Equilibrium isomeric mixtures: potential energy hypersurfaces as the origin of the overall thermodynamics and kinetics

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Abstract
Potential energy hypersurfaces are now most frequently represented by the location and characterization of their stationary (critical) points. Model studies have demonstrated that hundreds of stationary points can be present on some potential hypersurfaces of chemical interest. Any structure-dependent observed quantity can thus have a convolutional nature, being composed of contributions of relevant stationary points. This article focuses on equilibrium and rate processes; it deals with situations in which component of an equilibrium is represented by a group of several different local energy minima and/or when several different saddle points serve as activated complexes in a single rate process. Particular attention is paid to the case where two or more isomeric structures of comparable stability coexist, the isomers being indistinguishable under observational conditions. Evaluations of overall, convolutional standard and/or activation reaction terms based on the stationary-point hypersurface representation are discussed with emphasis on theory-observation comparisons, experiment interpretation, and prediction of behaviour.