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

Abstract

We have investigated the effects of relativity on the atomization energy of silane, SiH4, 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 SiH4 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, S5(sp3)–3P(s2p2), and the atomic cation, P4(sp2)–2P(s2p), 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 s2p2 in the neutral atom and s2p 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.