Competitive elimination of methane and ethylene from trimethylsilyl-substituted silylenium ions

Metastable dissociations have been studied for trimethylsilyldimethylsilyle nium ion, (CH3)(3)Si-Si+(CH3)(2), and four bridged analogs, (CH3)(3)Si-X-Si +(CH3)(2) (X = CH2, O, NH, C=C). Several dissociation pathways are observed , with branching ratios that vary significantly with the bridging group. Th is article focuses on two competing pathways: elimination of methane and of ethylene. The energetics of ethylene elimination from (CH3)(3)Si-Si+(CH3)( 2) have been characterized by molecular orbital and density functional theo ry, and the kinetic energy distributions of the products of the dissociatio n have been modeled with statistical phase space theory, yielding good agre ement with experiment. Methane elimination occurs across the two silicon ce nters, and is accompanied by a large release of kinetic energy, suggestive of a concerted reaction and a considerable reverse activation barrier in th e exit channel. Electronic structure calculations combined with statistical phase space modeling of the dissociation kinetics suggest that the product from methane elimination is a disilacyclobutyl cation. A mechanism for thi s elimination reaction is proposed. A comparison of the kinetic energy rele ase distributions observed for methane elimination from the bridged ions su ggests that an analogous mechanism is involved for the entire series. (C) 1 999 Elsevier Science B.V.