Laser flash photolysis–electron spin resonance kinetic studies of homolytic reactions in solution

Abstract

Laser Flash Photolytic (308 nm) generation coupled with time-resolved e.s.r. spectroscopic detection of radical intermediates has been used to obtain absolute rate coefficients and activation parameters for a number of uni- and bi-molecular homolytic reactions [(A)–(F)] in solution. The results accord well with Bu^tOĊO → Bu^t˙+ CO₂(A), (EtO)₃ṖOBu^t→ Bu^t˙+(EtO)₃PO (B), Bu^tO˙+ cyclo-C₅H₁₀→ cyclo-C₅H₉˙+ Bu^tOH (C), Bu^tO˙+ Bu^tOMe → Bu^tOĊH₂+ Bu^tOH (D), Me₃SiO˙+ cyclo-C₅H₁₀→ cyclo-C₅H₉˙+ Me₃SiOH (E), Me₃SiO˙+ Bu^tOMe → Bu^tOĊH₂+ Me₃SiOH (F) those obtained previously for some of these reactions using other techniques. The data for reaction (C) are in excellent agreement with those obtained by laser flash photolysis coupled with optical detection of transients, but differ markedly from results obtained previously by a flash photolysis/e.s.r. technique. The kinetic results for the hydrogen transfers (E) and (F) represent the first absolute rate data for reactions of trialkylsiloxyl radicals. The trimethylsiloxyl radical is more reactive in hydrogen abstraction and also more electrophilic than Bu^tO˙; these differences are attributable to the π-acceptor properties of the Me₃Si group.