A Time-Resolved EPR Study on the Photochemical α-Cleavage of Benzoin and Related Molecules

Article abstract of DOI:10.1246/bcsj.65.1679

The photochemical α-cleavage reactions of benzoin, benzoin methyl ether (BME) and deoxybenzoin (DOB) in methylcyclohexane and benzene have been studied by means of the time-resolved EPR technique.Strong emissive CIDEP spectra of the radicals produced by the α-cleavage reactions are observed in benzoin and BME.From the analysis of the hyperfine splittings they are assigned as benzoyl and α-hydroxybenzyl radicals in the case of benzoin and benzoyl and α-methoxybenzyl radicals in BME.It is clearly shown that the cleavage reaction takes place in the T₁(n?^*) states of these carbonyls and the triplet mechanism (TM) is mainly responsible for the CIDEP spectra.From the time profile of the CIDEP signals it is also shown that the produced radicals react almost exclusively with each other.In the case of DOB, the α-cleavage is found to be much less efficient and a hydrogen abstraction reaction is more dominant at room temperature.It is concluded that the cleavage rate constant (k_c) is in the order, k_c(BME)>k_c(benzoin)>>k_c(DOB).