Theoretical studies of clustering reactions. Sequential reactions of SiH+3 with silane

The mechanisms for the sequential clustering reactions of SiH+3 with SiH4 have been studied by means of accurate quantum chemical techniques using polarized basis sets and including zero-point corrections and the effects of electron correlation. 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, Reents, and Kolenbrander, the reactions lead to the formation of Si2H+5 and Si3H+7 without significant overall barriers. In addition, bimolecular attachment complexes (Si2H+7,Si3H+9 and Si4H+11) bound by bridging hydrogen atoms are involved as reaction intermediates and have also been observed by Mandich et al. as collisionally stabilized products. Unusual hydrogen bridged complexes play a key role in the termination of the growth sequence. The first bottleneck in the sequence of growth reactions is due to the formation of the particularly stable hydrogen bridged complex Si3H+7 containing a four-membered Si–Si–Si–H ring. Further growth beyond Si3H+7 other than bimolecular attachment involves transition states which have significant barriers and could only happen in the presence of excess energy. Isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism. Quantitative comparisons are made between our calculated transition state barriers and complexation energies 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 five different energetic quantities is only ≂3 kcal/mol.