Experimental and theoretical study of the formation of silicon-carbon ion species in gaseous silane/ethene mixtures

Results of gas-phase experiments and theoretical investigations are reported for ionic reactions in silane/ethene systems with the main interest in the formation and growth of species containing both silicon and carbon atoms. Ion/molecule reactions in different SiH4/C2H4 mixtures have been studied with an ion-trap mass spectrometer, determining variation of ion abundances with reaction time, reaction paths starting from primary ions of both reagents and reaction rate constants of the main processes. The best yield in formation of new Si-C bonds occurs in mixtures with an excess of silane, through processes of silicon-containing ions with ethene molecules. Since reactions of SiH2+ with ethene have been observed to play a major role in this system, they have been investigated by high-level ab initio methods. Structures and energies of intermediates (SiC2H6.+) and products (SiC2H5+, SiC2H4.+, SiCH3+), as well as energy profiles of the pathways observed experimentally, have been determined; The initial step is formation of a SiC2H6.+ adduct at -44 kcal mol(-1) with respect to the reactants, followed by isomerization reactions to four different structures through viable paths. Hydrogen atom loss to give SiC2H5+ occurs through homolytic cleavage of a Si-H or C-H bond without energy barriers for the inverse process. Four different structures have been computed for SiC2H4.+ ion species, but only three of them are attainable by H-2 elimination from SiC2H6.+ or by isomerization. Formation of SiCH3+ involves three isomerization steps of the SiC2H6.+ adduct before the cleavage of a Si-C bond. Enthalpies of formation of all the structures have also been computed, and a good agreement with previously reported experimental data is generally observed for the most stable isomers.