Elucidating the structures of isomeric silylenium ions (SiC3H9+, SiC4H11+, SiC5H13+) by using specific ion/molecule reactions

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Abstract
Specific ion/molecule reactions are demonstrated that distinguish the structures of the following isomeric organosilylenium ions: Si(CH3)3+ and SiH(CH3)(C2H5)+; Si(CH3)2(C2H5)+ and SiH(C2H5)2+; and Si(CH3)2(i−C3H7)+, Si(CH3)2(n−C3H7)+, Si(CH3)(C2H5)2+ , and Si(CH3)3(π−C2H4)+. Both methanol and isotopically labeled ethene yield structure-specific reactions with these ions. Methanol reacts with alkylsilylenium ions by competitive elimination of a corresponding alkane or dehydrogenation and yields a methoxysilylenium ion. Isotopically labeled ethene reacts specifically with alkylsilylenium ions containing a two-carbon or larger alkyl substituent by displacement of the corresponding olefin and yields an ethylsilylenium ion. Methanol reactions were found to be efficient for all systems, whereas isotopically labeled ethene reaction efficiencies were quite variable, with dialkylsilylenium ions reacting rapidly and trialkylsilylenium ions reacting much more slowly. Mechanisms for these reactions and differences in the kinetics are discussed.