Reactions of Silylene with Unreactive Molecules. I:  Carbon Dioxide; Gas-Phase Kinetic and Theoretical Studies

Abstract

Time-resolved studies of the reaction of silylene, SiH 2, with CO 2 have been carried out over the temperature range 298−681 K, using laser flash photolysis to generate and monitor SiH 2 . Between 339 and 681 K the reaction obeys second-order kinetics and the derived rate constants gave the following Arrhenius parameters: log( A /cm 3 molecule -1 s -1 ) = −11.89 ± 0.13, E a = +16.36 ± 1.23 kJ mol -1, where the uncertainties are single standard deviations. The reaction shows no overall pressure dependence. This reaction is unusual for SiH 2 in being extremely slow and having a positive activation energy. Ab-initio calculations at the G2 level suggest a mechanism occurring via the intermediacy of siloxiranone (formed via addition of SiH 2 to one of the C O double bonds) leading to overall formation of H 2 SiO + CO, consistent with kinetic findings. Direct abstraction of an O atom appears to be ruled out, as are other potential pathways. The mechanism resembles that of CH 2 ( 1 A 1 ) + CO 2 .