Silylanions: Inversion barriers and NMR chemical shifts

alpha-Substituent effects on inversion barriers and NMR chemical shifts have been studied on a set of silyl anions, [X3-nYnSi](-) (X, Y = H, CH3, and SiH3). The MP2/6-31 + G* optimized structures show a pattern of increasing inversion barriers with augmenting numbers of methyl substituents. The highest barrier of 48.5 kcal mol(-1) is obtained for the (CH3)(3)Si ion. The silyl group displays the opposite effect by decreasing the inversion barrier to a minimum of 16.3 kcal mol - 1 in (SiH3)(3)Si-. The influence of counterions on these barriers is probed by addition of a lithium or potassium cation. In most cases, a decrease of the energy barriers with respect to the bare anions is observed. The Si-29 NMR chemical shifts calculated at the IGLO-DFT and GIAO-MP2 level of theory are also analyzed in view of the substituents and counterions.