Captodative effect qualitative

The familiarity with qualitative valence bond descriptions of substituent effects in combination with the known substituent effects in carbocations and carbanions led Viehe and his group to the postulate of a captodative effect for free radicals (Stella et ai, 1978 Viehe et al., 1979). They did not seem to be aware of the earlier work which was of a more physical organic character. The fact that carbocations [8] are stabilized by + M substituents, and carbanions [9] by - M substituents, raised the idea that free radicals, as... 

All these examples, and it would be possible to quote more, are a manifestation that captor and donor subtituents stabilize radicals. Judged by the temperature range where dissociation occurs it seems as if captodative substitution stabilizes better than dicaptor substitution (Stella et al., 1981). Mostly, however, these are qualitative or semiquantitative observations which do not allow one to evaluate the magnitude of stabilization in kcal mol". In particular, the question of a synergetic action of the captor and the donor substituent cannot be answered satisfactorily. In part, the observed effects might be related to steric interactions of the substituents. 

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