SPECTRA AND STRUCTURE OF SILICON-CONTAINING COMPOUNDS .29. RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT, BARRIER TO INTERNAL-ROTATION, AND AB-INITIO CALCULATIONS OF ETHYLDIFLUOROSILANE

Raman (3200 to 10 cm-1) and infrared (3200 to 400 cm-1) spectra have b een recorded for gaseous and solid phases of ethyldifluorosilane, CH3C H2SiHF2. The Raman spectrum of the liquid was also recorded and quanti tative depolarization values were obtained. Both the trans and gauche conformers are present in the gas and liquid phases with the trans con former being the more stable rotamer and the only rotamer present in t he annealed solid spectrum. From a temperature study using the conform er pair observed at 714 and 646 cm-1 in the Raman spectrum of the liqu id, the enthalpy difference was found to be 41 +/- 9 cm-1 (117 +/- 25 cal mol-1) with the trans rotamer the more stable conformer. Similar v ariable-temperature studies of the infrared spectrum with the sample d issolved in liquid argon were carried out and the value of the enthalp y difference was determined to be 96 +/- 27 cm-1 (275 +/- 77 cal mol-1 ). This value should be near the DELTAH value for the gas. Assisted by ab initio calculations and normal coordinate analysis, the vibrationa l spectrum of ethyldifluorosilane has been assigned. Optimized structu ral parameters obtained using the RHF/3-21G, RHF/3-21G, RHF/6-31G*, M P2/6-31G and/or MP2/6-31+G* basis sets are reported for both conforme rs together with the total energies, dipole moments, rotational consta nts, unscaled and scaled force constants. The results are discussed an d compared to the corresponding quantities obtained for some similar m olecules.