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. • Thermoneutral forward and backward reactions R ⇌ ‡ ⇌ P are symmetric. • New quantum definition of reactant ( R ), transition state species ( ‡ ), product ( P ). • Neumark's transition state spectroscopy can observe the quantum species ‡. • The approach is exemplified for the Cope rearrangement of semibullvalene. • It corrects two compensating errors of non-symmetric transition state theory.