Infrared and raman spectra, conformational stability, barriers to internal rotation, ab initio calculations, ro structure, and vibrational assignment for methyl vinyl silane

Abstract The infrared (3250 to 40 cm −1 ) and Raman (3250 to 10 cm −1 ) spectra of gaseous and solid methyl vinyl silane (CH 2 CHSiH 2 CH 3 ) have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. From the far infrared spectrum of the gas, the fundamental asymmetric torsions for both the cis and gauche conformers have been observed at 98.68 and ∼73 cm −1 , respectively, with the cis conformer having several upper state transitions. From these transitions, the potential function to internal rotation has been determined with the following values: V 1 = 56 ± 11, V 2 = 79±6, V 3 = 600 ± 16, V 4 = 31 ± 6, and V 6 = 64 ± 6 cm −1 , with the cis conformer thermodynamically preferred by 124 ± 33 cm −1 (355 ± 94 cal mol −1 ). The cis to gauche , gauche to gauche , and gauche to cis barriers are 696 (1.99 kcal mol −1 ), 532 (1.52 kcal mol −1 ), and 572 cm −1 (1.64 kcal mol −1 ), respectively. A variable temperature study of the Raman spectrum of the liquid was carried out and the enthalpy difference was found to be 254 ± 16 cm −1 (726 ± 46 cal mol −1 ) with the cis conformer being the more stable form. A complete vibrational assignment is proposed based on infrared band contours, depolarization ratios, and group frequencies. The assignment is supported by an ab initio calculation utilizing the 3-21G* basis set to obtain the force constants and potential energy distribution. Additionally, ab initio calculations have been carried out utilizing the 6-31G* basis set to obtain the conformational stability, barriers to internal rotation and optimized structural parameters. These results are compared with the experimentally determined values as well as to corresponding results of some related molecules.