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We present in this paper the main structural features and enthalpy details for the energy profiles of the title reactions, both for the exothermic (forward) path to NH$_{3}$ formation and for the endothermic (reverse) reaction to NH$_{2}^{-}$ formation. Both systems 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 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 the experiments at room temperature, while the measured low temperature data down to 8 K are well described once we analyse in detail the physics of the reactions and modify accordingly the VTST approach. This is done by employing a T-dependent scaling, from room temperature conditions down to the lower ISM temperatures, which acknowledges the non-canonical behavior of the fast, barrierless exothermic reaction. This feature was also suggested in the earlier work discussed below in our main text. The physical reasons for the experimental behavior, and the need for improving on the VTST method when used away from room temperatures, are discussed in detail.