Theoretical studies of silicon-containing molecules

We report herein, a series of ab initio calculations on various SiH_n and Si₂H_n species using generalized valence bond and configuration interactions methods. These results allow systematic estimation of the heats of formation and the enthalpies of reactions for these species. For SiH_n compounds, the calculated and experimental energetics are in excellent agreement; we obtain D₃₀₀ (H₃SiH) ‡ 91.6 kcal mol^-1 (experiment, 90.3 ± 1), D₃₀₀ (SiH) = 68.2 kcal (experiment ≤70 kcal), and an energy of atomization for SiH₄ of 303.4 kcal mol^-1 (experiments yield 307.9). However, for silylene, we find Δ H_f = 67 kcal mol^-1, 9 kcal mol^-1 higher than the (indirect) experimental value and suggest re-examination of these experiments. We calculate D₃₀₀ (H₃SiSiH₃) = 71.7 kcal mol^-1, in good agreement with the experimental value of 73.7 ± 2 kcal mol^-1. Particularly interesting is disilene, where it is found that (i) the ground state is nonplanar (trans) (with a barrier to planarization of ≈ 1 kcal mol^-1), (ii) disilene is stable relative to the isomer silylsilylene (H₃SiSiH) by about 6 kcal mol^-1, and (iii) the SiSi double bond energy is 56 kcal mol^-1, which is 16 kcal mol^-1 weaker than the SiSi single bond in disilane! The origin of this effect is explained in terms of GVB bonding.