The Hinshelwood-Lindemann mechanism is the corner-stone of thermal unimolecular kinetics and provides the model for the thermal unimolecular rate constant versus T and P, kuni(T,P). RRKM theory is used for the unimolecular rate constant k(E) and the steady-state approximation is used to obtain the expression for kuni(T,P). Following Slater and Bunker,1,2 a more general approach may be used to obtain kuni(T,P), which involves a convolution between unimolecular decomposition and collisional deactivation probabilities. Using this approach, it is found that a bi-exponential decomposition probabilitiy, for intrinsic non-RRKM dynamics, gives an equation for kuni(T,P) that is in exact agreement with the RRKM kuni(T,P), for which the collision deactivation frequency is scaled by an energy transfer efficiency factor βc.3 This analysis indicates that fitting an experimental kuni(T,P) by the Hinshelwood-Lindemann-RRKM model does not unambiguously identify non-RRKM dynamics.

Comparison of RRKM and intrinsic non-RRKM kinetics for the Hinshelwood-Lindemann mechanism

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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