Cl+ and HCl+ in Reaction with H2 and Isotopologues: A Glance into H Abstraction and Indirect Exchange at Astrophysical Conditions

Abstract

Astrochemical models of interstellar clouds, the sites of stars, and planet formation require information about spin-state chemistry to allow quantitative comparison with spectroscopic observations. In particular, it is important to know if full scrambling or H abstraction (also known as proton hopping) takes place in ion-neutral reactions. The reaction of Cl⁺ and HCl⁺ with H₂ and isotopologues has been studied at cryogenic temperatures between 20 and 180 K using a 22 pole radio frequency ion trap. Isotopic exchange processes are used to probe the reaction mechanism of the HCl⁺ + H₂ reaction. The results are compared with previous measurements and theoretical predictions. The rate coefficients for the Cl⁺ + H₂ and HCl⁺ + H₂ reactions are found to be constant in the range of temperatures studied, except for the DCl⁺ + D₂ reaction, where a weak negative temperature dependence is observed, and reactions with D₂ are found to be significantly slower than the Langevin rate. No isotopic exchange reactions are observed to occur for the H₂Cl⁺ ion. The analysis of the products of the HCl⁺ + H₂ isotopic system clearly indicates that the reaction proceeds via simple hydrogen atom abstraction.