A reaction scheme is described allowing for magnetic field effects on the chemical kinetics of triplet reactions due to the selective decay of triplet sublevels. The theoretical treatment of this scheme is outlined on the basis of a stochastic Liouville equation, taking into account the rotational diffusion of molecules in liquid solution. Whereas the exact solution of the general case is obtained by a numerical procedure as described by Pedersen and Freed, an approximate analytical expression is also given. The magnetic field dependence of the radical yield in the electron transfer reaction between the thionine triplet and halogen-substituted anilines in methanol was measured up to 17 kG by a microprocessor-controlled laser flash spectrometer with a special averaging technique. The data were analyzed in terms of the triplet mechanism operating on the decay of triplet exciplexes formed as intermediates in the electron transfer reaction. Whereas the rate constant of intersystem crossing (Itisc) in these exciplexes depends on the halogen substituent and its position, the largest value ((7-10) X 1O hoch 10 s-l) being obtained for the p-iodoaniline exciplex, the rate constant of exciplex dissociation into free radicals (k,) is of the order of (5-8) X lo9 s-l for all halogen-substituted anilines investigated. Magnetic field effects on chemical kinetics according to the triplet mechanism appear to be especially useful for investigating very short-lived triplet intermediates not easily amenable to other methods of detection.
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Ulrich et al. (1983) studied this question.