Some Considerations of Unimolecular Rate Theory

Abstract

It is the purpose of this paper to examine the pressure dependence of the rate constant k of unimolecular decompositions within the framework of the generalized Lindemann mechanism. For trace amounts of reactive polyatomic molecules possessing very small equilibrium fractions of reactive states, the observed k may be identified with the lowest eigenvalue of the relaxation spectrum. After an examination of the Kassel and Slater microscopic decomposition frequencies (m.d.f.'s), this approach is applied to the strong collision mechanism which is a common feature of both theories. Finally, the lowest eigenvalue is calculated for the inefficient stepwise activation mechanism in combination with the simple Kassel m.d.f. The pressure dependence of k on a reduced basis is found to be relatively insensitive to the details of the activation mechanism employed. The effect of less efficient processes in the intermolecular energy transfer and of anharmonicities in the intramolecular energy transfer tend to broaden the transition range of k between the high‐ and low‐pressure limits.