On the Role of Hydrogen Bonding in Gas-Phase S_N2 Reactions at Silicon

The shape of the potential energy surface (PES) of gas-phase S_N2 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 S_N2 reactions: X^- + SiR₃X → XSiR₃ + X^-, where X = Cl or F and R = H, Me, or OMe. While the fluorine systems and the ClSiH₃Cl system only give single-well PESs, ClSiMe₃Cl and ClSi(OMe)₃Cl 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 XSiMe₃X and XSi(OMe)₃X 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 ClSiMe₃Cl and ClSi(OMe)₃Cl in the gas phase.