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

Prior research has shown that hydrogen isotope fractionation occurs in gas-phase reactions relevant to interstellar chemistry. However, the specific behavior of NH(+) + H(2) reactions under such conditions remains unclear. Earlier studies focused on general reaction mechanisms but lacked detailed statistical modeling of isotopic branching ratios. This gap motivated the need to explore how isotopic substitution affects product distributions in ammonia formation pathways. No prior work had resolved the statistical preference for NHD(+) in these reactions. The role of deuterium in such reactions is not fully understood. Existing models do not account for rovibrational energy levels in isotopologues. This paper addresses these uncertainties by introducing a new potential-energy surface and statistical analysis.

The aim of this study is to model hydrogen isotope fractionation in the NH(+) + H(2) reaction at interstellar temperatures. The specific problem is to determine how isotopic substitution affects product branching ratios. The motivation comes from the need to refine interstellar chemistry models. Current models lack detailed statistical data on isotopic preferences. This work uses a new potential-energy surface to calculate rovibrational energy levels. These energy levels are used to determine accessible product states. The study focuses on NH(3)(+) intermediates and their isotopic variants. The goal is to provide a statistical framework for predicting product distributions.

Adiabatic capture theory calculations were used to model the association reaction. A new potential-energy surface was developed for NH(2)(+) products. Rovibrational energy levels were calculated for NH(2)(+) isotopologues. These energy levels were used to determine accessible product states. Statistical branching fractions were derived from these states. The analysis considered all isotopically substituted variants of the reaction. Energy and angular momentum were resolved in the calculations. The results were averaged to produce branching fractions for each reaction.

The statistical branching fractions show a preference for NHD(+) when both H and D are present. The NHD(+) product is favored over other isotopologues in all cases. The new potential-energy surface provided accurate rovibrational energy levels. These energy levels enabled precise calculation of accessible states. The results indicate that deuterium significantly influences product distribution. No other product outcompetes NHD(+) in mixed H/D environments. The branching ratios were determined using energy- and momentum-resolved data. These findings align with the hypothesis that deuterium enhances stability in the product.

The authors propose that deuterium enhances the stability of NHD(+) in the reaction. The statistical model shows a clear preference for NHD(+) when H and D coexist. This preference is consistent across all isotopic variants studied. The new potential-energy surface supports these findings. The rovibrational energy levels are critical to the statistical analysis. The results suggest that isotopic substitution strongly affects product distribution. These findings may refine interstellar chemistry models. The study does not claim to resolve all uncertainties in isotope fractionation.