Isomerization reactions relaxation equation

Ion-derived products are also formed in the irradiation of bicyclic benzyl chlorides150,151. Compound 23, which is both a benzylic and a homoallylic halide, shows a comparable direct versus sensitized irradiation dichotomy as simple, less activated benzylic halides do (equation 32)150. Also, in the direct isomerization, substantial epimerization occurs, which is not observed upon sensitization. This means that the R+CT ion pairs produced in the singlet and triplet reaction are not identical and recombine or are captured before relaxing to identical species. Alternatively, the different behaviour may well be a manifestation of the intermediacy of an exciplex as product-directing intermediate in the sensitized reaction (cf equation 31). 

Velocity relaxation effects can be accounted for in an approximate fashion by going to a phase-space description in terms of Fokker-Planck or Langevin equations. Perhaps the best known study of this type is due to Kramers, who studied the escape of particles over potential barriers as a model for certain types of isomerization or dissociation reaction. 

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