Some Ion–Molecule Reactions of H3+ and the Proton Affinity of H2

Abstract

The flowing afterglow technique has been used to study the reactions of H₃⁺ with a number of neutral reactants at thermal energies. Proton transfer was the only primary reaction observed with N₂, CO, CO₂, N₂O, NO, CH₄, C₂H₂, H₂O, and NH₃. Both proton transfer and dissociative charge transfer were observed with C₂H₄ and C₂H₆, while dissociative charge transfer is the exclusive primary process with NO₂. Secondary reactions were observed with NO, C₂H₆, C₂H₄, and C₂H₂. Cluster ions were formed between NO⁺ and NO₂ and H₂O, between H₃O⁺ and H₂O, CO₂, and CO, and between NH₄⁺ and NH₃ and H₂O. Proton transfer was also observed between HN₂⁺ and CO₂, N₂O, CH₄, and H₂O, and between HO₂⁺ and H₂ and N₂. Rate constants were obtained for these reactions and are discussed. Limits could be placed on the proton affinity (P.A.) of H₂ from the failure to observe rapid proton transfer to O₂ and the observation of proton transfer to N₂. These indicate 4.2 < P.A. (H₂)< 4.7 eV with a recommended value of 4.4 eV. The technique can be used to measure relative proton affinities of gases.