Hg(63P1) photosensitization of 3-methylbut-1-ene. Part 2.—Intersystem crossing and cyclisation of the 2-methylbuta-1,3-diyl biradical

Abstract

Hg(6³P₁) photosensitization of 3-methylbut-1-ene (3MB) yields, among other products, trans- and cis-1,2-dimethylcyclopropane, in a strongly pressure dependent ratio, ranging from 1.37 at 2.5 Torr to 3.44 at 789.2 Torr. This observation is interpreted in terms of a mechanism involving intersystem crossing and ring closure of both vibrationally excited and thermalised triplet 2-methylbuta-1,3-diyl (MBD13) biradicals, produced by a 5pp hydrogen migration in excited triplet 3MB. Quantitative analysis of the data allows a value for the intersystem crossing rate constant for ³(MBD13)^* to be obtained, together with the ratio of dimethylcyclopropanes formed from thermalised MBD13. Addition of oxygen to the reaction system considerably simplifies the observed product spectrum and enables lower limit estimates of the rate constants for intersystem crossing and cyclisation of thermalised triplet and singlet MBD13 respectively, to be derived. The results are compared to those obtained in other systems, and their relevance to the implications of thermochemical and quantum chemical analyses of the energetics of propa-1,3-diyl biradicals discussed.