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⋅⋅⋅H2→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.