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 been recorded for gaseous and solid phases of ethyldifluorosilane, CH3CH2SiHF2. The Raman spectrum of the liquid was also recorded and quantitative depolarization values were obtained. Both the trans and gauche conformers are present in the gas and liquid phases with the trans conformer being the more stable rotamer and the only rotamer present in the annealed solid spectrum. From a temperature study using the conformer pair observed at 714 and 646 cm-1 in the Raman spectrum of the liquid, 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 variable-temperature studies of the infrared spectrum with the sample dissolved in liquid argon were carried out and the value of the enthalpy 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 vibrational spectrum of ethyldifluorosilane has been assigned. Optimized structural parameters obtained using the RHF/3-21G, RHF/3-21G*, RHF/6-31G*, MP2/6-31G* and/or MP2/6-31+G* basis sets are reported for both conformers together with the total energies, dipole moments, rotational constants, unscaled and scaled force constants. The results are discussed and compared to the corresponding quantities obtained for some similar molecules.