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 H3O+, N2H+, and H3 + 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 H3O+ are mostly nondissociative and those of H3 + are mostly dissociative, but many reactions, especially those of N2H+, have both dissociative and nondissociative channels. The dissociative channels result mostly in H2 and/or CH4 loss in the small hydrocarbons and in toluene, loss of C2H2 from acrylonitrile, and loss of HCN from pyrrole. Only nondissociative proton transfer is observed with benzene, pyridine, and larger aromatics. Drift tube studies of N2H+ reactions with propene and propyne showed that increased energy in the reactant ion enhances fragmentation. Some D3 + reactions were also investigated and the results suggest that reactions of H3 + 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 H3 + can convert C2−C6 hydrocarbons to smaller and less saturated molecules, but polycyclic aromatics are stable against decomposition by this mechanism. The dissociative reactions of H3 + can therefore favor the accumulation of small unsaturated hydrocarbons and aromatics in astrochemical environments.