Loss of benzene from methylphenylsilane and dimethylphenylsilane molecular cations

The dissociations of methylphenylsilane and dimethylphenylsilane molecular cations have been investigated by obtaining the 70-eV electron ionization mass spectra and the metastable ion decomposition spectra. Benzene loss from these molecular ions was noticeable compared to the dissociation of phenylsilane molecular cation. To understand the overall dissociation mechanisms, density functional theory calculations were performed at the B3LYP/6-311++G(d,p) level. It is proposed that the benzene loss occurs via the ion-molecule complexes formed by 1,2 shift of an α-H atom from the silicon to the ipso-carbon. The experimental observation that the dimethylphenylsilane ion loses benzene more easily than the methylphenylsilane ion can be understood by considering the stabilities of the product ions.