The ortho-to-para ratio of interstellar NH$_2$: Quasi-classical
  trajectory calculations and new simulations
  Based on recent $Herschel$ results, the ortho-to-para ratio (OPR) of NH$_2$
has been measured towards the following high-mass star-forming regions: W31C
(G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), and G34.3+0.1. The OPR at
thermal equilibrium ranges from the statistical limit of three at high
temperatures to infinity as the temperature tends toward zero, unlike the case
of H$_{2}$. Depending on the position observed along the lines-of-sight, the
OPR was found to lie either slightly below the high temperature limit of three
(in the range $2.2-2.9$) or above this limit ($\sim3.5$, $\gtrsim 4.2$, and
$\gtrsim 5.0$). In low temperature interstellar gas, where the H$_{2}$ is
para-enriched, our nearly pure gas-phase astrochemical models with nuclear-spin
chemistry can account for anomalously low observed NH$_2$-OPR values. We have
tentatively explained OPR values larger than three by assuming that spin
thermalization of NH$_2$ can proceed at least partially by H-atom exchange
collisions with atomic hydrogen, thus increasing the OPR with decreasing
temperature. In this paper, we present quasi-classical trajectory calculations
of the H-exchange reaction NH$_2$ + H, which show the reaction to proceed
without a barrier, confirming that the H-exchange will be efficient in the
temperature range of interest. With the inclusion of this process, our models
suggest both that OPR values below three arise in regions with temperatures
$\gtrsim20-25$~K, depending on time, and values above three but lower than the
thermal limit arise at still lower temperatures.
