Dimethylsilanone Enolate Anion: Competitive Fragmentation and Electron Autodetachment of Vibrationally Excited Siloxide Anions in the Gas Phase

Article abstract of DOI:10.1021/ja00270a009

Infrared multiple photon photochemical studies of gas-phase siloxide anions trapped in an ion cyclotron resonance spectrometer are reported.Upon CO2 laser irradiation, trimethylsiloxide anion, 1, and dimethylsiloxide anion, 3, eliminate methane and molecular hydrogen, respectively, resulting in the production of dimethylsilanone enolate anion, the silicon-containing analogue of acetone enolate anion.These fragmentation reactions are analogous to the unimolecular decompositions of gas-phase alkoxide anions which react via a stepwise mechanism involving initial heterolytic cleavage to an intermediate anion-ketone complex and a subsequent proton-transfer reaction within the ion-molecule complex.Extension of this mechanism to the siloxide systems suggests that dimethylsilanone is an intermediate in the decomposition.In contrast to the alkoxides, vibrationally excited siloxide anions also undergo electron loss (vibrationally induced electron detachment, VED).The use of both high-power pulsed and low-power continuous-wave (CW) CO2 lasers provides access to both available reaction channels and aids in the elucidation of the primary photochemical events.For 1, pulsed laser activation results in both methane elimination and VED, while activation with the CW laser leads to electron loss exclusively.Products resulting from both fragmentation and electron detachment are observed for 3 activated by either laser.A significant contribution from secondary photochemistry was found in the pulsed laser-induced reactions of 3.The observation of direct photochemical branching between fragmentation (H2 elimination) and electron detachment (VED) in the CW laser photolysis of 3 provides an estimate of 62 kcal/mol as an upper limit for the silicon-oxygen ? bond energy in dimethylsilanone.