Selectivity in gas-phase ion chemistry. Competitive fast reactions in a silane/propene system

Gas-phase selective reactions have been studied in silane/propene mixtures by ion-trap mass spectrometry and density functional calculations. Products from the third step of reaction of SiH+ ions with SiH4 and C3H6 were selected and reacted in the cell of a quadrupole ion trap. The prior steps have been the subject of a previous study by this research group. While ions Si3H5+, Si4H7+, and SimCnHp+ react selectively with propene, collisions with silane do not lead to any detectable product besides self-condensation of silane. Experimental and collisional rate constants of the Maine processes have been compared, determining reaction efficiencies. Chain propagation proceeds until the formation of large clusters, such as Si2C5H13+, Si3C2Hn+ (n = 9, 11), and Si4C2H11+, is attained. The critical points on the B3LYP/6-311G(d,p) potential energy surfaces of the Si2H3+/C3H6 and Si3H5+/C3H6 systems have been found, describing the structures of the most stable reaction products. The observed selectivity toward propene of species Si2CH5+ and Si3CH7+ with respect to two alternative reaction channels (silane or propene) is explained in terms of relative reaction entropies of the corresponding adducts.