Experimental and theoretical evidence for homogeneous catalysis in the gas-phase reaction of SiH(2) with H(2)O (and D(2)O): a combined kinetic and quantum chemical study

Time-resolved kinetic studies of the reaction of silylene, SiH(2), with H(2)O and with D(2)O have been carried out in the gas phase at 297 K and at 345 K, using laser flash photolysis to generate and monitor SiH(2). The reaction was studied independently as a function of H(2)O (or D(2)O) and SF(6) (bath gas) pressures. At a fixed pressure of SF(6) (5 Torr), SiH(2)] decay constants, k(obs), showed a quadratic dependence on H(2)O] or D(2)O]. At a fixed pressure of H(2)O or D(2)O, k(obs) values were strongly dependent on SF(6)]. The combined rate expression is consistent with a mechanism involving the reversible formation of a vibrationally excited zwitterionic donor-acceptor complex, H(2)Si...OH(2) (or H(2)Si...OD(2)). This complex can then either be stabilized by SF(6) or it reacts with a further molecule of H(2)O (or D(2)O) in the rate-determining step. Isotope effects are in the range 1.0-1.5 and are broadly consistent with this mechanism. The mechanism is further supported by RRKM theory, which shows the association reaction to be close to its third-order region of pressure (SF(6)) dependence. Ab initio quantum calculations, carried out at the G3 level, support the existence of a hydrated zwitterion H(2)Si...(OH(2))(2), which can rearrange to hydrated silanol, with an energy barrier below the reaction energy threshold. This is the first example of a gas-phase-catalyzed silylene reaction.