Modeling deuterium chemistry in starless cores: full scrambling versus
  proton hop
  We constructed two new models for deuterium and spin-state chemistry for the
purpose of modeling the low-temperature environment prevailing in starless and
pre-stellar cores. The fundamental difference between the two models is in the
treatment of ion-molecule proton-donation reactions of the form $\rm XH^+ + Y
\longrightarrow X + YH^+$, which are allowed to proceed either via full
scrambling or via direct proton hop, i.e., disregarding proton exchange. The
choice of the reaction mechanism affects both deuterium and spin-state
chemistry, and in this work our main interest is on the effect on deuterated
ammonia. We applied the new models to the starless core H-MM1, where several
deuterated forms of ammonia have been observed. Our investigation slightly
favors the proton hop mechanism over full scrambling because the ammonia D/H
ratios are better fit by the former model, although neither model can reproduce
the observed $\rm NH_2D$ ortho-to-para ratio of 3 (the models predict a value
of $\sim$2). Extending the proton hop scenario to hydrogen atom abstraction
reactions yields a good agreement for the spin-state abundance ratios, but
greatly overestimates the deuterium fractions of ammonia. However, one can find
a reasonably good agreement with the observations with this model by increasing
the cosmic-ray ionization rate over the commonly-adopted value of
$\sim$$10^{-17}\,\rm s^{-1}$. We also find that the deuterium fractions of
several other species, such as $\rm H_2CO$, $\rm H_2O$, and $\rm CH_3$, are
sensitive to the adopted proton-donation reaction mechanism. Whether the full
scrambling or proton hop mechanism dominates may be dependent on the reacting
system, and new laboratory and theoretical studies for various reacting systems
are needed to constrain chemical models.
