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

• Published in: International journal of mass spectrometry 1999, Vol.185, p.449-462
• Main Authors: Willard, Belinda B., Graul, Susan T.
• Format: Article
• Language: English
• Subjects: Metastable dissociation; Organosilicon; Theory; Unimolecular reaction
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/S1387-3806(98)14295-1

Metastable dissociations have been studied for trimethylsilyldimethylsilylenium ion, (CH 3) 3Si–Si +(CH 3) 2, and four bridged analogs, (CH 3) 3Si–X–Si +(CH 3) 2 (X = CH 2, O, NH, CC). Several dissociation pathways are observed, with branching ratios that vary significantly with the bridging group. This article focuses on two competing pathways: elimination of methane and of ethylene. The energetics of ethylene elimination from (CH 3) 3Si–Si +(CH 3) 2 have been characterized by molecular orbital and density functional theory, and the kinetic energy distributions of the products of the dissociation have been modeled with statistical phase space theory, yielding good agreement with experiment. Methane elimination occurs across the two silicon centers, and is accompanied by a large release of kinetic energy, suggestive of a concerted reaction and a considerable reverse activation barrier in the 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 this elimination reaction is proposed. A comparison of the kinetic energy release distributions observed for methane elimination from the bridged ions suggests that an analogous mechanism is involved for the entire series.