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We present in this paper the main structural features and enthalpy detailsfor the energy profiles of the title reactions, both for the exothermic(forward) path to NH$_{3}$ formation and for the endothermic (reverse) reactionto NH$_{2}^{-}$ formation. Both systems have relevance for the nitrogenchemistry in the interstellar medium (ISM). They are also helpful to documentthe possible role of H$^{-}$ in molecular clouds at temperatures well belowroom temperature. The structural calculations are carried out using ab initiomethods and are further employed to obtain the reaction rates down to theinterstellar temperatures detected in earlier experiments. The reaction ratesare obtained from the computed Minimum Energy Path (MEP) using the VariationalTransition State Theory (VTST) approach. The results indicate very good accordwith the experiments at room temperature, while the measured low temperaturedata down to 8 K are well described once we analyse in detail the physics ofthe reactions and modify accordingly the VTST approach. This is done byemploying a T-dependent scaling, from room temperature conditions down to thelower ISM temperatures, which acknowledges the non-canonical behavior of thefast, barrierless exothermic reaction. This feature was also suggested in theearlier work discussed below in our main text. The physical reasons for theexperimental behavior, and the need for improving on the VTST method when usedaway from room temperatures, are discussed in detail.

Modeling ionic reactions at interstellar temperatures: the case of NH2- + H2 <--> NH3 + H-