The rate constants for the H+H₂ reaction and its isotopic analogs at low temperatures: Wigner threshold law behavior

Variational transition-state theory rate constants for the H+H₂ reaction and its isotopic analogs at very low temperatures have been calculated using the LSTH potential surface. The Wigner threshold law is found to hold quantitatively below 1 K and to within 34% on average at 5 K. The transition-state theory rate constants are found to be consistent with the requirements of the Wigner threshold law provided that a tunneling probability having the correct threshold behavior is used in calculating the transmission coefficient. Using these calculated rate constants, decay rates of H and D atoms in solid H₂, D₂, and HD were estimated and compared with experiment. The good agreement found suggests that these decays occur through quantum mechanical tunneling exchange reactions.