By using the principle of microscopic reversibility, unimolecular decompositions are considered from the point of view of the reverse reaction, which is a bimolecular association. In cases where the immediate product of the decomposition is a pair of free radicals, it is assumed that the probability of these free radicals recombining upon collision is independent of their energy. An explicit expression for the activation energy in terms of the energy of reaction and the thermal properties of the molecules involved is thus obtained. From this one may calculate the energy of activation at absolute zero and the energy necessary to break the bond which falls apart in the reaction, quantities which may differ considerably from each other and from the energy of activation at the temperature of reaction. The results are applied to several cases of interest, and are critically compared with those of the earlier theories of Rice and Ramsperger, Kassel and Rice. A brief discussion is given of the calculation of the rate constant as a function of pressure, and its modification according to the point of view of the present paper.
