On the Role of Hydrogen Bonding in Gas-Phase S2 Reactions at Silicon.

The shape of the potential energy surface (PES) of gas-phase S2 reactions at silicon is determined by the type of nucleophile, the leaving group, and substituents which remain bonded to silicon. In this study, we present PES scans along the reaction coordinate of six symmetrical S2 reactions: X + SiRX → XSiR + X, where X = Cl or F and R = H, Me, or OMe. While the fluorine systems and the ClSiHCl system only give single-well PESs, ClSiMeCl and ClSi(OMe)Cl give triple- and double-well PESs with stable pre- and post-reaction complexes. A complementary bonding analysis (energy decomposition analysis, quantum theory of atoms in molecules, and natural bond orbitals) reveals that the leaving group (X) is stabilized by hydrogen bonding in the XSiMeX and XSi(OMe)X systems. It is shown that this so far neglected stabilizing contribution, along with σ-hole bonding, is responsible for the shapes of the PESs of ClSiMeCl and ClSi(OMe)Cl in the gas phase.