Elektronenspinresonanz freier Radikale bei photochemischen Reaktionen von Ketonen in Lösung

Transient alkyl and acyl radicals are detected and identified by electron spin resonance during UV.‐Irradiation of dialkyl and alkyl aryl ketones in liquid solutions. They are formed by the primary processes of α‐cleavage, intermolecular photoreduction or α‐chloro elimination. For several ketones a temperature dependent competition between α‐cleavage and photo‐reduction is observed indicating a higher energy of activation for the cleavage than for the reduction process. The results are discussed with respect to the photochemical behaviour of the ketones. Electron spin resonance parameters are reported for a variety of radicals. In particular 13C coupling constants support a sigma type structure of acyl radicals and require a nearly planar radical center of t‐butyl and isopropyl radicals.