Dissociative Proton Transfer Reactions of H3+, N2H+, and H3O+ with Acyclic, Cyclic, and Aromatic Hydrocarbons and Nitrogen Compounds, and Astrochemical Implications

A flowing afterglow-selected ion flow drift tube has been used to measure the rate coefficients and product ion distributions for reactions of H 3 O +, N 2 H +, and H 3 + with a series of 16 alkanes, alkenes, alkynes, and aromatic hydrocarbons as well as acrylonitrile, pyrrole, and pyridine. Exothermic proton transfer generally occurs close to the collision rate. The reactions of H 3 O + are mostly nondissociative and those of H 3 + are mostly dissociative, but many reactions, especially those of N 2 H +, have both dissociative and nondissociative channels. The dissociative channels result mostly in H 2 and/or CH 4 loss in the small hydrocarbons and in toluene, loss of C 2 H 2 from acrylonitrile, and loss of HCN from pyrrole. Only nondissociative proton transfer is observed with benzene, pyridine, and larger aromatics. Drift tube studies of N 2 H + reactions with propene and propyne showed that increased energy in the reactant ion enhances fragmentation. Some D 3 + reactions were also investigated and the results suggest that reactions of H 3 + with unsaturated hydrocarbons B proceed through proton transfer that forms excited (BH + )* intermediates. Pressure effects suggest that a fraction of the (BH + )* intermediates decomposes too rapidly to allow collisional stabilization in the flow tube ( t < 3 × 10 -8 s). The other low-energy (BH + )* intermediates are formed by the removal of up to 40% of the reaction exothermicity as translational energy, and these intermediates result in stable BH + products. The results suggest that, in hydrogen-dominated planetary and interstellar environments, the reactions of H 3 + can convert C 2 −C 6 hydrocarbons to smaller and less saturated molecules, but polycyclic aromatics are stable against decomposition by this mechanism. The dissociative reactions of H 3 + can therefore favor the accumulation of small unsaturated hydrocarbons and aromatics in astrochemical environments.