On the Role of Hydrogen Bonding in Gas-Phase SN2 Reactions at Silicon

( )The shape of the potential energy surface (PES) of gas-phase S(N)2 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(N)2 reactions: X- + SiR3X -> XSiR3 + X-, where X = Cl or F and R = H, Me, or OMe. While the fluorine systems and the CISiH3Cl system only give single-well PESs, CISiMe3Cl and CISi(OMe)(3)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 XSiMe3X and XSi(OMe)(3)X systems. It is shown that this so far neglected stabilizing contribution, along with sigma-hole bonding, is responsible for the shapes of the PESs of ClSiMe3Cl and CISi(OMe)(3)Cl in the gas phase.