The microcanonical Lindemann mechanism for unimolecular reactions

The microcanonical analog of the Lindemann mechanism for unimolecular reactions, or microcanonical Lindemann mechanism (MLM), is discussed. The mechanism makes explicit a central role of intramolecular vibrational redistribution (IVR) in mediating rates of unimolecular reactions. Under conditions of ergodic dynamics, the MLM predicts single-exponential kinetics with a rate constant that is generally smaller than the Rice-Ramsperger-Kassel-Marcus (RRKM) theory estimate. The RRKM theory rate is reached in the limit where rates of IVR from states of the activated complex to non-reactive states of the reactant are much greater than rates of transition from states of the activated complex to states of the product. We discuss the MLM in the context of experimental results for conformational isomerization, one irreversible, the photoisomerization of trans-stilbene, and another reversible, cyclohexane ringinversion. This Perspective aims to address and clarify misunderstandings that persist as to the role of IVR in unimolecular reaction kinetics. We also discuss, in the context of the MLM, a means to estimate rates of vibrational relaxation from states of the activated complex to non-reactive states from available energy-dependent unimolecular reaction rate data, which we believe is new.