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 selec ted 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 Si3 H5+, Si4H7+, and SimCnHp+ react selectively with propene, collisions with s ilane do not lead to any detectable product besides self-condensation of si lane. Experimental and collisional rate constants of the Maine processes ha ve been compared, determining reaction efficiencies. Chain propagation proc eeds until the formation of large clusters, such as Si2C5H13+, Si3C2Hn+ (n = 9, 11), and Si4C2H11+, is attained. The critical points on the B3LYP/6-31 1G(d,p) potential energy surfaces of the Si2H3+/C3H6 and Si3H5+/C3H6 system s have been found, describing the structures of the most stable reaction pr oducts. The observed selectivity toward propene of species Si2CH5+ and Si3C H7+ with respect to two alternative reaction channels (silane or propene) i s explained in terms of relative reaction entropies of the corresponding ad ducts.