Reaction rates of H(H₂), D(H₂), and H(D₂) van der Waals molecules and the threshold behavior of the bimolecular gas-phase rate coefficient

We study low-energy quantal phenomena in the rearrangement of three-atom systems composed of H and D. All calculations are carried out on the double many-body expansion potential energy surface for the hydrogen trimer. The unimolecular rearrangements of van der Waals molecules, such as D⋯H ₂→HD⋯H, are studied as a model for the exchange transfer reaction in condensed phases, and the gas-phase bimolecular reactions, such as D + H₂→HD + H, are studied to probe the limiting low-temperature threshold behavior, which is compared to that predicted by quantum mechanical threshold laws. The reaction rates are studied down to temperatures of 10 ^-3 K. We also predict the spectroscopic tunneling shift on the lowest energy levels of the H⋯H₂ complex.