On the search for the mechanism of photoreactions of some heterocyclic compounds
Photochemical inversion of the sequence of carbon and nitrogen atoms in five-membered heterocycles being a rather common phenomenon, the mechanism of a reaction of this type, the isomerization of some 2-alkylindazoles to the corresponding 1-alkyl-benzimidazoles has been investigated. The reaction is shown to be monomolecular and monophotonic. It starts from the lowest excited singlet state. The u.v, spectrum of an intermediate with lifetime in the order of seconds at room temperature could be determined. There is some evidence for the existence of a further short-lived precursor. Several theoretical models have been applied to speculate on the nature of the primary step.