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Raman spectra and conformational stability of 2-methylpropionyl chloride
265
of Mokwhr Structure, 266 (1992) 265-270 Elsevier Science Publishers B.V., Amsterdam
Journal
Raman spectra and conformational stability of 2-methylpropionyl chloride G. A. Guirgisa, H. V. Phanb, and J. R. DurigC %Analytical Research Laboratory, Mobay Corporation, Bushy Park Plant, Charleston, SC 29411 USA hEthy Corporation,
P.O. Box 1028, Orangeburg,
CDe artment of Chemistry and Biochemistry, CoPumbia, SC 29208 USA
SC 29115
USA
University of South Carolina,
Abstract
The Raman spectra from 3600 to 10 cm-1 of liquid and solid 2-methylpropionyl chloride (isobutyryl chloride), (CH&CHCClO, have been recorded. Additionally, qualitative depolarization ratios have been obtained from the Raman spectrum of the liquid. These data have been interpreted in that the gauche conformation is thermodynamically preferred over the higher energy truns conformation in the liquid and is the only rotamer present in the annealed solid. From the relative intensities of the Raman lines of the liquid at 687 cm-1 (gauche) and 563 cm-l (t~ans) as a function of temperature, the enthalpy difference is determined to be 345 f 93 cm-l (987 f 266 cal/mol). Vibrational assignments are proposed for the fundamentals based on depolarization values and group frequencies. The results are compared t.c the corresponding data for some similar molecules.
1. INTRODUCTION We132 have recently published vibrational and structural studies of 2methylpropanal, (CH&CHCHO. In the initial studyl, the analyses of the microwave and far infrared spectra of 2-methylpropanal-do and -d7 were reported. From this study, it was determined that the gauche (oxygen atom eclipsing one of the methyl groups) and the higher energy truns (oxygen atom eclipsing the secondary hydrogen atom) conformers are present in the gas phase at ambient temperature. From the analysis of the far infrared spectrum of the gas the asymmetric torsional potential function was determined. Additionally, the enthalpy difference between the gauche and tram conformers of the aas was determined to be 248 f 50 cm-l (709 f 143 cal/mol) from a temperature dependent study of the Raman spectrum. This study was subsequently followed by a complete vibrational analysis2 of 2-
0022-2860/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved
266
methylpropanal-do and -dT that included the calculations of ab initio structural parameters, force constants, and frequencies for the normal modes for both the gauche and tram conformers. Also utilizing the ab initio calculated differences of the structural parameter8 as constants, ro structural parameters were re-calculated. Additionally, the enthalpy difference between the au&e and tram conformers of the liquid was determined to be 440 f 17 cm-f (1.26 f 0.05 kca.l/mol)from the temperature dependent study of the Raman spectrum of the liquid. These studies of the aldehyde have been followed by a similar stud3 of the corresponding fluoride, 2-methylpropionyl fluoride, (CH$$HCFO. The a-type R-branch transitions were observed in the microwave spectrum and assigned for the gauche conformer (one of the methyl groups eclipses the oxygen atom), and ro structural parameters were estimated from a combination of ab initio calculated values and the fit of the rotational constants. Also, from the Raman and infrared spectra of the gas, liquid and solid, it was concluded that the gauche conformation is thermodynamically preferred over the higher energy truns conformation and is the only rotamer present in the spectra of the annealed solid. From the relative intensities of the Raman spectrum of the gas as a function of temperature, the enthalpy difference was determined to be 461 2 40 cm-l (1.32 f 0.11 kcalImo1) and a similar study of the liquid gave an enthalpy value of 434 f 24 cm-1 (1.24 f 0.07 kcal/mol). As a continuation of these studies we have initiated a study of the chloride, vibrational and structural data for 2-methylpropionyl (CH&CHCClO. In the present paper the Raman spectra of the liquid and annealed solid are presented. These data are compared to those obtained for 2-methylpropanal and 2-methylpropionyl fluoride. 2. EXF’ERIMENTAL The sample of 2-methylpropionyl chloride was purchased from Aldrich Chemical Co., Milwaukee, WI, and had a stated purity of 99%. ‘Phi8 sample was further purified by using a low pressure, low temperature fractionation column. The Raman spectra of 2-methylpropionyl chloride were recorded on a Cary model 82 spectrophotometer equipped with a Spectra-Physics model 171 argon ion laser. The laser was operated on the 5145 A line with the laser power at the sample being varied from 0.5 to 2 W depending on the physical state under investigation. The spectrum of the liquid was obtained by using a sealed glass spherical ce114,and the Raman spectrum of the solid was obtained by condensing the sample onto a blackened brass block that is fixed at 15” to the incident radiation and cooled by boiling liquid nitrogen. The frequencies measured for sharp lines should be accurate to at least f 2 cm-l. The variable temperature experiment of the liquid was carried out using a Miller-Harney cell5 with the SPEX model 1403 double monocbromator
267
equipped with a third monochromator model 164 argon ion laser (5145 A>.
3.
(model 1442U) and a Spectra-Physics
CONF’ORMATIONAL ENERGY DIFFERENCE
As can be seen in Fig. 1, the Raman lines at 687 and 563 cm-1 show a significant change with temperature variation where the line at 563 cm-1 does not appear in the spectrum of the annealed solid. These lines are assigned to the C-Cl stretches for the gauche and bans conformers, respectively. By using the equation -1n.K = (AIVKT) - (A&R), the enthalpy difference between the gauche and tmns conformers can be calculated. In this case, K = I t, where I is the intensity of the Raman line attributed to the gauche co J ormer and 8.t 1s the intensity of the Raman line of the trans conformer. By assuming that AH is invariant with temperature, AH can be determined by a plot of -1nK versus Vl?. Five sets of spectral data of the two lines of 2-methylpropionyl chloride were obtained at temperatures ranging from 27 to -24°C. The plot of these data gives an enthalpy difference for the liquid of 345 f 93 cm-l (987 f 266 cal/mol).
4.
VIBRATIONAL A!%IGNMENT
Although the gauche rotamer (Cl symmetry) is found to be the most stable rotamer, the assignments provided in Table 1 are consistent with a molecule of C, symmetry, i.e., the higher energy trans conformation, due to the tendency of the isopropyl moiety to exhibit “local” C, symmetry. For the C, rotamer, the thirty-three fundamental modes span the representations: 19 A’ and 14 A” with the A’ modes giving rise to polarized Kaman lines. For Cl symmetry the thirty-three fundamental modes should give rise to polarized Raman lines in the liquid. There are seven carbon-hydrogen stretches for 2-methylpropionyl chloride. The c&H stretch is assigned to the line of medium intensity at 2928 cm-l in the Raman spectrum of the liquid. This value is less than that for the a-CH stretch in 2-methylpropanal2 (2973 cm-l) as well as slightly less than that for the fluoride3 (2938 cm-l). The methyl CH stretches of a-methylpropionyl chloride are found in the characteristic 2890 to 3000 cm-l region, with the antisymmetric stretches being assigned at higher frequencies. The weak Raman line at 1810 cm-lis assigned to the C=O stretch. This frequency is much higher than that observed for this mode in 2methylpropanal2 (1752 cm-l) but lower than that for the fluoride3 (1856 cm-l). There are three skeletal stretches in 2-meth lpropionyl chloride. The CCC antisymmetric stretch is assigned at 1093 cm- 9. The CC stretch and the CCC symmetric stretches are assigned at 924 and 842 cm-l, respectively. In 2-methylpropana12 the CC stretch and the CCC symmetric stretch have likewise been assigned at 920 and 799 cm-l, respectively.
268
750
625 Wavenumber
500 (cm“)
Figure 1. Variation of the Raman spectra of liquid 2-methylpropionyl chloride with temperature.
269
Table 1 Raman dataa for liquid and solid 2-methylpropionyl chloride. Liquid
2985
m
2945 2928 2914 2878 1810
vs m m m w
1460 vw 1451 vw 1322 1287 1173 1111 1096 966 940 932 847 687 642 563 437 416 337 320
w w w w w vw w w w m m m 8 s w m
224 w
Solid
Assignmentb
3005 m 2990 m 2983 m, 2977m 2966 w, 2945 w 2919 8 2913 w 2873 m 1800 vw 1480 w 1473 m 1463 m 1455 w 1447 w, 1442m 1392 w, 1372 w 1323 w, 1320 w 1280 w, 1264 w 1170 w 1123 s 1093 m 968 m 946 m 924 m 850 w, 842m 685 m 625 vs
“20 “21 “1 “2 “3 “4 “22 “5 “6 “23 “I “24 “8 “25 “9 “26 “10 “11 “27 “28 “29 “12 “13 “14 “30
442 vs, 439 w 408 vs
“15 “16
333 8 310 w 238 m 228 vw 222 vw 110 vw
“17 “18 “31 “19 “32 “33
79 m, 66 m, 57 w, 50 vs, 33 vw, 30 vw
CH3 antisymmetric stretch CH3 antisymmetric stretch CH3 antisymmetric stretch CH3 antisymmetric stretch c&H stretch CH3 symmetric stretch CH3 symmetric stretch C=O stretch CH3 antisymmetric deformation CH3 antisymmetric deformation CH3 antisymmetric deformation CH3 antisymmetric deformation CH3 symmetric deformation CH3 symmetric deformation a-CH bend a-CH bend CH3 rock CH3 rock CCC antisymmetric stretch CH3 rock CH3 rock CC stretch CCC symmetric stretch C-Cl stretch CC10 wag C-Cl stretch (Conformer II) CC10 deformation CC10 rock CC2 bend (Conformer II) CC2 deformation CC2 wag cc2 twist CH3 torsion CH3 torsion asymmetric torsion lattice modes
aAbbreviations used: s, strong; m, moderate; w, weak; and v, very. bNumbered for C, symmetry but gauche conformer is the more stable form.
270
The CH bending modes are observed in the characteristic 1480 to 900 cm-l region. The CH3 deformations are assigned in the 1440 to 1480 cm-l region with the corresponding A” modes being assigned to depolarized Raman lines in the spectrum of the liquid. The c&H in-plane and out-ofplane bends are assigned at 1322 and 1287 cm-l, respectively. In the case of 2-methylpropanal,z the c&H in-plane bend has been assigned at 1329 cm-l in the infrared spectrum of the gas, and the cc-CH out-of-plane bend has been assigned at 1290 cm-1 in the Raman spectrum of the gas. The CH3 rocks in 2-methylpropionyl chloride are assigned at 1170 and 1123 cm-l for the A modes and at 968 and 946 cm-1 for the A” modes. In 2-methylpropanal2 and 2-methylpropionyl fluoride the A’ CH3 rocks have also been assigned at approximately 200 cm-l higher in frequency than the corresponding A” modes. The C-Cl stretch of the gauche conformer is assigned to the Raman line at 687 cm-l and for the truns conformer at 563 cm-l. The latter band is not present in the spectrum of the solid. The bending modes of the CC10 moiety are tentatively assigned at higher frequency than the skeletal CC2 bending modes. The CC10 wag is assigned at 642 cm-l, the CC10 deformation at 437 cm-l, and the CC10 rock at 416 cm-1. These modes are undoubtedly mixed and a normal coordinate analysis needs to be carried out in order to provide a more accurate description of them. The low frequency skeletal bending modes and the torsional modes remain to be assigned. The CC2 deformation and CC2 wag are assigned at 333 and 310 cm-1 and the CC2 twist is assigned at 238 cm-1 in the Raman spectrum of the solid. However, in 2-methylpropanal2 the CC2 deformation has been assigned as the lowest skeletal bending mode at 272 cm-l. The symmetric and antisymmetric CH3 torsions of 2-methylpropionyl chloride are assigned at 228 and 222 cm-1 in the Reman spectrum of the solid. The asymmetric torsion of 2-methylpropionyl fluoride is assigned3 at 54 cm-1 for the gauche conformer and it shifts to 73 cm-l in the spectrum of the solid. We have assigned the corresponding mode for the chloride at 110 cm-l and believe the shift may be larger for this mode for the chloride. Further studies need to be carried out using ab initio calculations to provide more definitive descriptions of the normal modes. 6. REFERENCES 1 J. R. Durig, G. A. Guirgis, T. S. Little, and 0. L. Stiefvater, J. Chem. Phys., 91(1989) 738. 2 J. R. Durig, G. A. Guirgis, W. E. Brewer, and T. S. Little, J. Mol. Struct., 248 (1991) 49. 3 J. R. Durig, G. A. Guirgis, W. E. Brewer, and G. Baranovic, J. Phys. Chem., 96 (1992) in press. 4 R. Furic and J. R. Durig, Appl. Spectrosc., 42 (1988) 175. 5 F. A. Miller and B. M. Harney, Appl. Spectrosc., 24 (1970) 291.
of Mokwhr Structure, 266 (1992) 265-270 Elsevier Science Publishers B.V., Amsterdam
Journal
Raman spectra and conformational stability of 2-methylpropionyl chloride G. A. Guirgisa, H. V. Phanb, and J. R. DurigC %Analytical Research Laboratory, Mobay Corporation, Bushy Park Plant, Charleston, SC 29411 USA hEthy Corporation,
P.O. Box 1028, Orangeburg,
CDe artment of Chemistry and Biochemistry, CoPumbia, SC 29208 USA
SC 29115
USA
University of South Carolina,
Abstract
The Raman spectra from 3600 to 10 cm-1 of liquid and solid 2-methylpropionyl chloride (isobutyryl chloride), (CH&CHCClO, have been recorded. Additionally, qualitative depolarization ratios have been obtained from the Raman spectrum of the liquid. These data have been interpreted in that the gauche conformation is thermodynamically preferred over the higher energy truns conformation in the liquid and is the only rotamer present in the annealed solid. From the relative intensities of the Raman lines of the liquid at 687 cm-1 (gauche) and 563 cm-l (t~ans) as a function of temperature, the enthalpy difference is determined to be 345 f 93 cm-l (987 f 266 cal/mol). Vibrational assignments are proposed for the fundamentals based on depolarization values and group frequencies. The results are compared t.c the corresponding data for some similar molecules.
1. INTRODUCTION We132 have recently published vibrational and structural studies of 2methylpropanal, (CH&CHCHO. In the initial studyl, the analyses of the microwave and far infrared spectra of 2-methylpropanal-do and -d7 were reported. From this study, it was determined that the gauche (oxygen atom eclipsing one of the methyl groups) and the higher energy truns (oxygen atom eclipsing the secondary hydrogen atom) conformers are present in the gas phase at ambient temperature. From the analysis of the far infrared spectrum of the gas the asymmetric torsional potential function was determined. Additionally, the enthalpy difference between the gauche and tram conformers of the aas was determined to be 248 f 50 cm-l (709 f 143 cal/mol) from a temperature dependent study of the Raman spectrum. This study was subsequently followed by a complete vibrational analysis2 of 2-
0022-2860/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved
266
methylpropanal-do and -dT that included the calculations of ab initio structural parameters, force constants, and frequencies for the normal modes for both the gauche and tram conformers. Also utilizing the ab initio calculated differences of the structural parameter8 as constants, ro structural parameters were re-calculated. Additionally, the enthalpy difference between the au&e and tram conformers of the liquid was determined to be 440 f 17 cm-f (1.26 f 0.05 kca.l/mol)from the temperature dependent study of the Raman spectrum of the liquid. These studies of the aldehyde have been followed by a similar stud3 of the corresponding fluoride, 2-methylpropionyl fluoride, (CH$$HCFO. The a-type R-branch transitions were observed in the microwave spectrum and assigned for the gauche conformer (one of the methyl groups eclipses the oxygen atom), and ro structural parameters were estimated from a combination of ab initio calculated values and the fit of the rotational constants. Also, from the Raman and infrared spectra of the gas, liquid and solid, it was concluded that the gauche conformation is thermodynamically preferred over the higher energy truns conformation and is the only rotamer present in the spectra of the annealed solid. From the relative intensities of the Raman spectrum of the gas as a function of temperature, the enthalpy difference was determined to be 461 2 40 cm-l (1.32 f 0.11 kcalImo1) and a similar study of the liquid gave an enthalpy value of 434 f 24 cm-1 (1.24 f 0.07 kcal/mol). As a continuation of these studies we have initiated a study of the chloride, vibrational and structural data for 2-methylpropionyl (CH&CHCClO. In the present paper the Raman spectra of the liquid and annealed solid are presented. These data are compared to those obtained for 2-methylpropanal and 2-methylpropionyl fluoride. 2. EXF’ERIMENTAL The sample of 2-methylpropionyl chloride was purchased from Aldrich Chemical Co., Milwaukee, WI, and had a stated purity of 99%. ‘Phi8 sample was further purified by using a low pressure, low temperature fractionation column. The Raman spectra of 2-methylpropionyl chloride were recorded on a Cary model 82 spectrophotometer equipped with a Spectra-Physics model 171 argon ion laser. The laser was operated on the 5145 A line with the laser power at the sample being varied from 0.5 to 2 W depending on the physical state under investigation. The spectrum of the liquid was obtained by using a sealed glass spherical ce114,and the Raman spectrum of the solid was obtained by condensing the sample onto a blackened brass block that is fixed at 15” to the incident radiation and cooled by boiling liquid nitrogen. The frequencies measured for sharp lines should be accurate to at least f 2 cm-l. The variable temperature experiment of the liquid was carried out using a Miller-Harney cell5 with the SPEX model 1403 double monocbromator
267
equipped with a third monochromator model 164 argon ion laser (5145 A>.
3.
(model 1442U) and a Spectra-Physics
CONF’ORMATIONAL ENERGY DIFFERENCE
As can be seen in Fig. 1, the Raman lines at 687 and 563 cm-1 show a significant change with temperature variation where the line at 563 cm-1 does not appear in the spectrum of the annealed solid. These lines are assigned to the C-Cl stretches for the gauche and bans conformers, respectively. By using the equation -1n.K = (AIVKT) - (A&R), the enthalpy difference between the gauche and tmns conformers can be calculated. In this case, K = I t, where I is the intensity of the Raman line attributed to the gauche co J ormer and 8.t 1s the intensity of the Raman line of the trans conformer. By assuming that AH is invariant with temperature, AH can be determined by a plot of -1nK versus Vl?. Five sets of spectral data of the two lines of 2-methylpropionyl chloride were obtained at temperatures ranging from 27 to -24°C. The plot of these data gives an enthalpy difference for the liquid of 345 f 93 cm-l (987 f 266 cal/mol).
4.
VIBRATIONAL A!%IGNMENT
Although the gauche rotamer (Cl symmetry) is found to be the most stable rotamer, the assignments provided in Table 1 are consistent with a molecule of C, symmetry, i.e., the higher energy trans conformation, due to the tendency of the isopropyl moiety to exhibit “local” C, symmetry. For the C, rotamer, the thirty-three fundamental modes span the representations: 19 A’ and 14 A” with the A’ modes giving rise to polarized Kaman lines. For Cl symmetry the thirty-three fundamental modes should give rise to polarized Raman lines in the liquid. There are seven carbon-hydrogen stretches for 2-methylpropionyl chloride. The c&H stretch is assigned to the line of medium intensity at 2928 cm-l in the Raman spectrum of the liquid. This value is less than that for the a-CH stretch in 2-methylpropanal2 (2973 cm-l) as well as slightly less than that for the fluoride3 (2938 cm-l). The methyl CH stretches of a-methylpropionyl chloride are found in the characteristic 2890 to 3000 cm-l region, with the antisymmetric stretches being assigned at higher frequencies. The weak Raman line at 1810 cm-lis assigned to the C=O stretch. This frequency is much higher than that observed for this mode in 2methylpropanal2 (1752 cm-l) but lower than that for the fluoride3 (1856 cm-l). There are three skeletal stretches in 2-meth lpropionyl chloride. The CCC antisymmetric stretch is assigned at 1093 cm- 9. The CC stretch and the CCC symmetric stretches are assigned at 924 and 842 cm-l, respectively. In 2-methylpropana12 the CC stretch and the CCC symmetric stretch have likewise been assigned at 920 and 799 cm-l, respectively.
268
750
625 Wavenumber
500 (cm“)
Figure 1. Variation of the Raman spectra of liquid 2-methylpropionyl chloride with temperature.
269
Table 1 Raman dataa for liquid and solid 2-methylpropionyl chloride. Liquid
2985
m
2945 2928 2914 2878 1810
vs m m m w
1460 vw 1451 vw 1322 1287 1173 1111 1096 966 940 932 847 687 642 563 437 416 337 320
w w w w w vw w w w m m m 8 s w m
224 w
Solid
Assignmentb
3005 m 2990 m 2983 m, 2977m 2966 w, 2945 w 2919 8 2913 w 2873 m 1800 vw 1480 w 1473 m 1463 m 1455 w 1447 w, 1442m 1392 w, 1372 w 1323 w, 1320 w 1280 w, 1264 w 1170 w 1123 s 1093 m 968 m 946 m 924 m 850 w, 842m 685 m 625 vs
“20 “21 “1 “2 “3 “4 “22 “5 “6 “23 “I “24 “8 “25 “9 “26 “10 “11 “27 “28 “29 “12 “13 “14 “30
442 vs, 439 w 408 vs
“15 “16
333 8 310 w 238 m 228 vw 222 vw 110 vw
“17 “18 “31 “19 “32 “33
79 m, 66 m, 57 w, 50 vs, 33 vw, 30 vw
CH3 antisymmetric stretch CH3 antisymmetric stretch CH3 antisymmetric stretch CH3 antisymmetric stretch c&H stretch CH3 symmetric stretch CH3 symmetric stretch C=O stretch CH3 antisymmetric deformation CH3 antisymmetric deformation CH3 antisymmetric deformation CH3 antisymmetric deformation CH3 symmetric deformation CH3 symmetric deformation a-CH bend a-CH bend CH3 rock CH3 rock CCC antisymmetric stretch CH3 rock CH3 rock CC stretch CCC symmetric stretch C-Cl stretch CC10 wag C-Cl stretch (Conformer II) CC10 deformation CC10 rock CC2 bend (Conformer II) CC2 deformation CC2 wag cc2 twist CH3 torsion CH3 torsion asymmetric torsion lattice modes
aAbbreviations used: s, strong; m, moderate; w, weak; and v, very. bNumbered for C, symmetry but gauche conformer is the more stable form.
270
The CH bending modes are observed in the characteristic 1480 to 900 cm-l region. The CH3 deformations are assigned in the 1440 to 1480 cm-l region with the corresponding A” modes being assigned to depolarized Raman lines in the spectrum of the liquid. The c&H in-plane and out-ofplane bends are assigned at 1322 and 1287 cm-l, respectively. In the case of 2-methylpropanal,z the c&H in-plane bend has been assigned at 1329 cm-l in the infrared spectrum of the gas, and the cc-CH out-of-plane bend has been assigned at 1290 cm-1 in the Raman spectrum of the gas. The CH3 rocks in 2-methylpropionyl chloride are assigned at 1170 and 1123 cm-l for the A modes and at 968 and 946 cm-1 for the A” modes. In 2-methylpropanal2 and 2-methylpropionyl fluoride the A’ CH3 rocks have also been assigned at approximately 200 cm-l higher in frequency than the corresponding A” modes. The C-Cl stretch of the gauche conformer is assigned to the Raman line at 687 cm-l and for the truns conformer at 563 cm-l. The latter band is not present in the spectrum of the solid. The bending modes of the CC10 moiety are tentatively assigned at higher frequency than the skeletal CC2 bending modes. The CC10 wag is assigned at 642 cm-l, the CC10 deformation at 437 cm-l, and the CC10 rock at 416 cm-1. These modes are undoubtedly mixed and a normal coordinate analysis needs to be carried out in order to provide a more accurate description of them. The low frequency skeletal bending modes and the torsional modes remain to be assigned. The CC2 deformation and CC2 wag are assigned at 333 and 310 cm-1 and the CC2 twist is assigned at 238 cm-1 in the Raman spectrum of the solid. However, in 2-methylpropanal2 the CC2 deformation has been assigned as the lowest skeletal bending mode at 272 cm-l. The symmetric and antisymmetric CH3 torsions of 2-methylpropionyl chloride are assigned at 228 and 222 cm-1 in the Reman spectrum of the solid. The asymmetric torsion of 2-methylpropionyl fluoride is assigned3 at 54 cm-1 for the gauche conformer and it shifts to 73 cm-l in the spectrum of the solid. We have assigned the corresponding mode for the chloride at 110 cm-l and believe the shift may be larger for this mode for the chloride. Further studies need to be carried out using ab initio calculations to provide more definitive descriptions of the normal modes. 6. REFERENCES 1 J. R. Durig, G. A. Guirgis, T. S. Little, and 0. L. Stiefvater, J. Chem. Phys., 91(1989) 738. 2 J. R. Durig, G. A. Guirgis, W. E. Brewer, and T. S. Little, J. Mol. Struct., 248 (1991) 49. 3 J. R. Durig, G. A. Guirgis, W. E. Brewer, and G. Baranovic, J. Phys. Chem., 96 (1992) in press. 4 R. Furic and J. R. Durig, Appl. Spectrosc., 42 (1988) 175. 5 F. A. Miller and B. M. Harney, Appl. Spectrosc., 24 (1970) 291.