Gas-Phase Acidities of Methylsilanes: C-H versus Si-H

The gas-phase acidities of silane, methylsilane, dimethylsilane, trimethylsilane, tetramethylsilane, and phenylsilane have been measured in the flowing afterglow by using standard techniques. In those methylsilanes having both C-H and Si-H bonds, abstraction occurs from both positions and, correspondingly, both C-H and Si-H acidities are reported. Quite unusually, the thermodynamically more stable products resulting from Si-H abstraction by base are not the first formed products. Thus, C-H abstraction occurs readily by a kinetically more accessible path. The measured gas-phase acidities are used to calculate electron affinities for the corresponding anions. The electron binding energies and basicity of the methyl-substituted silyl anions do not vary with methyl substitution. Furthermore, phenyl-substituted silyl anion has the same electron binding energy and basicity as the silyl anion itself.