A Simple Electrostatic Model for Trisilylamine: Theoretical Examinations of the n→σ* Negative Hyperconjugation, pπ→dπ Bonding, and Stereoelectronic Interaction

Abstract

A block-localized wave function method was used to examine the stereoelectronic effects on the origin of the structural difference between trisilylamine and trimethylamine. The pyramidal geometry of trimethylamine along with its high basicity is consistent with the traditional VSEPR (valence shell electron-pair repulsion) model for σ bonding. On the other hand, in trisilylamine, the silicon d orbitals make modest contribution to the electronic delocalization, although the key factor in charge delocalization is still nN→σSiH* negative hyperconjugation. Interestingly, the gain in pπ→dπ bonding stabilization is offset by a weaker negative hyperconjugation effect in trisilylamine, resulting in an overall smaller delocalization energy (−18.5 kcal/mol) than that in trimethylamine (−23.9 kcal/mol), which contains little pπ→dπ bonding character. Significantly, because of the relatively low electronegativity of silicon, the N−Si bond is much more polar than the N−C bond. Weinhold's natural population analyses of t...