Sequential clustering reactions of SiH+ with silane: An a b i n i t i o molecular orbital study

The mechanisms for the sequential clustering reactions of SiH+ with SiH4 have been studied by means of accurate quantum chemical techniques. Detailed reaction pathways including the necessary transition structures have been investigated to understand the structures and stabilities of the reaction intermediates and products. In accordance with the experimental observations of Mandich and Reents, the reactions lead to the formation of Si2H+3, Si3H+5, Si4H+7, and Si5H+11 without overall endothermic barriers. As seen experimentally, both reactive (dibridged) and unreactive (tribridged) isomers of Si2H+3 can be formed in the first reaction step. While Si2H+3 and Si3H+5 involve reactive structures with bridging hydrogens, Si4H+7 corresponds to a cyclic isomer (the second row analog of the cyclobutyl cation) which only forms a complex with silane. Silicon isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism.