Spectra and structure of silicon containing compounds. XLII. Conformational stability, ab initio calculations, r 0 structural parameters, and vibrational assignments for ethylmethylsilane and ethylmethylsilane-Si-d 2 (original) (raw)
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Journal of Molecular Structure
The infrared (3100 -40 cm 21 ) spectra of gaseous and solid and Raman (3200 -100 cm 21 ) spectra of liquid cyclopropylbromosilane, c-C 3 H 5 SiH 2 Br, have been recorded. Both the cis and gauche conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (2105 to 2 150 8C) studies of the infrared spectra of the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 126^25 cm 21 (1.50^0.30 kJ/mol) but with the cis conformer the more stable form in the rare gas solution. At ambient temperature it is estimated that there is 52^2% of the gauche conformer present. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, and ab initio predicted frequencies. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energy differences have been obtained for the cis and gauche conformers from ab initio MP2/631G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311 þ G(d,p) and 6-311 þ G(2d,2p) basis sets as well as these bases sets without diffuse functions. The predicted conformer stability and energy differences from these calculations agree with the experimentally determined stability and enthalpy difference except for the calculations at the Hartree-Fock level. Similar calculations by density functional theory (DFT) by the B3LYP method with eight different basis sets up to 6-311 þ G(2d,2p) gave incorrect conformer stabilities with the gauche conformer more stable by approximately 130 cm 21 from all calculations. Similar DFT calculations for cyclopropylchlorosilane also give incorrect predicted conformational stabilities. The results are discussed and compared to those obtained for some similar molecules. q
Journal of Molecular Structure
The infrared (3100 -40 cm 21 ) spectra of gaseous and solid and Raman (3200 -100 cm 21 ) spectra of liquid cyclopropylbromosilane, c-C 3 H 5 SiH 2 Br, have been recorded. Both the cis and gauche conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (2105 to 2 150 8C) studies of the infrared spectra of the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 126^25 cm 21 (1.50^0.30 kJ/mol) but with the cis conformer the more stable form in the rare gas solution. At ambient temperature it is estimated that there is 52^2% of the gauche conformer present. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, and ab initio predicted frequencies. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energy differences have been obtained for the cis and gauche conformers from ab initio MP2/631G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311 þ G(d,p) and 6-311 þ G(2d,2p) basis sets as well as these bases sets without diffuse functions. The predicted conformer stability and energy differences from these calculations agree with the experimentally determined stability and enthalpy difference except for the calculations at the Hartree-Fock level. Similar calculations by density functional theory (DFT) by the B3LYP method with eight different basis sets up to 6-311 þ G(2d,2p) gave incorrect conformer stabilities with the gauche conformer more stable by approximately 130 cm 21 from all calculations. Similar DFT calculations for cyclopropylchlorosilane also give incorrect predicted conformational stabilities. The results are discussed and compared to those obtained for some similar molecules. q
Journal of Molecular Structure, 2002
The Raman spectrum of liquid ethyldibromosilane, CH 3 CH 2 SiHBr 2 , was recorded at various temperatures between 298 and 218 K, and the spectrum of the crystal was obtained at 210 K from a sample sealed in a capillary. Additional spectra of the amorphous and annealed crystals, which were deposited on a copper finger cooled with boiling liquid nitrogen, were recorded. The infrared spectra were recorded of the vapor and amorphous and crystalline solid in the range 4000-50 cm −1 and mid-infrared spectra isolated at 4.8 K in argon and nitrogen matrices were also observed. These vibrational spectra show that two conformers, anti and gauche, are present in the vapor and in the liquid, but only the anti conformer remains in the crystalline solid. Three conformer pairs in the Raman spectrum of the liquid phase were used to obtain the enthalpy difference, which gave an average value of 128 ± 17 cm −1 (1.53 ± 0.2 kJ mol −1 ) with the anti form lower in energy. At ambient temperature it is estimated that there is 52 ± 2% of the gauche conformer present in the liquid. The optimized geometries, infrared and Raman intensities, and scaled vibrational wavenumbers for the anti and gauche conformers were obtained from ab initio MP2/6-31G(d) calculations with full electron correlation. The conformational energy difference was also obtained from ab initio MP2/6-311 + G(d,p) calculations which gave a predicted energy difference of 97 cm −1 with the anti form the conformer of lower energy. These spectroscopic and theoretical results are discussed and compared with the corresponding quantities for some similar molecules.
Journal of Molecular Structure, 2001
The infrared spectra of 1,2-diphenylethane were recorded as a melt and crystalline solid in capillary films and as a pellet in KBr and polyethylene in the 4000-50 cm 21 range. Moreover, the sample was isolated in an argon matrix at ca. 5 K, and the spectra recorded in the range 3100-450 cm 21 , before and after annealing. Raman spectra of the melt were recorded between 295 (supercooled) and 357 K, and spectra of the crystalline solid were obtained at ambient temperature.
Journal of Molecular Structure, 2001
The infrared spectra of 2-chloroethyl trifluorosilane (ClCH 2 CH 2 SiF 3 ) were recorded in the vapour, amorphous and crystalline solid phases in the range 4000-50 cm Ϫ1 . Spectra of the compound isolated in argon and nitrogen matrices at 10 K and variable temperature spectra in liquified xenon were obtained. Raman spectra of the liquid were recorded at various temperatures between 298 and 163 K, and the amorphous and crystalline solids studied. The spectra of 2-chloroethyl trifluorosilane showed the existence of two conformers-anti and gauche-present in the vapour and in the liquid. Large variations in the infrared and Raman spectra occurred during the annealing and an intermediate, probably metastable crystal appeared at ca. 125 K with pronounced crystal splitting and apparently containing both conformers. More than 15 infrared and Raman bands present in the fluid phases and in the 125 K crystal vanished in the stable crystal formed at ca. 160 K. From intensity variations with temperature of five Raman band pairs, a DHЊanti-gauche 0:8^0:3 kJ mol Ϫ1 was obtained in the liquid, and in liquid xenon under pressure a DH value of Ϫ0:7^0:1 kJ mol Ϫ1 was obtained from IR spectra. Annealing experiments indicate that the anti conformer has the lower energy in argon and nitrogen matrices as well as in xenon, and the barrier seemed to be ca. 8 kJ mol Ϫ1 . Ab initio calculations were performed using the Gaussian 94 program with the HF/6-311G* basis set and gave optimized geometries, infrared and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers. The conformational energy derived was 3.8 kJ mol Ϫ1 giving anti as the low energy conformer. ᭧
Journal of Raman Spectroscopy, 2001
The infrared spectra of dichloromethylmethyldichlorosilane (Cl 2 CHCH 3 SiCl 2 ) were recorded in the vapour, amorphous and partly crystalline phases and when isolated in argon, nitrogen and krypton matrices at 5 K. Raman spectra of the liquid were recorded at various temperatures between 295 and 147 K. Spectra of the amorphous and annealed crystal deposited on a copper finger at 80 K were obtained. The spectra showed the existence of two conformers, anti and gauche, in the vapour and in the liquid. Approximately 12 infrared bands were reduced in intensity, and six of the same bands present in the fluid phases in the Raman spectra vanished upon crystallization. From the intensity variations of two band pairs with temperature, a 1H • value of 0.4 ± 0.3 kJ mol −1 between the conformers was obtained in the liquid, anti being the low-energy conformer. Small increases and decreases in the IR band intensities were observed in the matrix spectra after annealing to 32 K (nitrogen), 36 K (argon) and 50 K (krypton). The enthalpy difference between the conformers is low in the matrices, but the anti conformer had lower energy than the gauche conformer. Ab initio calculations were performed at the HF/6-311G * level and gave optimized geometries, vibrational wavenumbers and infrared and Raman intensities for the anti and gauche conformers. The conformational energy derived was 4.4 kJ mol −1 with anti being the low-energy conformer. The dipole moments were calculated to be 1.2 and 3.1 D for the anti and gauche conformers, respectively. Correlation between the observed and calculated wavenumbers of both conformers revealed that anti was present in the crystal, and complete assignments of the spectra were carried out.
Journal of Molecular Structure, 2011
The infrared spectra (3500 -400 cm 21 ) of 3-methyl-1-butene, CH 2 yCHCH(CH 3 ) 2 , and dimethylvinylsilane, CH 2 yCHSiH(CH 3 ) 2 , dissolved in liquid xenon and krypton, respectively, have been recorded. From temperature (255 to 2 110 8C) dependent FT-IR spectra of xenon solutions, it is shown that the cis conformer (hydrogen atom eclipsing the double bond) of 3-methyl-1-butene is the more stable form with an enthalpy difference of 65^8 cm 21 (0.78^0.10 kJ/mol) which agrees with all of the theoretical predictions. Similar studies for dimethylvinylsilane in krypton solutions (2 105 to 2 150 8C) give an enthalpy difference of 55^5 cm 21 (0.66^0.06 kJ/mol) with the gauche conformer (methyl group eclipsing the double bond) the more stable rotamer which is consistent with the ab initio calculations except with the largest basis set MP2/6-311 þ G(2d,2p) utilized. Both ab initio calculations with full electron correlation by the perturbation method to the second order (MP2) and density functional theory (DFT) calculations by the B3LYP method have been carried out with a variety of basis sets up to 6-311G(2d,2p) with and without diffuse functions for both molecules to predict the optimized geometries and conformation stabilities. Utilizing previously reported microwave rotational constants for both conformers of 3-methyl-1butene along with ab initio predictions, adjusted r 0 parameters have been obtained. The r 0 Si -H bond distances of 1.489 and 1.490 Å have been obtained for the cis and gauche conformers, respectively, from the silicon-hydrogen stretching frequencies and the remaining parameters are estimated from the MP2/6-311 þ G(d,p) ab initio calculations. q
Journal of Molecular Structure, 2001
Chloromethyl methyl dichlorosilane (ClCH 2 CH 3 SiCl 2 ) was synthesized and the infrared spectra of its vapour, and of the amorphous and crystalline states, cooled to the temperature of liquid nitrogen were recorded. Additional mid IR spectra of the compound, isolated in both argon and nitrogen matrices, were taken at 5 K. Raman spectra of the liquid were recorded in a capillary at various temperatures between 298 and 223 K. The crystal was also studied in a capillary at 210 K and deposited onto a copper ®nger cooled with liquid nitrogen.
Journal of Molecular Structure, 2006
The Raman spectra (4000-100 cm K1) of liquid and solid and infrared spectra (4000-400 cm K1) of gaseous, liquid and solid cyclohexyl silane, cC 6 H 11 SiH 3 , have been recorded and assigned. These spectral data indicate the presence of two conformers in the fluid states. Variable temperature (21 to K71 8C) Raman spectra of the liquid were also recorded and by utilizing three conformer pairs, an enthalpy difference of 520G70 cm K1 (6.22G0.84 kJ/mol) was obtained with the chair-equatorial form the more stable conformer. It is calculated that at ambient temperature there is only 7.5G2.4% of the chair-axial form present in the liquid phase. Comparison with the spectra of the polycrystalline solid phase shows that the chair-equatorial conformer is the only form remaining in the solid. MP2 and DFT calculated conformational energy differences are slightly larger but in reasonable agreement with the experimental value. In addition, force constants, infrared intensities, Raman activities, depolarization ratios, scaled vibrational frequencies and potential energy distributions have been calculated from the MP2(full)/6-31G(d) results for both chair forms. These data support the complete vibrational assignment for the chair-equatorial form as well as the assignments for several of the fundamentals of the chair-axial form. By utilizing a series of sum and difference bands on the SiH stretching modes, the barrier to SiH 3 internal rotation has been determined to be 684G10 cm K1 (8.18G0.12 kJ/mol) for the chair-equatorial form. Estimated r 0 structural parameters have been obtained for both conformers by adjusting MP2(full)/6-311CG(d,p) structural predictions. Three additional twist forms (equatorial, axial and form III) were also predicted to be local minima since all calculated vibrational frequencies are real. However, all three twist forms are much higher in energy (2000-3000 cm K1) than the chair forms. Effects of electronegativity and steric effect on the conformational stability are compared among a series of mono-substituted cyclohexanes by NBO analyses of the donoracceptor delocalization interactions.
Journal of Molecular Structure, 1999
The infrared spectra of gaseous and solid chloromethyl methyl silane, ClCH 2 SiH 2 CH 3 , and chloromethyl methyl difluorosilane, ClCH 2 SiF 2 CH 3 , were recorded. Additionally, the variable temperature infrared spectra of the sample dissolved in liquid krypton and/or xenon were obtained. Both the trans and gauche conformers are present in the fluid phases but only the trans conformer is present in the solid for both molecules. The enthalpy difference between the two conformers for chloromethyl methyl silane was determined to be 180^15 cm Ϫ1 (2.16^0.18 kJ/mol) and for chloromethyl methyl difluorosilane 2361 5 cm Ϫ1 (2.82^0.17 kJ/mol) in xenon with the gauche conformers more stable for both molecules. The enthalpy value for the silane in krypton is 181^18 cm Ϫ1 (2.16^0.09 kJ/mol). Ab initio calculations were carried out at the RHF/6-31G* and MP2/6-31G* levels for both molecules from which structural parameters and conformational stabilities were determined. ᭧