Acidity, basicity, and ion-molecule reactions of arsine in the gas phase by ion cyclotron resonance spectroscopy

Abstract

The ion-molecule reactions of arsine, both pure and in binary mixtures with several other molecules, have been investigated by ion cyclotron resonance spectroscopy. Reaction pathways, product distributions, and rate constants have been determined for the ion-molecule reactions for both positive and, to a lesser extent, negative ions. Arsine fragment ions condense with neutral AsH_3 to generate product ions containing two and, on further reaction, three atoms of arsenic. In the process of condensation, one or two molecules of H_2 are expelled. The formation of AsH_4^+ occurs from AsH_3^+ which does not undergo condensation reactions to any significant extent. Where possible, thermochemical data have been determined, including the gas phase acidity, PA(AsH_2^-) = 356 ± 6 kcal/mole, and basicity, PA(AsH_3) = 175 ± 5 kcal/mole, of AsH_3. Observation of gas phase nucleophilic displacement reactions involving AsH_3 as a nucleophile have allowed limits to be placed on the basicity of AsH_3 toward a soft acid, CH_3^+. The implications of these results are discussed and the ion-molecule reactions of AsH_3 are compared with those of other hydrides.