  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.
