ABINITIO STUDY OF THE STRUCTURES, PROPERTIES, AND HEATS OF FORMATION OF FLUOROCHLOROMETHANES AND FLUOROCHLOROSILANES, CH4-M-NFMCLN AND SIH4-M-NFMCLN(M+N=0-4)

Equilibrium geometries and vibrational frequencies for CH4-m-nFmCln and SiH4-m-nFmCln (m + n - 0-4) were computed at the HF/6-31G* levels; electron correlation contributions were calculated at the MP4/6-31G** level. Heats of formation were obtained from isodesmic reactions using the experimental DELTA-H(f)-degrees for AH4, AF4, and ACl4 (A = C, Si). Good agreement between theory and experiment is found for CH4-m-nFmCln. For the mixed fluorochlorosilanes, the calculations predict the following heats of formation (in kcal/mol at 298 K and 1 atm): SiH2FCl, -129.3 +/- 3; SiHF2Cl,-230.3 +/- 3; SiHFCl2,-171.6 +/- 3; SiF3Cl, -329.5 +/- 3; SiF2Cl2,-272.9 +/- 3; SiFCl3, -215.8. Trends in the bond lengths, AH vibrational frequencies, and stabilization energies can be explained as a balance between electrostatic effects due to the electronegative substituents and negative hyperconjugation from the halogen lone pairs to adjacent sigma* orbitals.