Modeling Ionic Reactions at Interstellar Temperatures: The Case of NH₂^- + H₂ ⇔ NH₃ + H

Structural features and enthalpy details are presented for the title reactions, both for the exothermic (forward) path to NH₃ formation and for the endothermic (reverse) reaction to NH₂^- formation. Both pathways have relevance for the nitrogen chemistry in the interstellar medium (ISM). They are also helpful to document the possible role of H^- in molecular clouds at temperatures well below room temperature. The structural calculations are carried out using different ab initio methods and are further employed to obtain the reaction rates down to the interstellar temperatures detected in earlier experiments. The reaction rates are obtained from the computed minimum energy path (MEP) using the variational transition-state theory (VTST) approach. The results indicate very good accord with experiment results at room temperature, while measured low temperature data down to 8 K are well described using an appropriately modified VTST approach. This is done by employing a temperature-dependent scaling, from room temperature conditions down to the lower ISM temperatures, which acknowledges the noncanonical behavior of the fast, barrierless exothermic reaction. The reasons for this behavior and the need for improving on the VTST method when used away from room temperatures are discussed.