Skiing the Reaction Rate Slopes

On the way from reactants to products, a chemical reaction passes through what chemists term the transition state-for a brief moment, the participants in the reaction may look like one large molecule ready to fall apart. The nature of this elusive state is the subject of a number of exciting recent developments that involve theory and experiment the study of chemical reaction rates. They include time-resolved studies of molecules in the transition state region, spectrcopic observations of that region, reactive scattering state-to-state cross sections for such as H + H_2 → H_2 + H, and experimental/theoretical studies of electron transfer processes in the condensed phase. The experiments of et al. described in this issue of Science involve the fundamental question of the quantization of vibrational energy levels of the transition state and its for unimolecular reaction rate theory.