J . Phys. Chem. 1987, 91, 293-298 Rakov19 has shown that a molecule undergoing hindered librational movement can be expected to have a temperature-dependent bandwidth approximated by Av112 = a + b exp(-E,/RT). If we apply Rakov's expression to the v3 bandwidths listed above (larger cage), they can be used to estimate values for the three parameters: a = 3.5 cm-I, b = 241 cm-', and E, = 0.68 kcal/mol. This E, indicates a greater librational degree of freedom than C2D2 at room temperature in solvents such as CS2and n-pentane, for which Soussen-Jacob et aLZ0used Rakov's expression to deduce barriers to rotation of 1.5 and 1.0 kcal/mol, respectively. The bandwidths attributed to acetylene in the smaller cages are more difficult to measure because of the lower intensities. At 15,90.1, and 127 K, however, the bandwidths are about 5.3, 5, and 11 cm-l. The 11-cm-' bandwidth at 127 K immediately signals that the bamer to libration is larger than that for the larger cages. If we assume that the first two bandwidth values indicate the experimental uncertainty, f0.3 ad,the width at 127 K points to a barrier exceeding 1.5 kcal/mol. In any event, the band shapes become sufficiently narrow at low temperature to guarantee absence of free rotation and, instead, to indicate librational motion around fixed and well-defined orientations in the lattice cages. These orientations could place acetylene with its molecular axis directed toward oxygen atoms, toward the hydrogen atoms in cage hydrogen bonds, or toward opposing cage windows. The absence of the spectral signature of hydrogen bonding rules out the first alternative. The acidity of the acetylenic C-H bonds makes the second alternative implausible. Thus, we are led to favor the third alternative. In the smaller cages there are six equivalent directions that connect the centers of opposing pentagonal windows. These are likely to be the preferred orientations of acetylene guests in the smaller cages of Structure I (and 11). In the larger cages, there are opposing hexagonal windows in the direction of the cage minor axis. In the more spacious direction of the cage major axes, there are 12 possible orientations in which the acetylene molecular axis would intersect pentagonal windows on either side. In these positions, the acetylene molecules would be tilted with reference (19) (a) Rakov, A. V. Proc. P. N . Lebedeu, Phys. Imt. 1965,27, 110-148. (b) Rakov, A. V. Opt. Spectrosc. 1962, 13, 203-205. (20) Soussen-Jacob,J.; Bessiere, J.; Tsakiris, J.; Vincent-Goisse, J. Spectrochim. Acta 1977, 33, 805-8 13.
293
to the plane of the major axes and slightly displaced from the cage center. If these were occupied, fluxional motion might be involved. Bending Frequency Shifts. The observed v5 shifts from the gas-phase values present an anomaly since the C2Dzv5 frequencies are above those of gaseous C2H2,whereas the C2D2v5 frequencies are below those of gaseous C2D2. These shifts may signal coupling between the v5 motion with the clathrate lattice bending motion in the same spectral region. Since C2H2is studied in the D 2 0 clathrate, the v5 bending mode near 730 cm-' is coupling with a lattice vibration centered at lower frequencies, near 600 cm-I. Any coupling would tend to shift v5 toward higher frequency. The situation is reversed for C2D2in the H20clathrate. In this case, v5 is near 540 cm-I, and it is coupled with a lattice vibration centered at much higher frequencies, near 800 cm-'. Such coupling would tend to shift v5 toward lower frequency, as observed.
Conclusions The method of preparation of our clathrate samples, annealing under an ambient pressure of the guest molecule, significantly enhances the information content of their spectra. The method will, presumably, be applicable and beneficial in the spectral study of other clathrates. The distinctive spectra so obtained permit us to recognize acetylene in each of the two cages and to conclude that the acetylene molecules in the water clathrate undergo hindered librational movement around fixed orientations in the lattice. Since hydrogen bonding is not involved, this orientation may well place the acetylene molecular axis pointed at cage windows, an orientation that could relate to the prospect of clathrate acetylene polymerization (which we have not, as yet, been able to demonstrate). Acknowledgment. This work was partially supported by a grant-in-aid of research from Sigma Xi and partially by the Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U S . Department of Energy, under contract No. DE-AC03-76SF-00098. Additionally, we thank Mandy Chen for assistance in some preliminary experiments and J. P. Devlin for a prepublication copy of ref 7b. Registry NO.C2H2, 74-86-2;C2H2/(CH3)2CO (2:1), 104947-71-9; D2,
7782-39-0.
Overtone Spectral Investlgation of the Conformational Preference of Dichloromethyl and Dibromomethyl Groups in Benzal Chloride and Benzal Bromide M. Khalique Ahmed, David J. Swanton, and Bryan R. Henry* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received: August 4 , 1986) Liquid-phase CH-stretching overtone spectra of benzal chloride (C6HSCHC12)in the region of AucH = 3-6 and of benzal bromide (C6H5CHBr2)in the region of AUCH= 3-5 are measured. These measured spectra are decomposed into component Lorentzian peaks. The aryl region spectra for AuCH Z 4 consist of two peaks. At AuCH= 3, in addition to the two peaks, a combination peak of significant intensity is also observed. The alkyl regions of the overtone spectra of C6H5CHBr2are single symmetric peaks which are described by a single Lorentzian function. In contrast, the alkyl regions of the overtone spectra of C6HSCHC12are asymmetric bands which are fitted with two Lorentzian functions. From these observations it is concluded that the -CHBr2 group of C6H5CHBr2exists in a single conformation while two conformations (planar and orthogonal) are possible for the -CHC12group of C6HSCHCl2.The equilibrium geometry of benzal chloride is also determined from ab initio calculations at the SCF level with STO-3G and 4-31G basis sets.
Introduction The local mode model of stretching vibrational motions of X H bonds (X = C, 0, N, or S) of polyatomic molecules is well documented in the current literature on vibrational spectroscopy.14 (1) Henry, B.
R. Vib. Spectra Struct.
1981, 10, 269.
0022-3654/87/2091-0293$01.50/0
In the local mode model, the XH bonds of a polyatomic molecule are considered as an assembly of loosely coupled anharmonic (2) Sage, M. L.; Jortner, J. A d a Chem. Phys. 1981, 47, 293. (3) Child, M. S.;Halonen, L. Adu. Chem. Phys. 1984, 57, 1. (4) Child, M. S.Acc. Chem. Res. 1985, 18, 45.
0 1987 American Chemical Society
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Morse oscillators. Various studies have showns7 that the spectral features due to the coupling of the Morse oscillator type X H bonds are only observed at low overtones and in the fundamental spectrum. At higher overtones (AuXH L 3), the observed peaks correspond to transitions to vibrational states whose components have all of the vibrational energy associated with individual Morse potentials which represent the nonequivalent X H bonds of a polyatomic molecule. The energies of the peaks observed in the XH-stretching overtone spectra due to a particular type of X H bond fit very well to the energy equation of an anharmonic Morse oscilIator;'-' Le., AE(u) = wu
-W
+ U)
X ( ~
(1)
In eq 1, W , u, and w x are the harmonic frequency, vibrational quantum number, and the diagonal anharmonicity, respectively. In recent years, the local mode model has been used extensively to glean information about conformationally and structurally nonequivalent bond^.^-'^ As an example, consider the o-xylene molecule. The most stable conformer of this molecule is a planar one20-22where the methyl groups have one C H bond in the ring plane and two at 60'. The aryl C H bonds of o-xylene are nonequivalent. Two of the four aryl C H bonds experience ortho and meta influence from the methyl groups. The remaining two aryl CH bonds experience meta and para effects from the two methyl groups. Thus o-xylene has two types of methyl and two types of aryl C H bonds. Two peaks have been observed by Gough and HenryI4 in both the methyl and aryl regions of the gas-phase overtone spectra of o-xylene. In a recent Raman study,23Ribeiro-Claro and Teixeira-Dias have suggested that the -CHC12 group of C6H5CHC12exists in two conformers, one in which the CH bond lies in the ring plane (planar) and the other in which it makes an angle of 90' with the ring plane (orthogonal). The planar and orthogonal conformers have been associated with the two bands in the CHstretching region of the spectrum of the -CHC12 group. The authors state that the existence of two conformers is supported by solvent and temperature effects and by CNDO/2 calculations. Schaefer and Penner have used In another very recent geometry optimized STO-3G molecular orbital computations with the benzene framework constrained to be planar and hexagonal to show that the potential for rotation of the -CHC12 group of C6H5CHC12is twofold (single minimum), and that in the conformer of lowest energy the alkyl C H bond prefers the plane of benzene ring. A value of 11.6 f 0.1 kJ mol-' was derived for the internal rotational potential of -CHC12. ( 5 ) Mortensen, 0. S.; Henry, B. R.; Mohammadi, M. A. J . Chem. Phys. 1981, 75, 4800.
(6) Henry, B. R.; Tarr, A. W.; Mortensen, 0. S.; Murphy, W. F.; Compton, D. A. C. J. Chem. Phys. 1983, 79, 2583. (7) Tarr, A. W.; Henry, B. R. Chem. Phys. Lett. 1984, 112, 295. (8) Henry, B. R.; Hung, I. F.; MacPhail, R. A,; Strauss, H. L. J . Am. Chem. SOC.1980, 102, 515. (9) Wone. J. S.: Moore. C. B. J. Chem. Phvs. 1982. 77. 603. (10) Woig, J. S:; MacPhail, R. A,; Moore, C. B.; Strauss, H. L. J . Phys. Chem. 1982, 86, 1478. (11) Fang, H. L.; Swofford, R. L. Appl. Opt. 1982, 21, 5 5 . (12) Henry, B. R.; Gough, K.M. Laser Chem. 1983, 2, 309. (13) Henrv. B. R.: Gouah. K. M.: Sowa. M. G. In,. Reu. Phvs. Chem. 1986, 5, 133.' (14) Gough, K. M.; Henry, B. R. J. Phys. Chem. 1984, 88, 1298. (15) Fang, H. L.; Swofford, R. L.; Compton, D. A. C. Chem. Phys. Lett. 1984, 108, 539. (16) Fang, H. L.; Meister, D. M.; Swofford, R. L. J . Phys. Chem. 1984, RR - -, 405 .- - , 411-1 . .- . (17) Fang, H. L.; Swofford, R. L.; McDevitt, M.; Anderson, A. B. J . Phys. Chem. 1985, 89, 225. (18) Nakagaki, R.; Hanazaki, I. Chem. Phys. 1982, 72, 93. (19) Mizugai, Y.; Katayama, M. Chem. Phys. Lett. 1980, 73, 240. (20) Woolfenden, W. R.; Grant, D. M. J. Am. Chem. SOC.1966,88, 1496. (21) Rudolph, H. D.; Walzer, K.; Krutzik, I. J. Mol. Spectrosc. 1973, 47, 314. (22) Gough, K. M.; Henry, B. R.; Wildman, T. A. J. Mol. Struct. 1985, 124, 71. (23) Ribeiro-Claro, P. J. A,; Teixeira-Dias J. J. C. J . Raman Spectrosc. 1984, 15, 224. Ribeiro-Claro, R. J. A,; Rocha Gonsalves, A. M. D'A.; Teixeira-Dias, J. J. C. Spectrochim. Acta, Part A 1985, 41A, 1055. (24) Schaefer, T.; Penner, G. H. J . Raman Spectrosc. 1985, 16, 353. I
9200
9000
8800
8600
8400
(cm-9 Figure 1. Upper trace: liquid-phase overtone spectrum of C&&HC12 in the region of AvCH = 3. The spectrum was measured at room temperature with a path length of 3.0 cm. Middle trace: individual Lorentzian functions fit to the experimental spectrum. Lower trace: difference of the experimental and fit spectra.
O'I2
1
0 08
w
0
z
se
004
% m a
0 00
12000
I1500
I1000
Figure 2. Upper trace: liquid-phase overtone spectrum of C6HSCHCl2 in the region of AucH = 4. The spectrum was measured at room temperature with a path length of 3.0 cm. Middle trace: individual Lorentzian functions fit to the experimental spectrum. Lower trace: difference of the experimental and fit spectra.
In this paper we report the liquid-phase overtone spectra of C6H5CHC12(AuCH = 3-6) and C6H5CHBr2(AucH = 3-5). We analyze these spectra in terms of a local mode model. Our analysis of the alkyl regions of the overtone spectra of C6H5CHBr2shows that the -CHBr2 group in this molecule exists in a single conformation. However, in the alkyl regions of the overtone spectra of C6H5CHC12,we have identified spectral features which can be assigned to absorptions due to the planar and orthogonal conformations of the -CHC12 group. We have also extended the calculations of Schaefer and PennerZ4to the larger 4-3 1G basis set and examined the effect of restricting the benzene ring to a planar hexagonal geometry. Experimental Section Both C6H5CHC12and C6H5CHBr2were obtained from Aldrich Chemical Co. and had stated purities of 99% and 97%, respectively. The spectra of the liquid-phase samples were recorded on a Beckman 5270 spectrophotometer with the near-IR light source in the regions of AUCH = 3 and 4 and the visible light source for
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 295 I
I
17000
16500
16000
1
I
I
I
40.004 0 003
I%
0
m
0 002
m
a
0.001
9200
9000
8800
8600
8400 0.000
(cm-9 Figure 3. Upper trace: liquid-phaseovertone spectrum of C6H5CHBr2 in the region of AUCH= 3. The spectrum was measured at room temperature with a path length of 3.0 cm. Middle trace: individual Lor-
entzian functions fit to the experimental spectrum. Lower trace: difference of the experimental and fit spectra. 14500
0 08
14000
13500
Figure 5. Base line corrected liquid-phase overtone spectra of C6H5CHC12in the region of AuCH = 5 and 6 . The spectra were measured at room temperature with a path length of 10.0 cm. The upper abscissa and right-hand ordinate scales are for the AUCH= 6 spectrum.
-
-
8 1
12000
I1500
I1000
7 (cm-l) Figure 4. Upper trace: liquid-phaseovertone spectrum of C6HSCHBr2 in the region of AUCH = 4. The spectrum was measured at room temperature with a path length of 3.0 cm. Middle trace: individual Lor-
entzian functions fit to the experimental spectrum. Lower trace: difference of the experimental and fit spectra. the regions of AuCH = 5 and 6. The spectra at AuCH = 3 and 4 for both C6HsCHC12and C6HsCHBr2were recorded with a 3 cm path length cell. The spectra at AuCH = 5 and 6 for C6HSCHC12 were recorded with a 10 cm path length cell. The AUCH = 5 spectrum of C6H5CHBr2was recorded with a 5 cm path length cell. 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. The spectra were deconvoluted with a Fortran curve analysis program NIRCAg' which fitted Lorentzian peaks to the experimental bands. Calculations for C6H5CHC12were performed at the S C F level with the ab initio gradient program GAUSSIAN 8 x Z 6 The STO-3G and 4-31G basis sets were used. The calculations of Schaefer and PennerZ4with the STO-3G basis set were reproduced. We have ( 2 5 ) The NIRCAP program was written by R. K. Marat and modified by A. W. Tarr. (26) Binkley, J. S.; Frisch, M. J.; Defrees, D. J.; Raghavachari, K.; Whitside, R. A.; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. GAUSSIAN 82, Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, 1983.
L (cm-l)
Figure 6. Liquid-phase overtone spectrum of C6HSCHBr2in the region of AUCH= 5. This spectrum is the sum of four base line corrected scans. Individual scans were measured at room temperature with a 5.0 cm path
length cell. also extended these calculations in the sense that the benzene ring was not restricted to be planar and hexagonal. However, this restriction was retained in our calculation with the larger 4-31G basis set.
Results and Discussion The liquid-phase overtone spectra of C6HSCHClzand C6HsCHBr2 in the regions of AuCH= 3 and 4 are shown in Figures 1-4. In these Figures, we have also shown the Lorentzian functions which were fitted to the experimental spectra to obtain the individual peak positions. The liquid-phase overtone spectra of C6HSCHCI2in the regions of AuCH = 5 and AuCH = 6 are shown in Figure 5. The liquid-phase overtone spectrum of C6HsCHBrz in the region of AuCH = 5 is shown in Figure 6. The individual peak positions for AuCH = 5 and 6 of C6HSCHC1, and A u ~ H= 5 of C6HSCHBr2were also obtained through deconvolution but the corresponding Lorentzian functions for these regions are not shown. The sample of C6HSCHBr2had a slight yellow color. The overtone spectra of this molecule in the region of AuCH = 6 could not be obtained due to interference from the absorption tail of the visible electronic spectrum. The deconvoluted peak positions for the spectra of C6HSCHC12 in the regions of AuCH = 3-6 are given in Table I along with the
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Ahmed et al.
TABLE I: Deconvoluted Peak Positions (cm-I) and Assignments for the Overtone Peaks of C&CHC12 AvCH peak position assignment 8 545 8 619 8 753 8 800 8 845 11 142 11 247 1 1 479 11 551 13 509 13691 14 040 14 139 16046 16 528 16665
3
4
5 6
dichloromethyl dichloromethyl combination aryl C H aryl C H dichloromethyl dichloromethyl aryl C H aryl C H dichloromethyl dichloromethyl aryl C H aryl C H dichloromethyl aryl C H aryl C H
C H (90')" C H (0')'
I\
120.8
C H (90O)" C H (0')" C H (90')" C H (OO), C H (0')"
8 Y
See text.
H TABLE I 1 Deconvoluted Peak Positions (cm-I) and Assignments for the Overtone Peaks of C,&CHBr2 Avr~ peak position assignment 8 646 8 731 8 784 8 829 11 299 11 454 11 522 13788 14021 14 100
3 4 5
dibromomethyl C H combination aryl C H aryl C H dibromomethyl C H aryl C H aryl C H dibromomethyl C H aryl C H aryl C H
Figure 7. SCF-optimized geometry of benzal chloride with the STO-3G basis set. All parameters have been allowed to vary; the ring angles were all within f0.2' of the hexagonal angle (120'). The energy is -1 174.467 587 au. The bond lengths are given in angstroms and the angles are in degrees. The C-C1 bonds form an angle of 61.5' with the benzene plane.
,CI
TABLE III: Alkyl CH Bond Lengths (rC+,) in C6H5CHCI2as a Function of the Angle (8) Formed with the Benzene Plane, Calculated at the SCF Level with the STO-3Gand 4-31G Basis Sets TC-H, A
8. deg
STO-3G
4-31G
0 30 60 90
1.0940 1.0946 1.0963 1.0972
1.0687 1.0689 1.0696 1.0701
peak assignments. The deconvoluted peak positions and assignments for the spectra of C6H5CHBr2in the regions of AvCH = 3-5 are given in Table 11. The removal of the restrictions on the benzene ring to be planar and hexagonal does not make a significant difference in the calculated geometry at the STO-3G level. The largest bond length variation occurs for the ring C C bonds which changed by f0.002 to 0.005 A. The LCCC and LCCH angle variations were only * 0 . 2 O which sets an effective limit on the angular uncertainty in the calculation. Based on this relative stability of the ring system, and because of calculational limitations, we carried out geometry optimization with the 4-3 1G basis restricting the benzene ring to planarity and to hexagonal symmetry. The equilibrium geometries associated with both the unrestrained STO-3G calculation and the 4-31G calculation are summarized in Figures 7 and 8. In both calculations, the planar conformer with the alkyl C H bond in the plane of the benzene ring was found to be the most stable. The difference in energy between this conformer and the highest energy perpendicular conformer with the alkyl C H bond at 90' to the benzene ring was 11.42 and 10.99 kJ mol-' for the STO-3G and 4-3 1G calculations, respectively. The alkyl C H bond length is a function of angle. As in the case of toluene,14this bond length is shortest in the ring plane and longest at 90°. The values for this bond length at Oo, 30°, 60°, and 90° are given in Table 111 for the two calculations. As we have noted, the unrestrained STO-3G geometry is very similar to the restrained STO-3G geometry of Schaefer and Penner.24 However, there is one important difference between
H Figure 8. SCF-optimized geometry of benzal chloride with the 4-31G basis set. The benzene ring was constrained to be hexagonal. The energy of this conformer is -1 186.157 735 au. The bond lengths are given in angstroms and the angles in degrees. The C-C1 bonds form an angle of 60.5' with the benzene ring.
the STO-3G and 4-3 1G results. In the former, the aryl C H bond lengths are all very close with the C H bonds ortho and para to the -CHC12 group approximately the same, and longer than the two equal meta C H bonds by about 0.0006 A. In the 4-31G calculation, the aryl bond lengths are still close, but the largest difference occurs between the two ortho C H bonds with the one closer to the planar C H bond longer by 0.0018 A. There is an excellent correlation between C H bond lengths and overtone f r e q u e n c i e ~ . ~ J ~ ~However, ' ~ ~ ' ~ ~in~ ~the ~ ~most ~ stable planar conformer of C6H5CHC12and C6H5CHBr2,all five aryl C H bonds are nonequivalent. Moreover, the bond length differences are predicted to be small (see Figures 7 and 8). The spectra have been obtained in the liquid phase where the bands are undoubtedly broadened by intermolecular interactions. Under these circumstances, it is not surprising that the aryl peaks are unresolved and that an unambiguous assignment of the two ob(27) Hayward, R. J.; Henry, €3. R. Chem. Phys. 1976, 12, 387. (28) Gough, K. M.; Henry, B. R. J . Am. Chem. SOC.1984, 106, 2781.
Overtone Spectra of C6H5CHC12and C6H5CHBr2 served aryl features is not possible. The observed splitting between these two features increases with 4 v c H in the manner expectedz8 for contributions from two types of nonequivalent aryl CH bonds. Mizugai and K a t a ~ a m ahave ~ ~ .investigated ~ substituent effects on the 4 v C H = 6 overtones in the aryl region of the spectra of a series of monosubstituted benzenes using thermal lens techniques. For the spectrum of C&CHC12 they have reported a frequency of 16 520 cm-' at AuCH = 6. This value is in good agreement with our value of 16 528 cm-I for the lower energy, higher intensity aryl peak. However, it should be noted that Mizugai and Katayama did not attempt to deconvolute their spectra, and the frequency of 16 520 cm-I corresponds to the single asymmetric band due to all of the aryl C H bonds. In their study of the CH-stretching Raman spectrum of c6H5CHC12,on the basis of solvent and temperature effects, Ribeiro-Claro and Teixeira-Dias have assigned the two bands observed at 2992 and 3006 cm-' to the two conformers, planar and orthogonal. In overtone spectra, the frequency separation between peaks corresponding to nonequivalent CH bonds increases linearly with increasing AvcH.9,28 Thus if two rotational confomers were present, we would expect to observe an asymmetric band in the alkyl region of the spectrum, particularly at higher overtones. The alkyl region of each overtone of C6H5CHC12consists of an asymmetric band which is a result of two overlapping Lorentzian functions. This is apparent from the deconvoluted AuCH = 3 and 4 spectra of Figures 1 and 2. The geometry-optimized calculations on C6H5CHC12predict the C H bond of the -CHClZ group to be shortest when in the ring plane (planar conformation) (see Table 111). Thus on this basis and the overtone frequency bond length correlation, we have assigned the higher frequency, higher intensity methyl peak at each overtone of C6H5CHC12to the planar conformation. The lower energy, lower intensity component of the alkylic asymmetric band at each overtone of C6H5CHC12could possibly arise from a transition to a combination state of the type Iu - 1,2B), where u - 1 is the number of quanta in the pure local mode state and B refers to a C H bending mode. Such combination peaks have been observed previously in the overtone spectra of a number of polyatomic molecule^.^^^*^^-^^ However, such combination peaks change their relative position and intensity as a function of Av because of different anharmonicities for the interacting modes at different overtone levels. Neither of these effects is observed for the lower energy component peak of the alkylic band of C~H~CHC~Z. Our calculations on C6H5CHClzpredict the alkyl C H bond to be of maximum length when it makes an angle of 90° to the ring plane (orthogonal conformation). The predicted difference in the length of the C H bond of the planar and orthogonal conformations is 0.003 A (STO-3G) and 0.0014 A (4-31G) (see Table 111). For this bond length difference, the overtone frequency-bond length c o r r e l a t i ~ npredicts ~ , ~ ~ energy separations of 99, 132, and 165 cm-' (STO-3G) and 46, 62, and 77 cm-' (4-31G) between the alkyl peaks of the planar and orthogonal conformations at A v c H = 3, 4, and 5, respectively. The observed energy separations (74, 105, and 18238cm-I at AuCH = 3, 4, and 5, respectively) are in reasonably good agreement with the predicted values. Thus it appears that the lower energy component peak of the alkylic band of C6H5CHClZarises from the orthogonal conformation of the (29) Mizugai, Y.; Katayama, M. Bull. Chem. Sot. Jpn. 1980, 53, 2081. (30) Mizugai, Y.; Katayama, M. J . Am. Chem. Sot. 1980, 102, 6425. (31) Henry, B. R.; Mohammadi, M. A. Chem. Phys. 1981, 55, 385. (32) Fang, H. L.;Swofford, R. L. J . Chem. Phys. 1980, 72, 6382. (33) Fang, H. L.; Swofford, R. L. J . Chem. Phys. 1980, 73, 2607. (34) Henry, B. R.; Mohammadi, M. A.; Hanazaki, I.; Nakagaki, R. J . Phys. Chem. 1983,87,4827. (35) Voth, G. A.; Marcus, R. A,; Zewail, A. H. J . Chem. Phys. 1984,81, 5494. (36) Peyerimhoff, S.; Lewerenz, M.; Quack, M. Chem. Phys. Lett. 1984, 109, 563. (37) Perry, J. W.; Moll, D. J.; Kuppermann, A.; Zewail, A. H. J . Chem. Phys. 1985, 82, 1195. (38) The observed energy separation of 182 cm-' at AvCH = 5 is subject to greater error because of poor signal to noise ratio in this region of spectrum.
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 297 TABLE I V Local Mode Parameters (cm-') for the Nonequivalent Bonds of C&,CHCl, and C6H5CHBr2 molecule
bond type
C6HSCHCI2 dichloromethyl CH (90') dichloromethyl C H (0') aryl" aryl C6HSCHBr2 dibromomethyl C H aryl" aryl
w
WX
3145 f 25 3143 f 7.6 3170 f 8.0 3175 f 8.3 3132 f 12.0 3175 f 6.3 3189 f 2.4
73 f 6.0 67.0 f 1.6 59.8 f 1.7 57.2 f 1.8 62.2 f 2.9 61.9 f 1.5 61.5 f 0.6
'This value refers to the higher frequency aryl peak.
-CHC12 group. The approximate intensity ratios of these peaks with respect to the higher energy alkyl peaks are 1:11, 1:9, and 1:lO at AuCH = 3, 4, and 5, respectively. On this basis, if one assumes equal intrinsic intensities for the two conformers, on the average about 10% of the molecules would be in the orthogonal conformation in the liquid phase.39 Both our calculations and the earlier STO-3G calculations of Schaefer and PennerZ4predict only a single stable planar conformation. However, both our experimental results and those of Ribeiro-Claro and Teixeira-Dias indicate the existence of two stable conformers. The calculations relate to the isolated gas-phase molecules. It is conceivable, as pointed out by Schaefer and P e n r ~ e r that , ~ ~ intermolecular interactions could stabilize the orthogonal conformer in the liquid phase. The results of our overtone spectral study agree with the Raman studyz3of Ribeiro-Claro and Teixeira-Dias in the sense that both studies predict two stable conformations for C6H5CHC12. However, in principle, our assignments for the alkyl regions of the overtone spectra are in contradiction to the assignments by Ribeiro-Claro and Teixeira-Dias of the alkyl region of the fundamental Raman spectrum. They assign the observed lower and higher frequency Raman peaks (2992 and 3006 cm-I) to the planar and orthogonal conformation, respectively, whereas our assignments are in the reverse order. In the fundamental spectrum, due to extensive coupling between vibrational states and the large number of transitions, there is the possibility of ambiguity in the assignment of peaks to different conformers. This ambiguity might be responsible for the disagreement with our overtone results. In the region of bCH = 3 in the spectrum of C6H5CHCl2,there is an additional peak at 8753 cm-1 between the aryl and alkyl C H peaks. We assign this peak to a combination which involves three quanta of alkyl C H stretching (8619 cm-I) and one quantum of ~ ~ peak has significant intensity, -CHC12 torsion (1 18 ~ m - l ) .This probably due to a near-resonant interaction with the aryl CHstretching local mode state(s). It is evident from Figures 3 and 4 that the alkyl regions of both the A u c H = 3 and 4 spectra of C6H5CHBr2are fitted very well by a single Lorentzian function. A similar result was obtained for the AuCH = 5 spectrum. These results provide strong evidence for the existence of a single stable conformation for the -CHBrz group of C6H5CHBrz. Such a result is in keeping with the expected larger barrier to internal rotation in benzal bromide as compared to benzal chloride. Assignments of the peaks, other than the alkyl peaks, at AuCH = 3-5 in the spectra of C6H5CHBr2 are analogous to the corresponding peaks in the spectra of c6H5CHC12(see Tables I and 11). The local mode parameters (harmonic frequency, w, and diagonal anharmonicity, w x ) of the nonequivalent bonds of c.5H5CHC12and C6H5CHBr2can be obtained by fitting the energies of the pure CH-stretching peaks from Tables I and I1 to eq 1. These parameters are listed in Table IV. The uncertainties in
the w and w x values for the alkyl CH oscillator of the orthogonal conformation of C6H5CHC12(also to a lesser degree for the alkyl (39) Overtone intensities depend in a complicated fashion on a number of factors.@ The danger of assigning overtone spectra on thd basis of relative intensities has been emphasized recently by Findsen et al.4' (40) Mortensen, 0.S.; Ahmed, M. K.; Henry, B. R.; Tarr, A. W. J . Chem. Phys. 1985, 82, 3903. (41) Findsen, L. A,; Fang, H . L.; Swofford, R. L.; Birge, R. R. J . Chem. Phys. 1986, 84, 16.
J. Phys. Chem. 1987, 91, 298-305
298
C H oscillator of C&15CHBr2) are significantly higher than usual.' These greater uncertainties are undoubtedly due to the uncertainty in the peak positions of these low-intensity peaks, particularly at AvCH = 5 (both molecules). Therefore, the w and w x values for the alkyl C H oscillator of the orthogonal conformer of C6H5CHClz and the alkyl C H oscillator of C6H5CHBr2should be regarded only as estimates. We also note that the values of w and w x associated with the two aryl peaks are averages in the sense that these peaks correspond to unresolved contributions from nonequivalent aryl C H bonds. The transition frequencies (see Tables I and 11) of the aryl C H oscillators of C6HSCHC12and C6H5CHBr2are higher than the corresponding transition frequencies for the CH oscillator of benzene (AE(benzene) = 8760, 11442, 14015, and 16467 cm-' = 3,4, 5, and 6, re~pectively~~). Both the -CHCI2 and for bCH (42) Patel, C. K. N.; Tam, A. C.; Kerl, R. J. J . Chem. Phys. 1979, 71, 1470.
-CHBr2 groups are electron-attracting relative to hydrogen. Thus, they strengthen the aryl CH bonds of C6H5CHC12and C6H5CHBr2 and increase their vibrational frequencies relative to benzene. It would be particularly useful to observe the CH-stretching overtone spectrum of C6H5CHCI2in the gas phase. There is the possibility that peaks due to the nonequivalent aryl C H bonds would be resolved. Of even greater interest would be an indication of the role played by liquid-phase intermolecular interactions in stabilizing the orthogonal conformer. We are currently attempting to build a high-temperature facility for an intracavity dye laser photoacoustic spectrometer to investigate these possibilities.
Acknowledgment. We are grateful to Professor Ted Schaefer and Mr. Glenn Penner for helpful discussions. We are also grateful to the University of Manitoba for the generous provision of computer time, and to the Natural Sciences and Engineering Research Council of Canada for financial support. Registry No. C6H5CHCI2,98-87-3; C6HSCHBr2,618-3 1-5.
Single Pulse Laser Induced Reactlons of Hexafluorobenzene/Siiane Mixtures at 1027 and 944 em-' 'I*
Yoshinori K ~ g a , Robert ~* M. Serino,3bRuth C t ~ e nand , ~ ~Philip M. Keehn* Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254 (Received: December 16, 1985; In Final Form: August 18, 1986)
C6F6/SiH4mixtures were irradiated with a single pulse of a megawatt cozinfrared laser at 1027 and 944 cm-I, using fluences which ranged from 0.26 to 2.0 J/cmZ. Neat C6F6 (7.5 Torr, 1027 cm-I) underwent decomposition to CzF4 at a fluence of 0.7 J/cm2 with a conversion per flash (CPF) of 4.5%. At 0.3 J/cmZ no reaction was observed, setting a fluence threshold for the laser-induced decomposition of C6F6 between 0.3 and 0.7J/cmZ. In the presence of SiH4 explosive reactions occurred with conversion of C6F6 as high as 70%! Different decomposition products were observed depending upon the amount of SiH4 present. At constant C6F6 pressure (7.3 Torr) and at high C6F6 mole fraction (R = PCsFs/(PCaF6 PSiH4) 2 OS), fluorinated carbonaceous products were observed (CzF4, C2Fs). At low C6F6 mole fraction (R 5 0.55), non-fluorinated carbonaceous products were observed (C2H2,C4H2). SiF, was the major gaseous product in both regions, while SiF3H was observed when R values were lower than 0.55. A polymeric black material was also deposited in the cell in both R zones. The highest CPF of C6F6 was obtained when the mole fraction of C6F6 was 0.55 and under these conditions only SiF4, SiF3H, and polymeric material were observed. Irradiation of C6F6/SiH4mixtures (constant C6F6 pressure, 7.5 Torr) at 944 cm-I using fluences below 1 J/cm2 did not induce reaction. At 1.6 J/cm2 no reaction was observed at C6F6 mole fractions above 0.3. However, between 0.3 and 0.1 mole fraction values identical products as those obtained in this zone under 1027-cm-' irradiation were observed. Identical products were also obtained when the C6F6 mole fraction was varied by adding C6F6 to SiH4 (constant SiH4 pressure, 30 Torr). However, the threshold for reaction was observed at a C6F6 mole fraction of R = 0.5 and no reaction was observed when R > 0.5. At higher overall pressure (PCsF6 = 72 Torr; PSiH4 = 37 Torr, with R = 0.66) irradiation at 944 cm-' gave the same products as those observed in the high R zone of the 1027-cm-' irradiation. A higher fluence (2.05/cm2) was necessary, however, to induce the reaction at 944 cm-I. These results are discussed in terms of (a) the low fluence threshold observed for the laser-induced decomposition of C6F6,(b) the effects that added gases have on the decomposition of C6F6, (c) the use of C6F6 as a sensitizer for laser-induced reactions, and (d) the potential for using SiH, for the laser-induced reduction of C-F bonds (C-F + Si-H C-H Si-F).
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Introduction Infrared-laser-induced bimolecular reactions have been given limited attention compared with their unimolecular counterparts. In 1981, Danen and Jang4 suggested that products of bimolecular reactions might be generated by the simultaneous laser irradiation of both reactants. When infrared absorption bands of both ~
(1) For previous paper in this series see: Madison,
J. Anal. Appl. Pyrol. 1986, 9, 237-246.
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S. A,; Keehn, P. M.
(2) This work was presented at the 187th National Meeting of the American Chemical Society, April 1984. Abstract ORGN 43. (3) (a) Permanent address: National Chemical Laboratory for Industry, Tsukuba, Japan. (b) Present address: USA Foreign Science and Technology Center, Charlottesville, VA 22901, (c) Present address: National Bureau of Standards, Gaithersburg, MD 20899. (4) Danen, W. C.; Jang, J. C. In Laser Induced Chemical Processes; Steinfeld, J., Ed.; Plenum: New York, 1981; p 78.
0022-3654/87/2091-0298$01.50/0
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reactants coincide with one of the emission bands of the laser, the reaction may be. influenced by a single laser irradiating wavelength. Bauer and Habermad reported on an explosive reaction between silane (SiH4) and sulfur hexafluoride (SF,) initiated by C W C 0 2 laser irradiation at 944 cm-', in which both molecular species absorbed energy. Similarly, Haggerty and Cannon6 succeeded in synthesizing Si3N4by irradiating mixtures of SiH4and N H 3 at 944 cm-' using focused and unfocused C W C 0 2 techniques. When the absorption bands of the reactant molecules are not coincident, the reaction might be induced by the simultaneous irradiation at two wavelengths by using two lasers. Alternatively, ( 5 ) Bauer, S. H.; Haberman, J. A., IEEE J . Quantum Electron. 1978 QE-14, 233. (6) Haggerty, J. S.; Cannon, W. R. In Laser Induced Chemical Processes; Steinfeld, J., Ed.; Plenum: New York, 1981; p 170.
0 1987 American Chemical Society
