The Role of Vibrational, Excitation in the Formation of 4‐Center Transition States

The experimental results describing the kinetics of the interaction between hydrogen and deuterium obtained by Farkas and Farkas are discussed in the light of the recently derived rate constants of the reactions: H 2 + M → H + H + M and H + D 2 → HD + D. It is suggested that in the early study of this exchange reaction, the dissociation of hydrogen occurred under heterogeneous conditions, so that the conclusions regarding the relative rates of the various steps in the exchange scheme are not entirely correct. A technique which provides purely homogeneous conditions at high temperatures is briefly described, and recent experimental results obtained in various exchange studies are presented. It is shown that under homogeneous conditions, at the temperature range over which exchange readily occurs, dissociation processes are slow, and the free radicals are present in too small concentrations for a free radical mechanism to contribute to the observed rate of exchange. The experimental results suggest that exchange occurs via a 4‐center intermediate, but not as a result of a hard collision between the exchanging molecules. A reaction may occur only if one of the molecules is excited to a critical vibrational state before colliding with its exchange partner. In a few diatomic molecules where the efficiency of translation → vibration energy exchange is not very high, the rate of vibrational excitation determines the rate of exchange. For one typical study, an evaluation of the rate of vibrational excitation from available data on translation → vibration transition probabilities is described. The agreement between the calculated and the experimental rates is excellent.