Binding interaction of the trimethylsilyl cation with oxygen and nitrogen bases in the gas phase. Acetopheones, benzaldehydes, pyridines, anilines, and N,N-dimethylanilines

The gas-phase basicities toward the trimethylsilyl cation, GB(Me3Si+), were determined for a series of acetophenones, benzaldehydes, pyridines, anilin es, and N,N-dimethylanilines by measuring the equilibrium constants of trim ethylsilyl cation transfer reactions. GB(Me3Si+) increases in the order ani line < benzaldehyde < acetophenone < N,N-dimethylaniline < pyridine, while GB(H+) increases as benzaldehyde < acetophenone < aniline < pyridine < N,N- dimethylaniline. The effects of ring substituents on the GB(Me3Si+) of the respective bases are, however, linearly correlated with the corresponding p roton basicities with a slope of 0.8 to 1.0, showing family-dependent linea r relationships. Furthermore, an analysis of the substituent effects on the GB(Me3Si+) of acetophenone and benzaldehyde showed that the rho values are close to those for the proton basicities, though the resonance demand of M e3Si+ adduct ions is slightly reduced compared with that for the protonated ones. DFT calculations at the B3LYP/6-31+G(d) level of theory reproduced s uch a trend in substituent effects as well as the family-dependent linear c orrelations between the H+ and Me3Si+ basicities. Both the experimental and theoretical results lead to the conclusion that the binding interactions o f Me3Si+ with O and N atoms of organic bases have a pronounced covalent cha racter to a similar degree as that of a proton.