Rate constants and product branching fractions for the reactions of H3O+ and NO+ with C2-C12 alkanes

Gas-phase ion molecule reactions of H3O+ and NO+ with a number of C-2-C-12 alkanes have been studied using a variable temperature-selected ion flow tube instrument. Reaction rate constants and product branching fractions were measured at 300 K, and temperature dependences of the rate constants and product branching fractions for reactions of C-7 and C-8 alkanes were examined from 300 to 500 K. The threshold size for H3O+ reaction occurs at hexane, with the reaction rate constants increasing steadily with alkane size. Association is the predominant reaction channel at 300 K for C-3-C-12 alkanes, although these products thermally dissociate back into initial reactants between 300 and 400 K. The proton-transfer reaction channel is expected to be exothermic for large alkanes; however, this reaction channel does not proceed at the collision rate, nor are any direct proton-transfer product ions observed. The presence of smaller alkyl product ions suggests proton-transfer product ions may dissociate into smaller alkyl fragment ions and neutral alkanes. The threshold size for NO+ reaction with n-alkanes also occurs at hexane, and the reaction proceeds primarily via hydride transfer as previous experiments have indicated. However, a number of smaller product ions are also observed for the reactions of NO+ with all n-alkanes larger than butane, with the branching fraction of the nonhydride transfer product ions increasing with increasing alkane size. Together with our previously reported results regarding the reactions of various atmospheric ions with octane isomers, the present results indicate that H3O+, NO+, and O-2(+) are not suitable chemical ionization agents for analysis of hydrocarbon emissions.