Statistical modelling of NH+/ND+ + H2/HD/D2 branching ratios

Abstract

In this work we study hydrogen isotope fractionation along the proposed gas-phase ammonia formation pathway at temperatures relevant to interstellar modelling. Specifically we study the isotopically substituted variants of the NH+ + H2 reaction, for which the primary product is NH2+ + H. Adiabatic capture theory calculations have been performed for the association reaction. A new potential-energy surface has been determined for the NH2+ product. An extensive set of rovibrational energy levels has been calculated for the NH2+ isotopologues described by this potential-energy surface. These rovibrational energy levels have been used to determine energy- and angular-momentum-resolved numbers of accessible product states from the NH 3+ isotopologue intermediates, which in turn have been averaged to give statistical branching fractions for all isotopically- substituted NH+ + H2 reactions. It is determined that in all cases where both hydrogen and deuterium are present, the NHD+ product is preferred.