Ion–molecule association of H3O + and C2H2 : interstellar CH3CHO

Abstract

The C₂H₂· H₃O⁺ product of the ion–molecule association reaction between H₃O⁺ and C₂H₂ is found to consist of a ca. 50 : 50 mixture of two isomeric ions. These two isomeric ions are identified in a selected ion flow tube, by their different proton transfer behaviour with the neutral reagents C₂H₅Br, 4-fluorotoluene, CH₃OH and benzene, as protonated vinyl alcohol, CH₂CHOH₂⁺ and either protonated acetaldehyde, CH₃CHOH⁺ or the electrostatic complex H₃O⁺· C₂H₂. These conclusions are supported by Gaussian G2 level calculations based on ab initio molecular orbital theory, which are applied to calculate the proton affinities of CH₃CHO, CH₂CHOH, oxirane and acyclic CH₂OCH₂. Reaction rate coefficients and product ratios are also reported for the reactions of specific C₂H₅O⁺ isomers, viz: CH₃CHOH⁺, CH₃OCH₂⁺ and [graphic omitted] with CH₃OH, 4-fluorotoluene and C₆H₆. The implications of the current results to the interstellar synthesis of CH₃CHO are discussed briefly.