Nitroxide radicals. Part XVI. Unpaired electron distribution in para-substituted aryl t-butyl nitroxides and 2-naphthyl phenyl nitroxides

Abstract

N.m.r. and e.s.r. measurements on a number of p-methoxy-, p-phenoxy-, p-alkyl-, and p-phenyl-phenyl t-butyl nitroxides are reported. The empirical relationship a_H^p–OMe/G = 0·123a_H^p–H/G – 0·02 between the a_H^OMe value of p-methoxyaryl radicals and the a_p–H value of the corresponding unsubstituted radicals has been established. The positive coupling constants of the β-protons of the p-t-butyl and -isopropyl groups of p-t-butyl-, p-isopropyl- and p-ethyl-phenyl t-butyl nitroxides are attributed to homohyperconjugative interaction between positive free spin on the para-carbon atom and the electrons of the β-C–H bond. E.s.r. and n.m.r. spectra of 2-naphthyl and 8-t-butyl-2-naphthyl phenyl nitroxides have been measured and interpreted.