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A quantum model of restricted vibrational energy flow on the way to the transition state in unimolecular reactions

Recent spectroscopic experiments and theoretical work have shown how restricted or insufficiently rapid intramolecular vibrational redistribution (IVR) mediates the kinetics of conformational isomerisation, and that the rates can be very different from the RRKM theory rate. Here we use the state space picture to provide a quantum mechanical description of how bottlenecks to energy flow can influence reaction kinetics, giving rise to states above the IVR threshold that are localised to a conformer and potentially contribute to a lower rate compared with the RRKM theory estimate. We illustrate the effect of quantum bottlenecks with a numerical example based on an alkyl chain molecule, and discuss how spectroscopic interrogation of nearly isoenergetic vibrational states of two conformers with similar energy minima and separated by a modest barrier may reveal reaction in only one direction.