A selected ion-flow tube study of the reactions of O +, H + and HeH] + with several molecular gases at 300 K

Abstract

The rate coefficients, k, and product ion distributions have been determined for the reactions of O +, H + (D +) and HeH + (HeD +) with 17 molecular gases at 300 K using a selected-ion flow tube apparatus. The 17 molecular gases are NO, O 2, NO 2, NH 3, H 2S, SO 2, H 2O, N 2O, CO 2, OCS, CH 4, C 2H 2, C 2H 4, C 2H 6, CH 3OH, CH 3SH and CH 3CN. This study was primarily undertaken to compare the ion chemistry of O + and H + which have nearly the same recombination energies. It is revealed that of these 34 reactions only four (O + + NO, O 2, C 2H 2 and H + + C 2H 2) are slow (i.e. k ⪡ k c, the collisional rate coefficient), this being due variously to unfavourable state-to-state correlations, short complex interaction lifetimes and unfavourable Franck—Condon factors. The other 30 reactions are very efficient (i.e. k ≈ k c). The product distributions for the reactions of both O + and H + with about half of the 17 molecular species are the same (within measurement errors), implying that the available energy in the reactions has a major influence. For the other half of the reactions, the products and the product distributions are quite different, suggestive of stronger chemical interactions. The 17 reactions of HeH + all occur at the collisional rate (i.e. k = k c in all cases). Proton transfer is exothermic in all the reactions, and in some reactions sufficiently so that dissociative proton transfer occurs. In only five of the HeH + reactions (i.e. those with O 2, NH 3, SO 2, H 2O and CO 2) is the protonated parent molecule the only product ion. For most of the reactions there is more than one product, a common product being the ionized parent molecule. This results from charge transfer reactions which are energetically allowed because of the relative large recombination energy of HeH + (to He and H).