Experimental and theoretical studies of gas-phase ion/molecule reactions in SiF4 forming SiFm+(SiF4)(n) clusters (m = 0-3 and n = 0-2)

Gas-phase ion/molecule reactions in SiF4 were studied using a pulsed-electr on beam mass spectrometer. The thermochemical stabilities of SiFm+(SiF4)(n) have been determined. Owing to the serious charging of the ion source, onl y up to n = 2 thermochemical data could be obtained experimentally. To eval uate bond energies that were difficult to obtain experimentally, ab initio calculations were carried out. Si+SiF4 and SiF+SiF4 have similar bond energ ies, 13 kcal/mol. SiF2+SiF4 and SiF3+SiF4 have 25 and 38 kcal/mol, respecti vely. While the bond energies of SiFm+(SiF4)(1) varied substantially with m , those of SiFm+(SiF4)(2) are all ca. 10 kcal/mol. The Si+SiF4 geometry is calculated to be a bidentate (bridged) one, where the Sif atom is located m idway between two Si-F bonds. On the other hand, SiF3+SiF4 has a symmetric D-3d structure where an F atom is shared equally by two SiF3 fragments. The oretical bond energies are in excellent agreement with experimental ones.