Hg(63P1) photosensitization of cyclohexanone. Role of triplet biradical intermediates

Abstract

Hg(6³P₁) photosensitization of cyclohexanone results in the formation of pent-1-ene, cyclopentane and hex-5-enal, just as observed in the direct photolysis. As the pressure of added SF₆ or Ar bath gas is increased, both the total product quantum yield and that of the hydrocarbon products decrease, while that of hex-5-enal increases. A comprehensive mechanism, differing in detail from those previously proposed, is now formulated to account quantitatively for the experimental observations. Two sequentially formed, energy randomized, vibrationally excited triplet biradicals are believed to be the important intermediates that lead to product formation. Thus α-C—C bond cleavage of triplet cyclohexanone yields an acyl-alkyl biradical, ³B*, that can give rise to both hex-5-enal and also, by loss of CO, to the penta-1,5-diyl biradical, ³PD*, the precursor of the hydrocarbon products. Rate constants for intersystem crossing of the two biradicals are deduced along with that for the fragmentation of ³B*. An RRKM treatment of this decomposition suggests that the observed rate constant is best fitted using a biradical excitation energy calculated by assuming a heat of formation for ³B greater than the value computed by conventional methods, which neglect electronic interaction. In addition, relative rate constants are obtained for the intramolecular disproportionation and cyclisation of ¹B and ¹PD when both vibrationally excited and thermalized. It is concluded that the critical energy for ring closure of ¹PD is greater than that for isomerization to pent-1-ene.