J. Phys. Chem. 1983, 87, 4027-4032
Overtone Spectra and Spectral Bandwidths in Liquid- and Vapor-Phase Tetramethylsilicon, -germanium, and -tin Bryan R. Henry,' M. All Mohammadl, hpattment of Chemistfy, University of Manitoba, Winnipeg, Manitoba, Canade R3T ZNZ
Ichlro Hanazakl, and Ryolchi Nakagakl
Instltute for Molecular Sclence, Myodaili. Okazakl 444, Japan (Received: May 5, 1983)
The overtone spectra of tetramethylsilicon, -tin, and -germanium are measured in the liquid and vapor phases in the spectral regions of the CH-stretching local-mode overtones corresponding to Au = 2 to Au = 6 (liquid), and to Au = 2 to Au = 5 (vapor). The observed spectral features are assigned on the basis of the local-mode model. Local-mode frequencies, wCH, and diagonal local-mode anharmonicities,X C H , are obtained from an analysis of the spectra. These parameters are used to discuss how the differences in vapor- and liquid-phase behavior vary from molecule to molecule. The bandwidths of the pure local-mode overtones are determined in the liquid and vapor phases. A consistent decrease in the vapor-phase bandwidth is observed from Au = 4 to Au = 5. The bandwidth results are related to recent theories of vibrational redistribution from highly excited local-mode states.
Introduction The local-mode model has been used to explain the relatively narrow single bands observed in the CHstretching overtone spectra of molecules with several equivalent CH oscillaton.1*2 For transitions corresponding to Au Z 3 ( u will be used to refer to the CH-stretching vibrational quantum number) the vibrational state preferentially excited by the radiation field can be described in terms of virtually degenerate symmetrized combinations of components of the type lv,O,O...), IO,u,O,O ...), etc., where all of the vibrational quanta are localized in a single CH Because the components, lu,O,O...), etc., are only very weakly the energies of these transitions fit the equation for a diatomic anharmonic CH oscillator, i.e. h E / u = OCH + V X C H (1) where wCH is the local-mode frequency in cm-' and X C H is the diagonal local-mode anharmonicity constant. These local-mode parameters, u C H and X C H , are extremely sensitive to the physical and chemical environment of the CH oscillator. On the basis of these parameters, local oscillators have been used as probes to study chemical environment? molecular conf~rmation,~-~ and nonbonded intra- and intermolecular forces.lOJ1 There have been several experimental and theoretical investigations of the vibrational properties of the symmetrical molecules (CH3)4M(M = C, Si, Ge, Sn, and Pb). Graham12and Biirger and Biedermann13have investigated (1) B. R. Henry, Acc. Chem. Res., 10, 207 (1977), and references therein. f2) B. R. Henrv. Vib. Smctra Struct.. 10. 269 (1981). (3) R. L. Swofiord, M. F. Long, and A. C;Albrecht, J. Chern. Phys., 65, 179 (1976). (4) I. A. Wataon, B. R. Henry, and I. G. Ross, Spectrochirn. Acta, Part A. 37. 857 (1981). ' ( 5 j 0. S.'Mor&nsen, B. R. Henry, and M. A. Mohammadi, J. Chem. Phys., 75, 4800 (1981). (6) W. R. A. Greenlay and B. R. Henry, J. Chem. Phys., 69,82 (1978). (7) B. R. Henry, I. F. Hung, R. A. MacPhail, and H. L. Strauss, J.Am. Chem. Soc., 102, 515 (1980). (8) B. R. Henry and M. A. Mohammadi, Chem. Phys., 55,385 (1981). (9) R. Nakagaki and I. Hanazaki, Chern. Phys. Lett., 83, 512 (1981). (10) W. R. A. Greenlay and B. R. Henry, Chern. Phys. Lett., 53, 325 (1978).
(li) B. R. Henry, M. A. Mohammadi, and J. A. Thomson, J. Chern. Phys., 75, 3165 (1981). ~~
the CH-stretching vibrations of these molecules in the fundamental region and have analyzed their results on the basis of the traditional normal-mode description. The liquid-phase CH-stretching overtone spectra of tetramethylsilane have been investigated by Burberry and Albrecht from Au = 2 to Au = 6.14 Their work focused on the local mode-local mode combination bands. They assigned the splittings in these bands on the basis of the energy differences between A, and E symmetrized combinations of Components corresponding to the distribution of energy over the three CH methyl local oscillators. We have investigated the liquid-15J6 and gas-phase17 overtone spectra of neopentane. For both phases, the spectra had very narrow bandwidths for the pure localmode overtones. These bandwidths were considerably narrower than those for the corresponding methyl transitions in liquid-phase alkanes15or even in gaseous ethane.6 In fact, the gas-phase bandwidth at Au = 5 (64 cm-l)17 was narrower than the corresponding bandwidth in gaseous benzene (109 cm-l).18 We pointed out" that, if the bandwidth arose from homogeneous broadening due to vibrational redistribution, then the narrower bandwidth in neopentane could not be rationalized by either local mode-local mode couplinglg or direct coupling to the full normal-mode bath.20 In neopentane, the CH oscillators share a common carbon and so local-mode coupling would be stronger than in benzene. Neopentane contains more atoms than benzene and so the overall density of states would be larger in neopentane. (12) S. C. Graham, Spectrochirn. Acta, Part A , 26, 345 (1970). (13) H. Burger and S. Biedermann, Spectrochirn. Acta, Part A , 28, 2283 (1972). (14) M. S. Burberry and A. C. Albrecht, J. Chern. Phys., 71, 4631 (1979). (15) B. R. Henry and W. R. A. Greenlay, J. Chem. Phys., 72, 5516 (1980). (16) B. R. Henry, A. W. Tarr,0. S. Mortensen, W. F. Murphy, and D. A. C. Compton, J. Chem. Phys., 79, 2583 (1983). (17) B. R. Henry and M. A. Mohammadi, Chem. Phys. Lett., 75,99 (1980). (18) R. G. Bray and M. J. Berry, J. Chern. Phys., 71, 4909 (1979); K. V. Reddy, D. F. Heller, and M. J. Berry, ibid., 76, 2814 (1982). (19) D. F. Heller and S. Mukamel, J. Chem. Phys., 70, 463 (1979). (20) M. L. Sage and J . Jortner, Chern. Phys. Lett., 62, 451 (1979).
0022-3654/03/2007-4027$0 1.50/0 0 1903
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4828
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Henry et ai.
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A(nm) Figure 1. Vaporphase overtone spectra of (CH3),Si (105-ton pressure, 2.25-m path length), (CH,),Ge (110-torr pressure, 2.25-m path length), and (CH,),Sn (58-torr pressure, 3.75-m path length) at room temperature in the region of Av,, = 2. The (CH3),Ge and (CH,),Sn absorbances have been offset by 0.3 and 0.7 absorbance units, respectively.
In the present work, we investigate the overtone spectra of (CH3),M (M = Si, Ge, Sn) in both liquid and vapor phases. In particular, we compare the bandwidths for the two phases and compare the results to those for neopentane. We attempt to explain the results based on recent theories of the dynamics and coupling of highly vibrationally excited local-mode states.
Experimental Section Tetramethylsilane (99.9+ %) and tetramethyltin (99+%) were obtained from Aldrich Chemical Co. Tetramethylgermane (99+%) was obtained from Chemical Procurement Laboratories. The spectra of these molecules were recorded as colorless neat liquids at room temperature with a Beckman 5270 spectrophotometer. The spectra were also measured in the vapor phase with a variable path length gas cell (Wilks Scientific Corp., South Norwalk, CT, Model 5720) with KBr windows. The cell path length was adjusted in the range from 0.75 to 9.75 m. The spectra were digitized and deconvoluted in order to obtain the full width at half-maximum (fwhm) of the principal local-mode absorption band. Either 128 or 256 points were used. In cases where a rising base line was evident, a linear sloping base line correction was subtracted. The digitized bands were first converted to an energy scale (cm-'). The data were then entered into a Nicolet 1180 data system and deconvoluted with a standard curve analysis program, which fitted the band as a s u m of Lorentzian peaks. In all cases, the experimental and calculated band envelopes were compared to check the quality of the deconvolution fit.
Results The vapor-phase overtone spectra of (CH3),M (M = Si, Ge, and Sn) for Au = 2-5 are presented in Figures 1-4. The liquid-phase spectra for Au = 2-6 of the same molecules are given in Figures 5-9. For both the vapor- and liquid-phase spectra for Au 1 3, the principal spectral contribution for each overtone arises from a single peak. This peak corresponds to unresolved transitions to the A,
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A(nrn) Figure 2. Vaporphase overtone spectra of (CH,),Si (105-ton pressure, 2.25-m path length), (CH,),Ge (90-torr pressure, 3.75-m path length), and (CH,),Sn (58-torr pressure, 8.25-m path length) at room temperature In the region of Avcn = 3. The (CH3),Ge and (CH,),Sn absorbances have been offset by 0.06 and 0.15 absorbance units, respectively.
a
0.04 -
900
910
Figure 3. Vapor-phase overtone spectra of (CH,),Si (85-torr pressure, 5.25-m path length), (CH,),Ge (110-torr pressure, 5.25-m path length), and (CH,),Sn (58-torr pressure, 9.75-m path length) at room temperature in the region of AvCH= 4. The (CH,),Ge and (CH,),Sn absorbances have been offset by 0.02 and 0.04 absorbance units, respectively.
and E components of the pure local-mode overtone states. (We use AI and E symmetry labels appropriate to the C, symmetry of the local methyl groups, vide infra.) These states can be described almost totally in terms of symmetrized combinations of basis functions Iu,O,O),JO,u,O), and IO,O,u), i.e., contributions from functions where all of the vibrational quanta are localized in one of the methyl
Spectra of Tetramethylsilicon, -germanium, and -tin
0024
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983 4829
7 A
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Flgure 4. Vaporphase overtone spectra of (CH,),Si (120-torr pressure, 6.75-m path length), (CH3),Ge (110-torr pressure, 6.75-m path length), and (CH,),Sn (58-torr pressure, 9.75-m path length) at room temperature In the region of Av,, = 5. The (CH,),Sn absorbance has been offset by 0.004 absorbance unit. The parenthetical wavelength scale refers to (CH,),Si.
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Flgure 5. Liquid-phase overtone spectra of (CH,),SI, (CH.J4Ge, and (CH,),Sn at room temperature in the reglon of A v , = 2: 0.1-cm path length. The (CH,),Ge and (CH,),Sn absorbances have been ofset by 0.2 and 0.4 absorbance units, respectively.
Flgure 7. Llqubphase overtone spectra of (CH,),SI ( M m path length), (CH,),Ge (8.1-cm path length), and (CH3),Sn (10-cm path length) at room temperature in the region of A v C H = 4. The (CH,),Ge and (CH3),Sn absorbances have been offset by 0.3 and 0.7 absorbance units, respectively.
CH bonds. For Au Z 3, local mode-local mode (LM) combination bands and local mode-normal mode (NM) combinations also occur but are generally of much lower relative intensity. The spectra for Au = 2 (Figures 1 and 5) are notably different. They correspond, in all cases, to a single rather asymmetric peak and a higher energy doublet of comparable intensity. The local-mode contributions to the single peak are A I and E components of local-mode states (i.e., combinations of 12,0,0),(0,2,0),and 10,0,2)).14J6By comparison to neopentane,16 we would expect these two com-
ponents to be split by -10 cm-l and therefore to be unresolved. Since both components will carry intensity, the resultant single band is expected to be asymmetric. In neopentane, there is extensive normal-mode combination band activity in this region.16 The relative intensity of normal-mode combinations appears to be much lower for these three molecules; nevertheless, similar combinations could be contributing to the intensity within the single band envelopes. The doublet components correspond to the A, and E local-mode combinations of p , l , O ) , ll,O,l), and lO,1,l).l4J6
4830
The Journal of Physical Chemistry, Vol. 87, No. 24, 7983
Henry et al.
TABLE I: Observed Local-Mode Band Maxima ( c m - ' ) in the Overtone Spectra of (CH,),M (M = C, Si, Ge, Sn) AU
molecule (CH,),C (CH,),Si (CH,),Ge (CH,),Sn From ref 1 7 .
0 12
phase
2
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57456 5742: 5723 ?: 5758 r 5739 t 5782f 5760 f
From ref 16.
3
1 0.5 1 0.8 1 0.4
8402 f 8401' 8432 f 8395 f 8468f 8445 k 8490 t 8459f
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4
5
6
10 959 f 2 10 92OC 11 025 i: 3 10 970 t 2 11 072 f 2 11 027 f 1 11 091 f 1 11 029 i: 3
13421 t 4 13 367' 13 508 i: 3 13 449 f 2 1 3 558 f 2 13 496 i: 0.6 13 564 i: 1 1 3 506 i: 2
15 740
f
2
15 818 f 2 15867
i:
1
' From ref 15. TABLE 11: Diagonal Local-Mode CH-Stretching Anharmonicity Constants, Local-Mode CH-Stretching Frequencies Calculated from the Peak Maxima of Table I, and Dissociation Energies Calculated from W C H and XCH(cm-')
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-59 t 1 - 6 7 i: 3 -56.3 t 1 -59.1 t 0 . 9 -55.7 f 0.2 -58.2 t 0.8 -59.2 f 0.5 - 5 9 . 0 f 0.7
2976 f 6 3003 r 1 0 2982 i: 4 2979 r 4 2990.2 * 0 . 9 2988 r 4 3009 r 2 2996 r 3
31 000 28 580 32 770 31 010 33 350 31 710 31 570 31 450
(CH,),Si (CH,),Ge (CH,),Sn
TABLE 111: Fwhm (cm-l) for CH-Stretching Overtones in (CH,),M AU
molecule
phase
3
40 ( 3 ) 61 ( 3 ) 46 ( 3 ) 45 ( 2 ) 76(3) 73(2)
54 49 50 ( 2 ) 48(2) 52 ( 2 ) 54 ( 2 ) 72(2) 97(3)
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740
750
A(nm) Flgure 8. Liquid-phase overtone spectra of (CH,),Si (10-crn path length), (CH,),Ge ( k r n path length), and (CH,),Sn (10-m path length) at room temperature in the region of AvcH = 5. The (CH3),Ge and (CH,),Sn absorbances have been offset by 0.04 and 0.05 absorbance units, respectively.
The observed spectral splitting of about 40 cm-' is similar to the splitting of 33 cm-' observed in neopentane.16 In the liquid phase (Figure 5) each of these components is well described by a single Lorentzian curve. The asymmetry observed in the doublet components in the gas-phase spectra (Figure 1)most probably arises from unresolved rotational fine structure. For all three molecules, there is a low-intensity peak observed just to the low-energy side of the intense peak at around 1740 nm. This peak corresponds to a combination. The splitting from the 1740-nm band is 125, 147, and 214 cm-' for (CH3)4Si,(CH3)4Ge,and (CH3)4Sn,respectively. Such combination bands draw intensity from the local-mode band. Thus, the increasing energy separation is also reflected in a decreasing relative intensity of this combination from (CH3)4Sito (CH3)4Geto (CHd4Sn. At Au = 3 (Figures 2 and 61, the local-mode peak corresponding to the virtually degenerate Al and E 13,0,0)type states is predominant. The local-mode combinations involving A, and 2E 12,1,0).type states are of much lower intensity and located in the region of 1150 nm. As expected: the splitting between these different local-mode combination Components is evident, especially in (CH3)4Si. An unresolved shoulder about 100 cm-' away from the pure local-mode peak can be seen in the gas-phase spectra (Figure 2) of both (CH3)4Geand (CH3)4Sn. This peak also appears in neopentane and has been assigned to a com-
vapor6 liquid' (CH,),Si vapor liquid (CH,),Ge vapor liquid (CH,),Sn vapor liquid
2'
a Estimate only (see text). ref 15.
4
6
5
62(2) 64(3) 74 119 49 ( 2 ) 39 ( 2 ) 65(2) 54(2) 66(3) 64 ( 2 ) 56 (1) 6 4 ( 2 ) 85 ( 2 ) 98 ( 2 ) 6 2 ( 2 ) 47 ( 2 ) 78(3) 86(3) 88(3)
From ref 17.
From
bination involving the Au = 3 local-mode state and one quantum of a very low-frequency normal mode, possibly a CH3 torsional mode.15J7 The Au = 4 spectra for all three molecules (Figures 3 and 7) consist essentially of a single peak. The marked asymmetry on the high-energy side of the peak arises from the unresolved contribution of the combination involving the 100-cm-l mode. The Au = 5 bands show pronounced shoulders, on the high-frequency side of the local-mode peak in (CH3)4Si, and on the low-frequency side in (CHJ4Ge. Such an effect has been observed in the dihal~methanes~ where it arose from a resonance interaction between a state with u quanta in the pure local mode and a combination state involving u - 1 local-quanta and two quanta of CH bending. The observed positions of the pure local-mode peaks for (CH3)4M,(M = Si, Ge, and Sn) are listed in Table I along with similar data for neopentane from previous studies.'"17 The band maxima are obtained as the average of at least five scans and the listed uncertainties correspond to least mean squares deviations from these averages. The data of Table I have been fitted to eq 1with a least mean squares procedure in order to evaluate the localmode parameters wCH and X C H . These parameters are listed in Table I1 along with the corresponding uncertainties from the least mean squares fit. Table I11 lists the fwhm for the local-mode overtone bands in both the liquid
-
Spectra of Tetramethylsilicon, germanium, and -tin
i(nm)
Flgure 9, Liquid-phase overtone spectra of (CH&Si (10-cm path length), (CHJ4Ge ( W m path length), and (CH,),Sn (10-cm path length) at room temperature in the region of Avcn = 6. The (CH3),Ge and (CH,),Sn absorbances have been offset by 0.006 and 0.008absorbance units, respectively.
and vapor phases. The numbers in parentheses correspond to the number of Lorentzian bands used in the deconvolution procedure. In all cases the bandwidths correspond to the most intense component, which we interpret as the unresolved transitions to the Al and E pure local-mode overtones. For Au = 2 the Lorentzian fitting was notably poorer than for the higher overtones. This is not surprising given the expected splitting of -10 cm-' for the Al and E pure local-mode components. Therefore, the numbers for Au = 2 should be interpreted as a rough estimate only.
Discussion Local-Mode Parameters. The local-mode parameters, XcHand wCH, in Table I1 have been used to calculate the dissociation energy of CH bonds along a local methyl CH-stretching coordinate. The calculation is based on the assumption of a Lippincott-Schroeder potential functionz1 for the local CH oscillator. On the basis of this potential, a simplified expression can be obtained for the dissociation energyll D(cm-') = -wCHz/(5.333xCH + 28.93) (2) Dissociation energies calculated from eq 2 are also listed in Table 111. In the carbon, silicon, and germanium tetramethyl molecules, the magnitude of the diagonal local-mode anharmonicity is significantly greater in the liquid phase than in the vapor phase. The anharmonicity difference between the phases decreases from (CH3)4Cto (CH3),Si to (CH,),Ge. In (CH3),Sn the anharmonicities for the two phases are within experimental error. Generally the local-mode frequencies do not change appreciably between the liquid and vapor phases. Therefore, the dissociation energy is consistently greater in the vapor phase and the differences in D follow the same trend as the differences in the anharmonicity. Thus, it appears that the effect of intermolecular interactions on the local methyl CH potential decreases with increase in the atomic weight of M in the molecules (CH3)4M,which is opposite to the trend expected on the basis of the magnitude of the dispersive interactions. Increased intermolecular interactions result in increased spectral broadening in the liquid phase over the vapor phase. As u increases, the local CH oscillator becomes a (21)E.R. Lippincott and R. Schroeder, J . Chem. Phys., 23, 1131 (1955).
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983 4031
more sensitive probe of its environment. For these molecules the increased spectral broadening in the liquid phase becomes significant at Au = 5 (Table 111). On the basis of the differences in XcHand D,one would have expected that the increase in liquid-phase spectral bandwidth over that in the vapor phase should also decrease as the atomic weight of M increases. However, although the bandwidths of Table I11 indicate that, at Au = 5, the change in fwhm is greatest for neopentane (55 cm-'), the remaining molecules follow the order for A(fwhm) of (CH3),Si < (CH3),Ge < (CH3),Sn. A partial answer to this apparent dilemma may lie in molecular shape. (CH3)4Siis more nearly spherical than (CH,),Ge than (CH,),Sn (vide infra). Therefore, the intermolecularly induced spectral broadening may be reduced in (CH3)4Sidue to the isotopic nature of the interactions. Local-Mode Bandwidths. In interpreting our earlier studies of the overtone spectra of neopentane,16 and in our present assignment of spectral features in (CH3),M (M = Si, Ge, and Sn), we have assumed, as a fiist approximation, that the four CH3 units are effectively independent. We will adopt the same approach in interpreting the fwhm in Table 111. Two consistent trends can be noted from the data of Table 111. The first involves the marked increase in liquid-phase over vapor-phase bandwidths at Au = 5. We have already commented on this effect, and how it differs from molecule to molecule, in the preceding section. The second is the vapor-phase overtone band narrowing from Au = 4 to Au = 5 observed for tetramethylsilicon, -germanium, and -tin. (The bandwidths for neopentane are the same, within experimental error, for these two overtones.) This is a fascinating result which has implications for vibrational energy redistribution in these molecules. A similar effect has been observed in benzene by Berry and co-workers18where, with the exception of an anamoly at Au = 8, the overtone bandwidth decreases from Au = 5 to Au = 9. These authors have attributed the bandwidths to homogeneous broadening caused by vibrational energy redistribution. The first attempts to explain vibrational redistribution from overtone states were made by Heller and Mukamellg and by Sage and Jortner.*O Heller and Mukamel have identified the predominant coupling route in benzene as involving off-resonance coupling of the initially prepared pure local-mode overtone state to higher energy local-mode combination states of the form ~U-~,~,O,O,O,O).~~ Sage and Jortner, on the other hand, have ascribed the dominant coupling route to a direct resonance interaction of the initially prepared state to the full normal-mode bath.20 As pointed out in the Introduction, neither of these theories can explain the much narrower bandwidths observed at Au = 5 for the four tetramethyl molecules as compared to the value of 109 cm-' observed for benzene.18 More recently Stannard and GelbartZ2and Sibert et alaz3 have identified the predominant coupling route in benzene as the coupling of the initially prepared local-mode state to a normal-mode combination state with u - 1 quanta in a local CH oscillator and the remainder in lower frequency normal modes. It is important to realize, however, as has been noted by Stannard and Gelbart,22that the directly coupled state must involve only a small number of strongly interacting normal modes and not the full normal-mode bath. Otherwise the bandwidths should increase rapidly (22)P. R. Stannard and W. M. Gelbart, J. Phys. Chem., 85, 3592 (1981). (23)E.L. Sibert, W. P. Reinhardt, and J. T. Hyne,Chem. Phys. Lett., 92,455 (1982).
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The Journal of Physlcal Chemistry, Vol. 87, No. 24, 1983
with increasing u, in parallel with the increase in the total vibrational-state density. Sibert et al. specifically identify kinetic energy coupling in benzene of the u quanta local-mode overtone state to states with u - 1 local-mode quanta and two quanta in a CCH in-plane wag.23 Bandwidth narrowing from u = 5 to u = 9 occurs because the density of these “doorway” states in the energy region of interaction decreases from Au = 5 to Au = 9. The bandwidth narrowing observed for (CH3)4M(M = Si, Ge, and Sn) supports a similar explanation. Here the likely modes involved in the interaction are HCH bending modes. These modes are slightly higher in frequencyz4 than the corresponding CCH= modes in benzene, and this fact explains the earlier onset of bandwidth narrowing (Au = 4 for the tetramethyl molecules as compared to Av = 5 for benzene). In fact, the wider vapor-phase bandwidth of neopentane at Au = 5 in comparison to the other three tetramethyl molecules can also be rationalized on the basis of this explanation. As we have noted, normal-mode combination activity in the lower overtones of neopentane is more pronounced than for the other molecules. Therefore, coupling of the 15,0,0) type state to states of the type 14,0,0; nm), where nm refers to two quanta of HCH bend, would be expected to be stronger in neopentane. Therefore, the narrower bandwidth in the four tetramethyl molecules as compared to benzene is not dependent on either the relative couplings between the local CHstretching oscillators or the relative total densities of states. Rather it depends on the relative magnitudes of the kinetic coupling between the CH stretch and the CCH wag in benzene, and between the CH stretch and the HCH bend in the tetramethyl molecules. It could also reflect the relative densities of the two types of doorway states in the coupling region. Finally, we note that, despite the marked similarity in the spectra and in the vapor-phase bandwidths of the tetramethylsilicon, -germanium, and -tin molecules, there are small differences. These differences probably arise (24) C. W. Young, J. S. Koehler, and D. S. McKinney, J . Am. Chem.
Soc., 69, 1410 (1947).
(25) S. Brodersen and A. Langseth, K.Dan. Vidensk. Selsk., Mat.-Fys. Skr., 1, 1 (1959).
Henry et al.
primarily from differences in molecular structure and the resultant effects on the methyl CH vibrational properties. The bond length between the central atom and the carbon atom of the methyl group increases in the following fashion:26 r = 1.54 A, rStC = 1.93 A, r G d = 1.98 A, and rSnX = 2.18 The longer is the M-C bond length, the smaller are the interactions between the methyl groups and the lower are the barrier heights to methyl rotation.27 The marked increase of the M-C bond length from neopentane to the rest of the molecules could indicate that the difference in spectral properties between neopentane and the other three molecules arises because of a small, but significant, effect of methyl-methyl interactions on the CH vibrational properties in neopentane. Clearly it would be of great interest to observe the vapor-phase overtone bandwidths of all of these molecules for higher overtones. We are currently building a photoacoustically detected spectrometer to attempt to obtain these results.
k.
Conclusion The overtone spectra of the three molecules (CH3)4M (M = Si, Ge, and Sn) consist primarily of single peaks for Au 13. The vapor-phase bandwidths at Au = 5 are much narrower than the corresponding bandwidths in benzene. This result and the observed decrease in vapor-phase bandwidth from Au = 4 to Au = 5 are consistent with a model involving primary coupling of the local-mode overtone state to states with one fewer local-mode CHstretching quantum and two quanta of HCH bending.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We are also grateful for the financial support of the National Sciences and Engineering Research Council of Canada. B.R.H. is grateful to the members of the Institute of Molecular Science for their kind hospitality during a recent visit. Registry No. (CH3)4Si,75-76-3; (CH3),Sn,594-27-4;(CH3)4Ge, 865-52-1. (26) L. 0. Brockway and H. 0. Jenkins, J. Am. Chem. SOC.,58,2036 (1936). (27) J. R. Durig, S. M. Craven, and J. Bragin, J. Chem. Phys., 52,2046 (1970).
