A theoretical study of silyl anions and radicals. Change of mechanism from vertex inversion to edge inversion with increasing substitution by halogens

Abstract

Structures for trivalent silyl anions [Formula: see text] and [Formula: see text], where n takes values from 0 to 3, have been optimized at the HF/6-31 + +G* level and single point calculations made at the MP2/6-31 + +G* level (core included). SiH3− and ions containing one halogen invert by the vertex mechanism in which the lone-pair has π-symmetry, and the monosubstituted ions have high barriers (SiH2F− 45.2 kcal/mol and SiH2Cl− 44.0 kcal/mol). Further substitution by halogens results in a change to the edge inversion mechanism involving a T-shaped transition structure with the lone-pair coplanar with the ligands. Barriers (kcal/mol) at the MP2/6-31 + +G* level including ZPE are lower than for the monosubstituted ions and are SiHF2− 35.0, SiF3− 35.9, SiHCl2− 28.4, and SiCl3−32.5. In SiLi3− edge inversion is preferred, but the surface is much flatter and the barrier is low (9.8 kcal/mol). Trivalent silyl radicals SiHnF(3−n), SiHnCl(3−n) and SiFnCl(3−n) (with n having values 0 to 3) have also been examined with the 6-31 + +G* basis set, with optimization at the UHF level and single point calculations at the UMP2 level. Radicals SiH3, SiH2F, SiH2Cl, and SiHCl2 all invert by the vertex mechanism. Increased halogenation results in a change of mechanism and SiF3, SiCl3, SiF2Cl, and SiFCl2 invert by the edge mechanism. For radical SiHF2 the calculated barriers for the two mechanisms are almost identical with the higher level of theory slightly favouring edge inversion. Keywords: inversion mechanism, halogenated silyl radicals and anions.