An electron spin resonance flash photolysis and chemically induced nuclear polarisation study of the photolysis of benzaldehyde in solution

The photolysis of benzaldehyde and its ring-deuterated analogue has been studied in various solvents by the combined techniques of flash photolysis with time-resolved e.s.r. detection and chemically induced dynamic nuclear polarisation (CIDNP). In efficient hydrogen donors triplet benzaldehyde abstracts hydrogen to give the ketyl radical, which decays with first order kinetics. In n-hexane abstraction occurs from benzaldehyde itself to give both the ketyl and benzoyl radicals; benzoyl disappears by a pseudo first-order reaction with the solvent (k= 1.6 × 103 s–1). Using naphthalene as quencher shows that all the benzoyl originated in the triplet reaction, and that the rate of the triplet abstraction reaction is diffusion controlled (k= 1.64 × 109 l. mol–1 s–1). In inert fluorocarbon solvents (PP2 and PP9) and in the undiluted liquid a third radical is produced whose identity is uncertain. In PP2 benzoyl disappears by dimerisation (k= 1.0 × 109 l. mol–1 s–1) and the third radical by slow first-order reaction (k= 85 ± 20 s–1). The ketyl spectrum was too weak for its decay kinetics to be determined in both PP2 and n-hexane. CIDNP experiments showed that the same primary radical pair was produced in a variety of solvents and were used to elucidate the radical reaction pathways; in only the strong hydrogen donor propan-2-ol did the primary pair include a solvent radical.