Bimolecular Reaction Dynamics in the Phenyl-Silane System: Exploring the Prototype of a Radical Substitution Mechanism.

We present a combined experimental and theoretical investigation of the bimolecular gas-phase reaction of the phenyl radical (CH) with silane (SiH) under single collision conditions to investigate the chemical dynamics of forming phenylsilane (CHSiH) via a bimolecular radical substitution mechanism at a tetracoordinated silicon atom. Verified by electronic structure and quasiclassical trajectory calculations, the replacement of a single carbon atom in methane by silicon lowers the barrier to substitution, thus defying conventional wisdom that tetracoordinated hydrides undergo preferentially hydrogen abstraction. This reaction mechanism provides fundamental insights into the hitherto unexplored gas-phase chemical dynamics of radical substitution reactions of mononuclear main group hydrides under single collision conditions and highlights the distinct reactivity of silicon compared to its isovalent carbon. This mechanism might be also involved in the synthesis of cyanosilane (SiHCN) and methylsilane (CHSiH) probed in the circumstellar envelope of the carbon star IRC+10216.