J . Phys. Chem. 1987, 91, 3741-3747
3741
Uncoupled Methyl CH Oscillators: A Local-Mode Analysis of the Overtone Spectra of 2-Chloro-2-methylpropane and Chlorotrimethylstlane M. Khalique Ahmed and Bryan R. Henry* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received: January 20, 1987)
The CH-stretching overtone spectra of 2-chloro-2-methylpropane ((CH,),CCl) and chlorotrimethylsilane ((CH,),SiCI) are measured at room temperature in both the liquid phase (AUCH= 1-6) and the gas phase (AuCH = 1-4). Ab initio geometry optimization is carried out at the SCF level with 4-31G and STO-3G basis sets for (CH3),CCI and with a STO-3Gbasis set for (CH,),SiCI. In agreement with earlier studies of the fundamental gas-phase spectra of partially deuteriated (CH,),CCI, the calculationspredict two types of methyl CH bonds. Two well-resolved peaks dominate the overtone spectra of both (CH,),CCl and (CH3),SiCI, and we assign these peaks to the two nonequivalent methyl CH bonds. We analyze the spectra by assuming that the unique methyl CH bond (trans to the C-CI (Si-C1) bond) is uncoupled from the two identical methyl CH bonds. The vibrations of these latter two bonds are described in terms of a harmonically coupled local-mode model. From the local-mode analysis and the overtone frequencies, we predict that the CH bond trans to the C-C1 (Si-CI) bond is longer than CH bonds trans to another methyl group. This bond length difference from the overtone frequenciesis compared to bond length differences obtained from the ab initio calculations and from an earlier fundamental spectroscopic study.
Introduction Local-mode analyses of high-energy X H stretching overtone spectra have been used successfully to obtain information about structure, conformation, and relative X H bond lengths of polyatomic molecules.’-20 In these spectra the higher overtones are dominated by peaks which are equal in number to the number of nonequivalent XH bonds. The energy position of the local-mode peak a t each overtone level, corresponding to a particular type of X H bond, can be fitted to the energy equation of a Morse oscillator; i.e. AE =
VU(1
- X ) - V’WX
(1)
Here, w and wx are the harmonic frequency and the diagonal local-mode anharmonicity constant, respectively. Information about structurally nonequivalent X H bonds can also be obtained from the fundamental spectrum by the selective deuteriation method of McKean and his c o l l a b ~ r a t o r s . ~ l In -~~ Henry, B. R. Vib. Spectra Struct. 1981, 10, 269. Hayward, R. J.; Henry, B. R. Chem. Phys. 1976, 12, 387. Greenlay, W. R. A.; Henry, B. R. J. Chem. Phys. 1978, 69, 82. Henry, B. R.; Greenlay, W. R. A. J . Chem. Phys. 1980, 72, 5516. (5) Henry, B. R.; Thomson, J. A. Chem. Phys. Lett. 1980, 69, 275. (6) Henry, B. R.; Mohammadi, M. A. Chem. Phys. 1981, 55, 385. (7) Henry, B. R.; Hung, I. F.; McPhail, R. A.; Strauss, H.L. J. Am. Chem. (1) (2) (3) (4)
Soc. 1980. 102. 515.
(10) Fang, H. L.; Meister, D. M.; Swofford, R. L. J . Phys. Chem. 1984, 88, 410. (1 1) Fang, H. L.; Swofford, R. L.; Compton, D. A. C. Chem. Phys. Lett. 1984, 108, 539. (12) Fang. H. L.;Swofford. R. L.: McDevitt, M.;Anderson,A. B. J. Phys. Chem.’ 1985; 89, 225. (13) Henry, B. R.; Gough, K. M. Laser Chem. 1983, 2, 309. (14) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1983, 87, 3433. (15) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1983, 87, 3804. (16) Gough, K. M.; Henry, B. R. J . A m . Chem. Soc. 1984, 106, 2781. (17) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1984,88, 1298. (18) Henry, B. R.; Gough, K. M.; Sowa, M. G. Int. Reu. Phys. Chem. 1986, 5, 133. (19) Wong, J. S.; Moore, C. B. J. Chem. Phys. 1982, 77, 603. (20) Mizugai, Y.;Katayama, M. Chem. Phys. Lett. 1980, 73, 240. (21) McKean, D. C.; Duncan, J. L.; Batt, L. Spectrochim. Acta, Purr A 1973, 29A, 1037. (22) McKean, D. C. Chem. SOC.Rev. 1978, 7, 399. (23) McKean, D. C.; Biedermann, S.; Burger, H. Spectrochim. Acta, Purr A 1974, 30A, 845.
0022-3654/87/2091-3741$01.50/0
this method, all of the hydrogens of a polyatomic molecule but one are replaced by deuterium. The lone XH stretch is decoupled from the XH stretching vibrations as well as from the first overtone of bending modes. In a sense, one can think of the XH oscillators in such selectively deuteriated molecules as chemically produced local modes. Through measurements of the fundamental gas-phase C H stretching spectrum of the deuteriated molecule (CD3)2(CD,H)CCl, McKean et al. have shown that there are two kinds of structurally nonequivalent C H bonds in (CH3),CC1.23 In the present work, we are investigating the overtone spectra of (CH3),CC1 and (CH,),SiCI. The expected structure2, of these two molecules is shown in Figure 1, where the nonequivalent methyl CH bonds are labeled by an “a” and an “s” as was done by McKean et al.23 These authors predicted the CH, bonds of (CH3),CC1 to be 0.003 A longer than the CH, bonds through their correlation of fundamental frequencies with bond lengths in the partially deuteriated molecule. In this paper, the C H stretching fundamental and overtone spectra of (CH3),CCI and (CH,),SiCl in the liquid (AvCH = 1-6) and the gas phase (AvCH = 1-4) will be reported. We will analyze these spectra in terms of the local-mode model with the assumption of independent methyl groups with local C, symmetry. In the analysis, we will investigate the degree of coupling between the stretching motions of the unique CH, bond and the CH, bond within a methyl group. We will also use the excellent correlation between overtone frequencies and C H bond length^'^,'^-^^ to investigate the bond length difference between CH, and CH, bonds. We will compare our overtone results to results obtained from ab initio gradient calculations at the 4-31G and STO-3G levels, as well as to the partial deuteriation results of McKean et at.23
Experimental Section The two compounds, (CH3),CCI (98%) and (CH3),SiCI (99.5%), were obtained from Aldrich Chemical Co. and Fisher Scientific Co., respectively, and were used without further purification. The spectra in the region of the CH-stretching fundamentals were measured with a Nicolet MX-1 FTIR spectrometer. The liquid-phase spectra in this region were measured as 5% solutions in carbon tetrachloride with a 0.1 mm path length cell. The gas-phase spectra in this region were measured with a 10 cm path length cell fitted with KBr windows, at a gas pressure of 10 Torr. The CH-stretching overtone spectra were recorded on a Beckman 5270 spectrophotometer with the near-IR light source in the regions of AvCH= 2, 3 and 4, and the visible light source 0 1987 American Chemical Society
3142
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
Ahmed and Henry
P W
2
5
W
(L
0
v
(c")
Figure 2. Liquid-phase (5% solutions in CCI,,) fundamental spectra of = I. (CH,),CCI (---) and (CH3)3SiC1(-) in the region of A U ~ H Spectra were measured at room temperature with a path length of 0.1 mm.
M = C , Si Figure 1. Structure of (CH3),MC1(M = C, Si) molecules based on the study of McKean et aI.*, The hydrogen atoms labeled by an "a" lie on opposite sides of a u" plane, while those labeled by "s" lie on this plane.
for the regions of AuCH = 5 and 6. The overtone spectra of the pure liquids in the regions corresponding to AvCH= 2 and 3 were measured with cells of path lengths 0.1 and 1 cm, respectively. The overtone spectra of the pure liquids in the regions corresponding to AacH = 4-6 were measured with 10 cm path length cells. The gas-phase spectra in the regions corresponding to AvCH = 2-4 were measured with a Wilks variable path length gas cell (Wilks Scientific Corp., South Norwalk, CT, Model 5720) with KBr windows. The cell path length and vapor pressures were varied to obtain optimally measurable signals for the various overtones. All of the spectra, in a digital format, were transferred to a Nicolet 1280 computer and converted to a linear energy scale. The spectra were plotted in wavenumber units with a Nicolet Zeta 160 plotter. All of the overtone spectra were decomposed with a Fortran 77 curve analysis program, NIRCAP,'~ which fitted Lorentzian peaks to the experimental bands. The experimental and fitted spectra were plotted and compared to check the quality of the deconvolution fit. Calculations to determine geometry optimized C H bond lengths were performed at the S C F level with M O N S T E R G A U S S , ~ ~a modified version of the GAUSSIANSO program,26 on an Amdahl V8 system. Calculations for (CH3)3CC1used the 4-31G and STO-3G basis sets, whereas the calculations for (CH3)3SiC1used only the STO-3G basis set. In the geometry optimization procedure, all of the bond lengths and bond angles were allowed to vary except the dihedral angles between the C-CI (Si-CI) and CH, bonds (see Figure 1). Results and Discussion The liquid-phase fundamental and overtone spectra of (CH3)3CC1 and (CH3)3SiC1 in the regions Of "CH = are shown in Figures 2-7. The gas-phase spectra of these molecules (24) The NIRCAP program was written by R. K. Marat and modified by
A. W. Tarr. ( 2 5 ) Peterson,
M. R.; Poirier, R. A. MONSTERGAUSS; Department of Chemistry, University of Toronto, Toronto. ( 2 6 ) Krishnan, R,;Seeger, R.;DeFrees, D,J.; *hlegel, H.B.;Topiol, s,; Kahn, L. R.; Pople, J. A. GAUSSIAN 80; Department of Chemistry, Carnegie-Mellon University, Pittsburg, PA, 1980.
1/ (cm-') Figure 3. Liquid-phase overtone spectra of (CH3),CCI and (CH3),SiCI in the region of AuCH = 2. Spectra were measured at room temperature with a path length of 0.1 cm. The absorbance of (CH3)3SiCIhas been offset by 0.3 absorbance units.
u (cm-l) FiWe 4. Liquid-Phase overtone spectra of ( C H M C I and (CH,),SiCI in the region of AUCH= 3. Spectra were measured at room temperature with a path length of 1.0 cm. The absorbance of (CH3),SiCI has been offst by 0.3 absorbance units,
in the regions of AUCH = 1-4 are shown in Figures 8-1 1* The observed peak positions are listed in Tables I, 11, and 111. The values for the fundamentals correspond to experimental band maxima, whereas the overtone values correspond to the individual Lorentzian components which were obtained from curve decomposition.
Uncoupled Methyl
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3743
CH Oscillators
0.72 W
V
2
a 048
0 v)
m a
0 24
0 00
V (cm-1) Figure 5. Liquid-phase overtone spectra of (CH3)&C1 and (CH3),SiC1 in the region of AUCH= 4. Spectra were measured at room temperature with a path length of 10.0 cm. The absorbance of (CH3)3SiCIhas been offst by 0.18 absorbance units.
0 06 W 0
F (cm-') Figure 8. Gas-phase spectra of (CH,),CCI and (CH3),SiCI at room temperature in the region of AuCH = 1 (10 Torr pressure, 10 cm path length).
1
z
5
am
004
a
0 02 Oo0
5450
'
5j50
'
5550
5350
L (cm-')
000
14000
14400
13600
13200
F (cm-9 Figure 6. Liquid-phase overtone spectra of (CH3),CC1 and (CH3)pSiC1 in the region of AuCH = 5. Spectra were measured at room temperature with a path length of 10.0 cm. The absorbance of (CH3),SiCI has been offset by 0.022 absorbance units.
Figure 9. Gas-phase overtone spectra of (CH,),CCI (-) and (CH,),SiCI (---) at room temperature in the region of A u c H = 2 (40 Torr pressure, 10.80 m path length). The right-hand ordinate scale represents the absorbance of (CH3),SiC1.
L
t 0 IO
i-i (cm-1)
Ooo0
w 16200
15800
15400
B (cm-l) Figure 7. Liquid-phase overtone spectra of (CH3)3CCIand (CH3),SiCI in the region of AuCH = 6. The spectra were measured at room temperature with a path length of 10.0 cm. The spectrum of (CH3)3CCI is an average of four base line corrected scans. The absorbance of (CH3),SiCI has been offset by 0.022 absorbance units.
Spectral Analysis. T h e overtone spectra a r e dominated by purely CH-stretching peaks involving t h e methyl CH oscillators. These spectra could be assigned through t h e application of t h e
Figure 10. Gas-phase overtone spectra of (CH3)JCI (60 Torr pressure, 12.15 m path length) and (CH&SiCI (60 Torr pressure, 10.89 m path length) at room temperature in the region of AuCH = 3. The right-hand ordinate scale represents the absorbance of (CH,),SiCI. local-mode Hamiltonian of three coupled Morse o s ~ i l l a t o r s ~ ~ - ~ ~ to symmetrized states with components (u,O,O), JO,u,O), ltj-l,l,O), etc., a n d diagonalization of t h e resultant matrices. In such analyses, t h e three oscillators could be approximated as equiva-
(27) Henry, B. R.; Tarr, A. W.; Mortensen, 0. S.; Murphy, W. F.; Compton, D. A. C. J . Chem. Phys. 1983, 79, 2583. (28) Sage, M. L. J . Chem. Phys. 1984, 80, 2872. (29) Hanazaki, I.; Baba, M.; Nagashima, U. J . Pfiys. Cfiem. 1985, 89, 5637. (30)Findsen, L. A.; Fang, H. L.; Swofford, R. L.; Birge, R. R. J . Cfiem. Phys. 1986, 84, 16.
3744
Ahmed and Henry
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 I
I
I
I
I
I
TABLE 11: Observed and Calculated CH Stretching Peak Positions (cm-') for Chlorotrimethylsilane
0.018
liquid calcd 2930° 2930 2930" 293 1 2964 2964 5736 5735 5764 5774 5772 5774 5903 5927 8416 8420 8470(+) 8471 847 1 (-) 8709 8691 8754 8755 10972 10966 1 IO49 1 1049 1 l402(+) 1 I400 11424(-) 11567 13403 13378 13504 13524 13992(+) I4004 13995(-) 14207(+) 14165 14302(-) 15709 15712 15836 15842
AL~CW obsd 1 W 0
5
0.012
2
m
E
0
v,
m
a
3
0.006
4 0.000 I1300
10900
I1100
F
(cm-')
Figure 11. Gas-phase overtone spectra of (CH3)3CC1(100 Torr pressure, 10.53 m path length) and (CH3),SiCI (70 Torr pressure, 10.80 m path length) at r w m temperature in the region of AuCH = 4. The absorbance of (CH,),SiCI has been offset by 0.003 absorbance units. TABLE I: Observed and Calculated CH Stretching Peak Positions (cm-') for 2-Chloro-2-methvlpropane Auru 1
2O
3
4
5
6
liquid calcd 2928 2943 2972 5733 5785 5792 5921 8414 8500 8723 8780 I0976 11088 1 1444(+) 11462(-) 1 I601 13409 13413 I3556 13553 14033 14042(+) 14044(-) 15744 15728 15923 15894 obsd 2927 2946 2975 5751 5799 5799 5937 8406 8482 8737 8789 10965 I1080 11457
gas obsd 2937 2957 2981 5763 5817 5817 5953 8428 8510 8758 8806 10992 11129
calcd 2940 2957 2981 5752 5807 5811 5942 8435 8525 8758 8806 10992 11114 1 1485(+) 1 1497(-) 11635 13421 I3577 1408 1(+) 14082(-) 15722 15913
6
2970 5755 5790 5790 5934 8460 8 504 8740 8780 11022 11 102
calcd 2940 2939 2970 5757 5782 5790 5917 8453 8502(+) 8503(-) 8717 8778 11027 1 IO99 I1442(+) I1463(-) 1 I600 13478.5 13576 14049(+) I4052(-) 14258(+) 14340(-) 15808 15933
assignment
"Approximate position since this peak is quite broad (see Figure 2). assignment
'Some peaks in the gas-phase spectrum at AUCH= 2 show unresolved rotational fine structure. For these peaks, frequencies of the band centers are quoted.
lent2' or the nonequivalence of the a and s oscillators could be included from the outset.29 However, in the present case, we simplify the analysis by assuming that the stretching vibrations of the CH, oscillators can be decoupled from the stretching vibrations of the CH, oscillators. Clearly such a zeroth-order approximation cannot be rigorously correct. However, it will be used as a basis for the analysis of the spectra, and its justification will rest in the success of that analysis. Moreover, the analysis will enable us to examine the extent of coupling between the two types of methyl oscillators and to determine the dependence of that coupling on factors such as overtone energy. Under this assumption, the symmetrized vibrational states associated with the CH, bonds of (CH3),CC1 and (CH,),SiCI are31 Iu,O)+,, lu-1 , I )*,, etc. The CH-stretching vibrational states associated with the unique CH, bond can be represented by single ( 3 1 ) Mortensen, 0. S . ; Henry, B. R.; Mohammadi, M. A. J . Cfiem. Pfiys. 1981, 75, 4800.
5
gas obsd
TABLE 111: Positions (cm-') and Tentative Assignments for the Combination Peaks Observed in the Overtone Spectra of (CH3)$CI and (CH3)3SiCI (CH3)3CC1 (CH,)SiCI AuCH liq gas liq gas assignments'sb 2
4
;:;:I{
5587 5650 5688
5600 5664 5704
5904 5957 8141 18286 8584 8708
5916 5970 8167 8305 8604 8726
+
5405 5453 5607 5882
5419 5465 5624 5896
{8233
8254
8650
8660
11158
+
~l,O,l)c
ll,O)-a + 2986 (2993) 12)9(1230)*A+ 2B(+)
13,0),, {I0880
5 6
2865 2B(-) l l ) s + 2B(*) ll,O)-a + 2B(*) II), 2B(-)
13)s(i330)*a)+ 2W+) 14,0),,
11181
13456 15967
+ -I00
l l , l , l ) , ll,0,2). 12,0,l)'
+
-
100
+ 2B(-) 15), + 2f3-1
l4),
OB(+) and B(-) are symmetric and antisymmetric bends of CH3, B(-) = 1368 cm-', B(+) = 1238 cm-' for (CH,),CCI; and B(-) = 1333 cm-', B(+) = 1254 cm-' for (CH3),SiC1 (FTIR liquid-phase values from the current study). bWhen B appears with both + and signs, the + and - signify (CH,),SiCI and (CH,),CCI, respectively. rThese are vibrational states of the type I C , ~ , L ' ~ ~ . U ~ , ) (see text).
Morse oscillator states, 1 ~ ) ~ The . energies of the CH, states can be obtained simply from the vibrational energy expression given in eq I . The energies of t h e CH, states can b e obtained through an application of the harmonically coupled local-mode model for an XH2 system.31 The Hamiltonian is written in t h e form H = E,
+
+
( ~ 1 U ~ ) W- (uI' + uZ* + L I ~+ L I ~ ) W X+ da1+ - N a 2 + - a 2 b + 4(a1+ + all(%+ + a2)w (2)
In eq 2, u I and u2 are the quantum numbers of the two C H oscillators, Eo is the wavenumber energy of the ground state, and y and 4 characterize the off-diagonal kinetic and potential energy coupling, respectively. These parameters are defined in terms of Wilson G and F matrix elements (y = -('/2)(G12/Gl,), $J = (1/2)(F12/Fll)).The operators al+,a l , etc. have step-up, step-down properties and cause coupling between the two equivalent C H bonds.
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3745
Uncoupled Methyl C H Oscillators TABLE I V Local-Mode Parameters (cm-I) for (CH3),CCI and (CHa)SiCI
oscillator type CH, CH, CH, CH, CH, CH, CH, CH,
molecule phase (CHp),CCI liquid gas (CH3),SiC1 liquid' gas
wx
wy'
61.6 f 1.2 61.3 f 0.9 63.3 f 2.2 63.9 f 2.2 61.5 f 0.3 62.4 f 0.5 59.7 f 0.3 61.0 f 2.0
14.5
W
3081 3050 3096 3068 3071 3055 3074 3062
f 4.7 f 3.8
f 6.3 f 6.4 f 1.4 f 2.0 f 1.0 f 6.6
12.0 16.4
15.5
"The nucH= 5 local-mode peaks of (CH3)3SiCIwere not fit to the Morse oscillator equation in obtaining w and w x because a combination peak at 13 456 cm-I perturbs the energy positions of these peaks (see Figure 6). I
I
I
u
26000
2
4
6
v Figure 12. Plots of the vibrational energy equation of a single Morse oscillator for the nonequivalent CH bonds of liquid-phase (CH3)3CCI.
The details of this model have been described in a previous p~blication.~'The model accounts for all of the anharmonicity that is diagonal in a single C H oscillator. However, coupling between the simple product basis states is restricted to a given vibrational manifold and to the harmonic limit, Le., states Iul,u2) couple only with states ~ v l f l , u 2 f). l A calculation of the energies for transitions to the states I U ) ~ , I U , ~ ) ~ , , Iu-l,l)*,, etc., requires (i) the anharmonicity constant wx and the harmonic frequency w for both the CH, and CH, bonds; (ii) the effective coupling parameter y'w(y' = y - +), which appears in-the intramanifold coupling matrices3' of eq 2 for CH, bonds only. The parameters w and wx for CH, and CH, bonds are evaluated by fitting the observed energies of the I L ) , O ) ~ ,and Iu), peaks, respectively, to eq 1 with a least mean squares analysis. These values are given in Table IV. The quality of the fit, and the validity of the analysis, is illustrated in Figure 12 which displays a plot of eq 1 for the liquid-phase spectra of (CH3)3CCI. In Table IV, we have listed values of the coupling parameter, y'w, for both (CH3),CCI and (CH3)3SiCIin liquid and gas phases. In our previous work,3i the value of this parameter has been obtained from the observed splitting between the 11,0)+ and 11,O)peaks. The values for y'w for (CH3)3CCIwere obtained on this basis (2y'w = E(Il,O)-,) - E(Il,O)+,)). For (CH3)3SiCI, the Il,O)+, (and also the 11 ),) peak is not observed in the gas-phase spectrum, probably due to low intrinsic intensity. The intensity of CH stretching vibrations changes markedly on passing from the gas phase to solutions in CCl,; e.g., the intensity of the C H stretching vibration of chloroform has been observed to be 20 times higher in CCI4 solution than in the gas phase.32 In the fundamental spectrum of a 5% solution of (CH3),SiC1 in CCI4, a weak broad peak is observed at about 2930 cm-' (see Figure 2). The energy of this peak is approximately the same as is expected for the Il,O)+, and 11 ), peaks on the basis of the Birge-Sponer plots (32) Robins, C. C. Proc. R. Soc. A 1962, 269, 492.
(eq 1). However, due to the uncertainty involved in locating the position of the ll,O)+apeak, we have not used the position of this peak in combination with the position of the Il,O)-, peak to calculate the coupling parameter, y'w, for the CH, bonds of (CH3),SiCI. The values of this parameter for both liquid and gas phases of (CH3)3SiClthat are listed in Table IV were evaluated by fitting the observed energy of the 11,O)% peak to the expression for the state energy on the basis of our local-mode model for an XH2 system3I E(Il,O)-,) = (1 - 2x y')w). Substitution of the local-mode parameters of Table IV into the intramanifold coupling matrices3' of the Hamiltonian (eq 2), followed by diagonalization of these matrices, gives the calculated energies of the peaks corresponding to the CH, bonds. As we have noted, the energies of the peaks associated with the CH, bonds can be obtained straightforwardly from eq 1. The calculated and observed energies of the purely C H stretching peaks associated with either CH, or CH, bonds are given in Tables I and I1 for (CH3),CCI and (CH3)3SiC1,respectively, along with the peak assignments. The agreement between calculated and oberved peak energies is very good. Evidence for the consistency of our analysis can also be obtained by a comparison to the fundamental frequencies observed by McKean et aLz3 From the harmonic frequencies of the nonequivalent CH bonds (Table IV), the fundamental frequencies, v, can be obtained (Y = w - 2wx). The gas-phase values of u for (CH3)3CC1and (CH3)3SiCIare 2969 and 2940 cm-I, and 2955 and 2940 cm-I, respectively. The values of v for (CH3)$C1 are in excellent agreement with the fundamental frequencies of (CD3)2(CD2H)CC1(2968.2 and 2936.0 ~ m - ' ) . ~ ~ The only overtone peaks left for assignment are those peaks we have listed in Table 111 and labeled as combinations. The combination peaks in the regions of AuCH = 2-6 are relatively low in intensity, particularly at the higher overtones. These peaks are mainly of two types. One type involves u - 1 quanta in a purely C H stretching state (lo),, ~ u , O ) ~ and , ) two quanta in a symmetric or antisymmetric methyl group bending mode. Such peaks are commonly observed in the overtone spectra of several molec u l e ~ . ~ , ~The - ~ second ~ , ~ ~type - ~ of~ combination peak arises when the C H stretching vibrational quanta are distributed over at least two of the three methyl C H bonds of (CH3),CCI and (CH3),SiC1. Peaks where the quanta reside solely in the a oscillators are listed in Tables I and 11. However, Table 111 also identifies peaks ~ U ~ , , U ~ , , where U ~ ~ ) via + v2, + v3, = AvCH, and both vjs and one or both of u,, and u2, are not equal to zero. Finally, peaks are observed at AUCH = 3 and 4 of both liquid and gas phase (CH3),CC1 that correspond to Iv,O)*, plus a low-frequency mode of about 100 cm-I. Similar peaks have been observed in the overtone spectra of neopentane, (CH3)4C.4,39Energy positions and tentative assignments of all combination states except Iula,u2,,0) are given in Table 111. CH,, CH, Coupling. Our assumption that the stretching motion of the unique CH, bond of each methyl group is essentially decoupled from the stretching motion(s) of the other two CH, bonds is validated by the very good agreement of the calculated and observed peaks listed in Tables I and 11. However, these bonds are weakly coupled at low overtones as evidenced by the appearance of combination peaks involving both CH, and CH, oscillators in the regions of AuCH = 2 and Av,, = 3 in both the liquid- and gas-phase spectra (see Figures 3 , 4, 9, and 10, and Tables I and 11). If the anharmonicities of the CH, and CH, oscillators are equal (this is, in fact, very nearly the case, see Table IV), then the energy
+
(33) Fang, H. L.; Swofford, R. L. J . Chem. Phys. 1980, 72, 6382. (34) Fang, H. L.; Swofford, R. L. J . Chem. Phys. 1980, 73, 2607. (35) Henry, B. R.; Mohammadi, M. A,; Hanazaki, I.; Nakagaki, R. J . Phys. Chem. 1983, 87, 4827. (36) Voth, G.A.; Marcus, R. A.; Zewail, A. H. J . Chem. Phys. 1984, 81, 5494. (37) Peyerimhoff, S.; Lewerenz, M.; Quack, M. Chem. Phys. Left. 1984, 109, 563. (38) Perry, J. W.; Moll, D. J.; Kuppermann, A,; Zewail, A. H. J . Chem. Phys. 1985.82, 1195. (39) Henry, B. R.; Mohammadi, M. A . Chem. Phys. Lett. 1980, 75, 99.
Ahmed and Henry
3146 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 TABLE V: Bond Length Differences Ar = r(CH,) - r(CH,) (A) between the Nonequivalent Bonds of (CH,)&CI and (CH,)$iCI from Different Techniques method
(CH,),CCI
McKean et a]." local mode
0.003 0.003 0.005 0.0012
4-31G STO-3G
(CH,),SiCI 0.0017 0.00 1
From ref 23. separation between the CH, and CH, peaks will increase linearly with o; i.e. AE(CH,-CH,) = UAW (3) where AW is the difference between the harmonic frequencies of the nonequivalent bonds. Such an increase is observed and, therefore, we would expect the effective coupling between the CH, and CH, bonds to decrease as v increases. The absence of combination peaks involving both CH, and CH, for AvCH > 3 is in accord with this expectation. It should be noted that had we analyzed these spectra in terms of three equivalent C H methyl bonds, two principal components would also be expected, Le., the A , and E states involving symmetrized combinations of Iv,O,O),IO,v,O) and IO,O,v).27 However, these components would be expected to move closer in energy as u increased and to become effectively degenerate for AvCH> 3. Thus, in this case, the spectra of the higher overtones would be dominated by single peaks as in n e ~ p e n t a n e , ~contrary .,~ to what we have observed. CH Bond Lengths. As we have noted in the Introduction, McKean et have analyzed the fundamental gas-phase spectrum of (CD3)2(CD2H)CCIand, on the basis of the observed frequencies, have predicted a bond length difference of 0.003 A between the CH, bonds and the longer CH, bonds. As we have also noted in the Introduction, an accurate correlation exists as well between overtone frequencies and C H bond lengths in undeuteriated m o l e c ~ l e s . ' ~ItJ ~has ~ ~been shown that at AvCH= 6 a frequency shift of 69 cm-' corresponds to a bond length difference of 0.001 A.15J6 The gas-phase spectra of (CH3),CC1 and (CH3),SiCI in the region of AvCH = 4 consist of two well-resolved peaks separated by 138 and 80 cm-I, respectively. (See Figure 11 and Tables I and 11.) In our analysis, these peaks are associated with the CH, and CH, oscillators. From the bond length overtone frequency correlation,I6 these separations correspond to bond length differences of 0.003 and 0.002 A, respectively. In Table V, these bond length differences are compared to McKean's results23for (CH,),CCI and to the results obtained from our ab initio geometry optimization calculations. Note that the relative bond lengths for the nonequivalent C H bonds of (CH3),CCI from our overtone spectral study are in perfect agreement with the results of McKean et al.23 However, overtone spectral studies are more convenient for such structural investigations in the sense that the difficult synthetic procedures involved with the selective deuteriation process are not required. The ab initio calculations correctly predict that the CH, bonds are longer, but there is a significant difference between the 4-31G and STO-3G results for (CH,),CCI. Moreover, the agreement between the overtone results and the ab initio gradient calculations, while reasonable, it not as good as has been observed for other m o l e c ~ l e s . ' ~ J ~ We have based our assignment of the CH, and CH, peaks on the analysis given in the preceding paragraphs. However, we note that this assignment is also in accord with the expected intensity ratio of 2:1 for the two types of C H bonds in both molecules. When the gas-phase spectra (Figure 11) of (CH3)3CCI and (CH3),SiCI at vCH= 4 are decomposed into component Lorenzian peaks, the areas of the high- and low-frequency peaks are found to be in the ratio of 2 : l . As an example, the decomposition of the AvCH = 4 gas-phase spectrum of (CH3),SiC1 is illustrated in Figure 13. In some cases, it has been observed that relative peak intensities do not correspond to the number of XH oscillators of a given type. For example, in the overtone spectra of acetone and
V (cm-'1 Figure 13. Upper traces: the calculated (broken curve) and experimentally observed (solid curve) gas-phase overtone spectrum of (CH3)3SiCIin the region of AUC- = 4. The experimental spectrum was measured at room temperature (70 Torr pressure, 10.80 m path length). Lower trace: individual Lorentzian functions fit to the experimental spectrum.
acetaldehyde, the more numerous out-of-plane methyl C H bonds give rise to less intense peaks.29*30However, in general, the areas of spectral peaks at high overtones are proportional to the number of CH bonds of a particular type.'3J7J9The spectra of (CH,),CCI and (CH,),SiCl are in accord with this general observation. Trans and Inductive Effects on CH Bond Strengths. In their study of the fundamental C H stretching spectra of (CD3)2(CHD2)CX (X = F, C1, Br, I) molecule^,^^^^^ McKean et al. observed that the isolated frequency of the CH, bond trans to X decreases as X is replaced by a less electronegative halogen. However, the isolated frequency of the CH, bond remains almost invariant as X is changed from F to I. These observations led McKean et al. to suggest that an effect analogous to the "trans lone pair" effect40 is present in (CH3),CX molecules. Simplistically, the trans lone pair effect is the lengthening of a C H bond trans to a lone pair of oxygen, nitrogen, sulfur, According to Hamlow et al.,4' the lengthening of a C H bond trans to lone pair electrons occurs via interaction of the lone pair electron density with the antibonding orbital of the C H bond. The elegant infrared fundamental studies on nitrogen and oxygen containing molecules by McKean and his co-workersZ2and the overtone spectral studies by Fang et aL9-I2on similar compounds support the phenomenon of a trans lone pair effect. Williams et a]!* have also explained the influence of lone pair orbital interaction on molecular structure through ab initio studies of molecules containing -OH, -OCH3, -NH, and -NCH3 groups. For the tertbutyl halides, the explanation is that the lower the electronegativity of the halogen, the greater the electron density on carbon, the greater the trans effect, and the longer the trans CH, bond.23 The harmonic frequencies of the CH, and CH, bonds of (CH,),CCl and (CH3),SiC1 are given in Table IV. The data of Table IV show that within experimental uncertainty, the harmonic frequencies of the CH, bonds of (CH,),CCI and (CH,),SiCl are equal. The data of Table IV also show that the harmonic frequency of the CH, bonds of (CH,),CCI is higher than the harmonic frequency of the CH, bonds of (CH3),SiC1 in both liquid and gas phases. Two conclusions can be drawn from these observations. Firstly, the relative constancy of the harmonic frequency of CH, bonds reveals that the trans lone pair effect of the C-CI and Si-C1 bonds on the CH, bonds of (CH3),CC1 and (CH,),SiCI is approximately equal. Secondly, the higher frequency of the CH, bonds of (CH3),CC1 than the frequency of the similar bonds of (CH3),SiC1 is most likely due to an inductive effect of the chlorine atom on the strength of the CH, bonds. Since the Si-Cl and Si-C bond lengths in (CH,),SiCI (2.100 A, 1.859 (40) Bellamy, L. J; Mayo, D . W. J . Phys. Chem. 1976,80,1217, (41) Hamlow, H. P.; Okuda, S.; Nakagawa, N. Tetrahedron Lerl. 1964, 37, 2553. (42) Williams, J. 0.; Scarsdale, J. N.; Schafer, L. J . Mol. Strucr. 1981, 76, 1 1 .
J . Phys. Chem. 1987, 91, 3747-3750
A) are longer than the corresponding bond lengths in (CH3)3CC1 (C-CI = 1.843 A, C-C = 1.547 the bond strengthening effect of chlorine on the CH, bonds would be expected to be more pronounced in (CH3)3CCIand would lead to higher values of the CH, harmonic frequencies for this molecule. Summary
A simple model which assumes that the stretching motion of the unique CH, bond of each methyl group is essentially uncoupled (43) These bond lengths are from geometry optimization of (CH,),SiCI
and (CH,),CCI at the STO-3G level.
3141
from the stretching motions of the other two CH, bonds is capable of accounting for the details in the overtone spectra of (CH3)$C1 and (CH3)3SiCl. Within this description, the CH, bonds are treated as harmonically coupled local modes. The success of this simple model suggests that it would also be useful in analyzing the overtone spectra of other molecules with methyl groups of C, symmetry and in investigating structural and conformational differences between closely related X H bonds.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council for financial support. B.R.H is grateful to the Canada Council for a Killam Research Fellowship.
Experimental and Theoretical Approaches to the Optical Absorption Spectra of Sulfonyl Radicals’ C. Chatgilialoglu,*+D. Griller,$ and M. Guerra Consiglio Nazionale delle Ricerche, Ozzano Emilia, Italy 40064, and National Research Council of Canada, Ottawa, Ontario, Canada, K I A OR6 (Received: February 3, 1987)
The optical absorption spectra of sulfonyl radicals, RSOz, were measured by using modulation spectroscopy and showed strong absorption bands at ca. 340nm. Multiple scattering X, calculations showed that the dominant absorption detected in the UV-visible spectrum was due to an electronic transition from the oxygen atoms to the semioccupied molecular orbital. The calculations successfully predicted the energy of the transition and the effect of phenyl substitution on the spectra of sulfonyl radicals.
Sulfonyl radicals are important intermediates in many reactions of organosulfur compounds.2 Spectroscopic3-’ and theoretical studies3-10 suggest that they have a u-type structure with the unpaired electron localized on the SO2moiety and that they have a pyramidal configuration at sulfur which depends upon the electronegativity of the substituents. The UV-visible spectra of several sulfonyl radicals in solution have been reported.”-I4 However, so far as we are aware, no attempt has been made to rationalize them. All of the theoretical effort3-I0 seems to have focused on ground-state properties and structures. In this work, we have extended studies of the UVvisible spectra of sulfonyl radicals and have applied the multiple scattering X, method (MSX,) to interpret them. As we have already shown,I5 this method has a number of distinct advantages over the more traditional approaches for calculating the excited-state energies of polyatomic free radicals.
Experimental Section Materials. All of the materials used in this work were commercially available. Sulfonyl chlorides and triethylsilane were purified by distillation. Di-tert-butyl peroxide was washed with aqueous silver nitrate then with water so as to remove olefinic impurities. It was then dried over magnesium sulfate and was finally passed through a column of alumina to remove traces of the hydroperoxide. Solvents were spectroscopic grade and were used as received. Apparatus. The modulation spectrometer used in this work follows closely on the design originated by Fischer and Huggenberger16 and has been described in detail e1sewhere.l’ Briefly, solutions containing appropriate substrates were flowed through a standard UV flow cell (Starna 430) where they were photolyzed with a 1000-W mercury-xenon lamp (Hanovia 997B001) to produce the transient of interest. The output of the lamp was modulated with a mechanical chopper which was fitted with irises Consiglio Nazionale delle Ricerche. *National Research Council of Canada.
0022-3654/87/2091-3747$01.50/0
so that the light striking the sample rose and fell sinusoidally in intensity. This caused a corresponding sinusoidal modulation of the transient concentration. A detection system was arranged at right angles to the photolyzing beam and consisted of a 100-W xenon monitoring lamp, a system of lenses and filters, and a monochrometer fitted with a photomultiplier tube detector. At the detector the signal consisted of a large dc component, V, and a very small ac component, A?‘, due to the modulated absorption of the transient. These were separated by using a lock-in amplifier (PAR 124A) which took the operating frequency of the light chopper as a reference signal. The amplitude of the ac signal was normalized to take account (1) Issued as NRCC publication No. 27592.
(2) Freeman, F.; Keindel, M. C. Sulfur Rep. 1985, 4 , 231. (3) McDowell, C. A.; Hezzing, F. G.; Tait, J. C. J. Chem. Phys. 1975, 63, 3275. (4) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. 0. C. J . Chem. SOC., Perkin Trans. 2 1979, 770. ( 5 ) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. 0. C. J . Chem. SOC., Perkin Trans. 2 1980, 1929. ( 6 ) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. 0. C.; Symons. M. C. 185. (M) 2610. R. J. Chem. Res. 1980. (7) Alberti, A.; ChatgiGaloglu; C.; Guerra, M. J . Chem. SOC.,Perkin Trans. 2 1986, 1179. ( 8 ) Boyd, R. J.; Gupta, A,; Langer, R. F.; Lowhie, S. P.; Pincock, J. A. Can. J . Chem. 1980, 58. 331. (9) Hinchliffe, A. J . Mol. Struct. 1981, 71, 349. (10) Chatgilialoglu, C.; Guerra, M., manuscript in preparation. (11) Bjellqvist, B.; Reitbezger, T. Nucl. Sci. Absrr. 1974, 30, 12. (12) Eriksen, T. E.; Lind, L. Radiochem. Radioanal. Lett. 1976, 25, 11. (13) Thoi, H. H.; Ito, 0.;Iino, M.; Matsuda, M. J . Phys. Chem. 1978,82, 314. (14) Nikolaer, A. I.; Safiullin, R. L.; Komissazov, V. D. Khim. Fir. 1984, 3, 257. (15) Chatgilialoglu, - C.; Guerra, M. J . Am. Chem. SOC.,submitted for publication. (16) Huggenberger, C.; Fischer, H. Helu. Chim. Acta 1981, 64, 3 3 8 . (17) Burkey, T. J.: Griller, D. Rev. Chem. Intermed. 1984,5, 21. Girard, M.; Griller, D: J . Phys. Chem. 1986, 90, 6801
Published 1987 by the American Chemical Society
