Does Hydride Ion Transfer from Silanes to Carbenium Ions Proceed Via a Rate‐Determining Formation of a Silicenium Ion or Via a Rate‐Determining Electron Transfer? An Ab Initio Quantum Mechanical Study and a Curve‐Crossing Analysis

The hydride transfer reactions from simple silanes to carbenium ions are studied by ab initio calculations. The simplest reaction, H₄Si + CH₃⁺ → H₃Si⁺ + CH₄, is also studied with inclusion of the solvent effect (with the SCRF method) in the ab initio scheme. Under all conditions the preferred mechanism is the synchronous hydride transfer (SHT), which is barrierless in the gas phase but possesses small barriers in solution. The mechanistic alternative involving a rate‐determining single electron transfer (SET) step followed by H‐atom abstraction is found to be of very high energy. Modelling of the primary isotope effect for the SHT process of H₃SiH(D) + CH₃* → H₃Si⁺ + H₃CH(D) shows that the primary isotope effect is small, between ca. 1.1 and 2.7, for the entire relevant range of Si—H(D) distances (1.5–2.3 Å). Furthermore, the pattern of the computed primary isotope effect shows it to be an insensitive probe of the SHT mechanism. The curve‐crossing method is used to model the mechanistic dichotomy. It is shown that the reaction profiles for both SHT and SET arise from an avoided crossing between the ground state and a charge transfer state of the R₃SiH//R′₃C⁺ reactant pair. Thus, in the SHT mechanism a single electron switches sites in synchronicity with bond reorganization, while in SET the electron switch precedes the bond coupling. This avoided bond coupling is the foremost disadvantage of the SET mechanism. The common origin of the avoided crossing elucidates the reason why SHT exhibits characteristics of an electron transfer process without actually being a SET process.