Heterocyclic free radicals. Part 10. Phenothiazine cation-radicals as probes of the inductive effect

Abstract

Phenothiazine cation-radicals, functionalised at nitrogen by polymethylene chains which terminate in polar substituents, exhibit nitrogen hyperfine splittings which vary regularly with the length of the polymethylene chain and with the polarity of the terminal substituent, as measured by σ_I. The cation-radicals are thus probes of the inductive effect which give quantitative insight into its transmission mechanisms and its attenuation by intervening bonds. For chain-lengths longer than a single methylene group, the field effects of the polar terminal substituents are correlated by a cosθ/r³ function which implies that any terminal substituent may be modelled as a point-dipole whose mean orientation and mean separation from the radical aminium centre are determined by the conformational preferences of the polymethylene chain. Literature values of σ_I for certain substituents are questioned in the light of the results obtained and inductive substituent constants, in general, are discussed.