Experimental and theoretical studies of silacycloheptatrienyl cation formation from phenylsilane

The formation of C6SiH7+ from phenylsilane was studied using Fourier transform ion cyclotron resonance spectrometry and by ab initio calculations. The (parent - H or D)(+) ions were produced by electron impact (EI) dissociation of C6H5SiH3 and C6H5SiD3, and their bimolecular reactivities toward phenylsilane, cycloheptatriene, benzyl chloride, benzene-d(6), and toluene-d(8) were examined. The reactive component that abstracts hydride from phenylsilane and cycloheptatriene or chloride from benzyl chloride is mostly the phenylsilyl cation C6H5SiH2+. The identity of the unreactive component was characterized by collision-induced dissociation and bimolecular chemical reactivity. The low-energy collision-induced dissociation of the unreactive C6SiH7+ ion with argon yielded SiH+ with a loss of C6H6 and C6SiH5+ with a loss of H-2 as the primary fragments. Vibrationally hot C6SiH7+ ions from EI of C6H5SiH3 reacted with benzene-d(6) to form C6D6 . SiH+ adducts, which after few seconds of cooling delay remained unreactive toward cycloheptatriene. Ab initio calculations predict the hydride affinity of C6H6 . SiH+ that forms C6H6 + SiH2 to be comparable to or lower than that of tropylium ion. The chloride affinity of C6H6 . SiH+ that forms C6H6 + SiHCl is estimated to be similar to 4 kcal mol(-1) lower than that of the benzyl cation. Both experiment and theory suggest the C6H6 . SiH+ adduct as the unreactive C6SiH7+ component and not the silacycloheptatrienyl cation. The mechanism of the formation of C6H6 . SiH+ is presented based on the theoretical energetics of radical cations and the transition state for the [1,2] sigmatropic migration of an alpha-H.