Sequential Clustering Reactions of Si sup + with Silane: A Theoretical Study of the Reaction Mechanisms.

The mechanisms of the sequential clustering reactions of Si sup + with SiH sub 4 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 Si sub 2 H + over 2, Si sub 3 H + over 4, Si sub 4 H + over 6, and finally Si sub 5 + over 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 H sub 2 Si- Si sup +, H sub 3 Si-SiH-Si sup +, (H sub 3 Si) sub 2 Si-Si sup + and (H sub 3 Si) sub 3 Si-SiH sup +. The bottleneck in the reaction sequence is due to the formation of the branched product Si sub 5 H + over 10 where elimination of H sub 2 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.