On the Mechanism of Reversible Unimolecular Reactions and the Canonical (“High Pressure”) Limit of the Rate Coefficient at Low Pressures*)

Jost's [1] treatment of generalized first order kinetics is used for a new discussion of the mechanism of reversible unimolecular reactions. A detailed analysis of time dependent average microscopic rate coefficients and level populations is presented using exact solutions for a realistic model of an isomerization reaction. It is shown that unimolecular fall‐off for the macroscopic relaxation is not mainly due to a reduced population of reactive levels as suggested by the irreversible Lindemann mechanism, but rather due to a fast reverse reaction even at early times. It is furthermore shown that the average microscopic rate coefficients can be measured directly, in principle. At equilibrium they are equal to the canonical (“high pressure”) limit of the rate coefficient even for arbitrarily low pressures. Practical experimental schemes are suggested for thus obtaining high pressure limiting rate coefficients without extrapolation procedures.