Gas-phase ion chemistry and ion thermochemistry of phenyltrifluorosilane

Abstract

The ion/molecule reactions of phenyltrifluorosilane with F −, CH 3O − and OH − and the thermochemical properties of the resulting ions were investigated by using a flowing afterglow—triple quadrupole mass spectrometer. The silane reacts with fluoride and methoxide ions by addition, whereas the reaction with hydroxide proceeds by addition and loss of HF to form the phenyldifluorosiloxide ion. Collision-induced dissociation (CID) of the adduct ions produces predominantly phenide ion, although rearrangement processes are also observed for the methoxide adduct. CID of the siloxide ion results in direct cleavage to form the SiF 2O − ion, in addition to products formed by rearrangement. We also report the formation of the hydroxide adduct of phenyltrifluorosilane, and provide the first confirmation that activated hydroxide adducts of silanes dissociate by loss of RH. The hydride adduct of phenyltrifluorosilane is generated indirectly by the addition of fluoride to phenyldifluorosilane. Bond dissociation energies and rearrangement barriers were obtained for all ions from the energy-resolved CID cross sections. From the measured bond dissociation energies, the fluoride, hydride and methoxide affinities of phenyltrifluorosilane are determined to be 79.4±3.3 and 64.0±3.2, and 76.6±5.9 kcal/mol, respectively, and the fluoride affinity of phenyldifluorosilane is found to be 72.3±4.4 kcal/mol. The hydride affinity (HA) of phenyltrifluorosilane is comparable to that of borane, suggesting that C 6H 5SiHF 3 − is a potential hydride reducing agent in the condensed phase.