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| - Influence of the Geminal Biscyclopropyl Group in the Reactions ofTrimethylenemethane: Reversible Intersystem Crossing
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| - The azo compound precursor to Berson's trimethylenemethane (TMM)biradical system, 2-methylene-1,3-cyclopentanediyl, was substituted with two cyclopropyl groupson the exomethylene.Generation of the TMM species by pyrolysis, direct photolysis, andbenzophenone-sensitizedphotolysis resulted in formation of both cyclopropane ring-openedmonomer and cyclopropane ring-unopened dimers whose ratio depended directly on temperature and extentof dilution. The groundstate appears to be triplet on the basis of ESR spectrometry. Theresults contrast with previouswork (Waldemar Adam) showing that monocyclopropyl substitution onBerson's TMM species givesboth ring-opened monomer and ring-unopened dimers upon pyrolysis andgives mostly ring-unopened dimer upon direct photolysis at 0−20 °C in what appear tobe concentration-independentreactions. The observations with the biscyclopropyl-substitutedspecies are consistent with stericeffects retarding dimerization of the triplet allowing reversibleintersystem crossing back to singletand perhaps with reduction in the singlet−triplet gap aswell.
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