Impetus for upward revision in the heats of formation of the ethyl (primary), isopropyl (secondary), and t-butyl (tertiary) free radicals has gained sufficient momentum in the last 6 yr to prompt an illustrative assessment of the far-ranging implications for interpretation of thermal reorganizations. Previous obstacles to consideration of diradicals as transition states in rearrangements of cyclopropanes and cyclobutanes are removed. The energetic relation of cyclohexa-1,4-diyl as an intermediate diradical in the Cope rearrangement to the transition state of a concerted mechanism is clarified.
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W. von E. Doering (1981) studied this question.