Gas-phase reactions of SiH(n)+ (n = 1,2) with NF3: a computational investigation on the detailed mechanistic aspects.

The mechanism of the gas-phase reactions of SiH(n)(+) (n = 1,2) with NF(3) were investigated by ab initio calculations at the MP2 and CAS-MCSCF level of theory. In the reaction of SiH(+), the kinetically relevant intermediates are the two isomeric forms of fluorine-coordinated intermediate HSi-F-NF(2)(+). These species arise from the exoergic attack of SiH(+) to one of the F atoms of NF(3) and undergo two competitive processes, namely an isomerization and subsequent dissociation into SiF(+) + HNF(2) , and a singlet-triplet crossing so to form the spin-forbidden products HSiF(+) + NF(2). The reaction of SiH(2)(+) with NF(3) involves instead the concomitant formation of the nitrogen-coordinated complex H(2)Si-NF(3)(+) and of the fluorine-coordinated complex H(2)Si-F-NF(2)(+). The latter isomer directly dissociates into NF(2)(+) + H(2)SiF, whereas the former species preferably undergoes the passage through a conical intersection point so to form a H(2) SiF-NF(2)(+) isomer, which eventually dissociates into H(2)SiF(+) and NF(2).