzian functions in the cases where there seems to be only one component. Where there are several components present, a deconvolution would be possible but was not carried out inasmuch as there is considerable uncertainty as to the assignments and, in some cases, the number of components present. However, the bandwidths of the components can be estimated and are roughly found to be the same as the C-H stretching fundamentals but without discernible rotational branch structure. For example, in the case of MelSi (Figure 5), the intense lower energy peak of the AUCH= 5 overtone feature has a fwhm of at most 37 cm-', and the AucH= 7 overtone feature has a fwhm of 39 cm-'. The split peak of the AuCH = 6 overtone feature of Me4Si has an overall fwhm of 90 cm-' which can be attributed to two vibrational bands which each have a fwhm of -40 cm-' separated by 50 cm-I. These bandwidth values are all very close to the value of -35 cm-' observed for the C-H stretching fundamentals of Me,Si at lower pressures where rotational features are present. The narrow bandwidth of the AuCH = 5 overtone in Me4Si has also been observed and commented upon in another gas-phase overtone While the bandwidths are not substantially broader than the rotationally featured fundamentals, the shapes indicate that the overtones are exhibiting damped oscillator behavior. A similar observation can be made for each of the other methylsilanes. In trimethylsilane the fundamental has a fwhm of about 35 cm-I, and the overtone features are in the 30-40-cm-' range, although not well resolved in some cases. In methylsilane and dimethylsilane the fundamentals are of the order of 50 cm-I broad, and the resolved overtone regions exhibit peaks of approximately the same width. Summary The vibrational overtones of the methylsilanes have been investigated in the visible spectral region by using the techniques of photoacoustic spectroscopy. The main findings are that the Si-H and C-H stretching overtones are very well described by the local-mode model, and molecular constants in the form of effective harmonic frequencies and diagonal anharmonic constants can be found. While the Si-H and C-H overtones do not exhibit rotational branch structure, the bandwidths are comparable to the fundamentals which do have rotational features.
Acknowledgment. This research was supported in part by the U.S. Department of Energy and the National Science Foundation. Registry No. CH3SiH3,992-94-9; (CH,),SiH,, 1 1 11-74-6; (CH3),SiH, 993-07-7; (CH3)&3i, 75-76-3.
