Radiationless transitions and photochemical reactivity

The potential energy along a reaction coordinate can be calculated in terms of the reaction energy, the force constants and the equilibrium bond lengths of the reactive bonds, the bond order at the transition state and, for very exothermic or endothermic reactions, the so called "configuration entropy". Such potential energy curves allows one to assess the importance of quantum mechanical tunnelling in Habstraction, a-cleavage and cycloaddition photochemical reactions of excited lzetones. Experimental recognition of tunnelling can he made by the study of the efficiency of the photoreactions as a function of molecular parameters and medium properties. The theory allows the establishement of structure-efficiency relationships and criteria to distinguish between the mechanisms of nuclear tunnelling and thermal activat ion.