Sequential clustering reactions of Si+ with silane: A theoretical study of the reaction mechanisms

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Abstract
The mechanisms of the sequential clustering reactions of Si+ with SiH4 have been studied by means of accurate quantum chemical techniques using polarized basis sets and including the effects of electron correlation and zero‐point corrections. In accordance with the experimental results of Mandich, Reents, and Jarrold, the reactions lead to the formation of Si2H+2, SiH+4, Si4H+6, and finally Si5H+10. The study of the detailed reaction pathways including the necessary transition structures leads to the assignment of specific isomeric products in each reaction step. The specific isomers assigned are H2Si–Si+, H3Si–SiH–Si+, (H3Si)2Si–Si+, and (H3Si)3Si–SiH+. The bottleneck in the reaction sequence is due to the formation of the branched product Si5H+10 where elimination of H2 is not easily possible due to the saturation of the bonding involving the central silicon atom. Isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism. Quantitative comparisons are made between our calculated energy barriers and those derived by Mandich et al. from phase space calculations using the experimental reaction rates. The mean absolute deviation between the two results for seven different energetic quantities is only ≂3 kcal/mol.