Attractive Force-Induced Isotope Effects through Ring-Polymer Molecular Dynamics Simulations for the Barrierless Reaction between HNCO and H3+ Isotopologues: H3+, H2D+, HD2+, and D3.

We performed ring-polymer molecular dynamics (RPMD) simulations at temperatures of 100, 200, and 300 K to investigate the H₃⁺ + HNCO, H₂D⁺ + HNCO, HD₂⁺ + HNCO, and D₃⁺ + HNCO reactions on the recently developed potential energy surface. Thermal rate coefficients and branching fractions were obtained, showing a decrease in rate coefficients with lower temperatures, largely influenced by the Maxwell-Boltzmann averaged velocities and nuclear masses of the reactants. In the H₂D⁺ + HNCO and HD₂⁺ + HNCO reactions, the abstraction of the lighter proton was favored over that of the heavier deuteron at lower temperatures due to the attractive forces derived from the potential energy surface in the barrierless reaction. Our findings suggest that in barrierless reactions, the heavier deuteron, due to its mass and reduced mobility, is less likely to collide with molecules, leading to deuteron dissociation over proton dissociation. These results emphasize the crucial role of entrance channels in barrierless reactions for deuterium enrichment under low-temperature conditions. This deuterium enrichment arises from the nonequilibrium isotope effect.