A model reaction coordinate potential energy curve for a fluxioral molecule

Rice-Ramsperger-Kassel-Marcus (RRKM) theory is widely used in interpreting the rates of unimolecular reactions. The theory assumes that intramolecular vibrational energy redistribution (IVR) occurs on a time-scale much faster than the unimolecular reaction. The X ^----CH₃Y  S_N2 complexes and trimethylene dissociation are two unimolecular systems where the rapid IVR assumption of RRKM theory is not valid. There is much interest in developing theoretical models to represent the intramolecular and unimolecular dynamics of such systenms.
Work is also in progress to improve the accuracy of RRKM calculations. Often anharmonic corrections are necessary, as for the decouporition of a fluxioral molecule with many potential energy minima (see figure). There is also the question of whether the K rotational quantum number should be treated as an active or adiabatic degree of freedom.