AB-INITIO CALCULATIONS OF VIBRATIONAL PROPERTIES AND CONFORMER STABILITIES IN 1,2-SI2H4CL2, 1,1,2-SI2H3CL3 AND 1,1,1-SI2H3CL3, AND COMPARISONS WITH OBSERVED INFRARED AND RAMAN SPECTRAL

Ab initio calculations of vibration frequencies and intensities have b een made for 1,2-Si2H4Cl2 (conformers 12G, 12T), 1,1,2-Si2H3Cl3 (confo rmers 112C(S), 112C(1)) and 1,1,1-Si2H3Cl3 (111), at an HF/6-31G leve l. Energies and abundancies of both conformers have been obtained for 12 and 112 at this level, and for 12 also at HF/tzvp, MP2/6-31G and M P2/tzvp levels. Similar energies are also reported for 1,2-Si2H4F2 at HF/6-31G and HF/tzvp levels. A simple method of scaling the HF/6-31G* frequencies permits assignment of a number of bands observed in the i nfrared and Raman spectra of 1,2-Si2H4Cl2, and of a few in infrared sp ectra of 112 and 111. Positions of unobserved bands are predicted to w ithin an expected accuracy of approximate to 10 cm(-1) The T conformer is slightly more stable than the G one in both 1,2-dichloro and 1,2-d ifluoro compounds, unlike the situation in the corresponding haloethan es. However, the relative stability of the 12G and 12T conformers is s omewhat sensitive to basis set and level of treatment. The best estima te for the abundance X(G) of 12G is approximate to 0.6, in rough agree ment with observed spectra. For the 112C(1) conformer, an HF/6-31G ca lculation gives X(C-1) approximate to 0.9, a likely upper limit.