Dynamics of the Molecular and Atomic Mechanisms for the Hydrogen-Iodine Exchange Reaction

Theoretical treatment of both the molecular and atomic mechanisms for the hydrogen-iodine exchange reaction (H2+I2→ 2HI) is accomplished by means of extensive classical trajectories calculated on a reasonable potential-energy surface in which the single adjustable parameter is the iodine-core effective charge. The analysis shows the molecular mechanism to be dynamically forbidden, but gives an over-all rate constant for the atomic mechanism in substantial agreement with the experimental values. The formation of a weak H2I complex is predicted to play an important dynamical role if the atomic mechanism is limited to reactions with collision complexes involving no more than two hydrogen atoms and two iodine atoms. Excellent agreement with experiment is obtained for the rate constant for the recombination I+I+H2→ I2+H2 and its negative temperature coefficient. Trajectories for the latter reaction are rich in multiple exchange of internal energy between the molecular species, but the formation of H2I is predicted to be an unimportant process in recombination of iodine atoms with hydrogen molecule as the third body.