Evolution from unimolecular to bimolecular reactions. Reactions range from unimolecular reactions where a single, stable adduct is formed or broken (type I) to bimolecular reactions, which proceed over a simple barrier (type VI). In between are reactions with multiple transition states with varying energies and entropies; both profoundly affect the observed rate constant. The evolution from one reaction type to another is goverened by the need to rearrange electron density during the reaction (MO diagrams).