On the forward-backward symmetry of reversible thermoneutral unimolecular reactions

The rate constants of forward and backward thermoneutral isomerizations are symmetric. The symmetry is imposed by the equivalence of the reactants ($R$) and products ($P$). We discover consequences of this symmetry, using a simple kinetic model of the reactions from $R$ via the transition state ($‡$) to $P$, and back, with corresponding quantum mechanical definitions of R, $‡$ and P. The transition state species $‡$ is observable by Neumark's transition state spectroscopy. The model corrects compensating errors of the standard derivation of transition state theory. The general results are exemplified for the Cope rearrangement of semibullvalene.