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Certain recent experiments on the decomposition of propionic aldehyde1 show an actual falling off at low pressures in the rate of a reaction which is unimolecular at high pressures. It would seem that some kind of collision hypothesis must be used to account for activation in such a case.The old form of the collision hypothesis as developed by Langmuir,2 Christiansen and Kramers,3 and Tolman4 appears entirely inadequate even in the case just cited.5 It seems possible that the explanation may be found in the suggestion, originally made by Lewis and Smith6 and somewhat extended since by Christiansen7 and by Hinshelwood and Lindemann,5 that the internal degrees of freedom may be of some help in causing activation.8This theory may be put in several different forms, not all of which have been considered in this connection. These will be investigated, and equations for the reaction rate at low pressures will be developed, using the methods and results of classical statistical mechanics.

the chief means of activation at low pressure, and unless the mechanism of transfer of energy is very specific, we would expect the reaction rate to fall off during the course of a run as the products of reaction accumulate but practically this is not the case, though the run falls off in most cases to a very small extent. What the effects of simple local heating might be, and whether such an effect could be combined with our theory to give satisfactory results, we cannot at present decide. SummaryTwo theories based on collision hypotheses of quasi-unimolecular reactions, which we have previously developed, are applied to the decomposition of azomethane. It is possible to explain the falling off of the rate with pressure and to decide between the two theories. The efficiency of activation and deactivation at collision is considered, and the following are discussed: (1) the number of degrees of freedom involved in the various cases; (2) the effect of heat of reaction in the case of azomethane. The decomposition of diethyl ether is also considered briefly.

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