Relativistic effects in silicon chemistry: Are the experimental heats of formation of the silicon atom and SiH4 compatible?

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Abstract
We have investigated the effects of relativity on the atomization energy of silane, SiH 4 , to attempt to resolve an earlier discrepancy between theory and experiment. Using a spin-free no-pair Hamiltonian that is based on a second-order Douglas–Kroll transformation, we find that relativity reduces the atomization energy of SiH 4 by 0.7 kcal mol −1 : a small change, but sufficient to bring theory and experiment into agreement when we include experimental uncertainties. Excitation energies in the silicon atom, 5 S(sp 3 )– 3 P(s 2 p 2 ), and the atomic cation, 4 P(sp 2 )– 2 P(s 2 p), which involve a reduction in the number of s -electrons, increase ∼1.2 kcal mol −1 when we include relativity. These excitation energies show an even larger increase, about 2.5 kcal mol −1 , when we include core correlation. By contrast, the ionization potential, which involves no change in the number of s -electrons—electron configurations s 2 p 2 in the neutral atom and s 2 p in the cation—changes ∼0.2 kcal mol −1 when we include relativity. These predictions are consistent with the notion that s -electrons are the most affected by relativity, and that changes in the amount of s -character are related, qualitatively, to differential relativistic effects.