Solvent effects in the electron spin resonance spectra of nitric oxide radicals and para-substituted nitrobenzene anions

It is shown that the nitrogen hyperfine coupling parameters for nitric oxide radicals depend on the solvent. An equilibrium constant is calculated for the interaction of diphenyl nitric oxide and p-p′-dianisyl nitric oxide with di-tert-butyl peroxide and tert-butyl hydroperoxide in a mixture of the two solvents. By plotting the nitrogen splitting found in the peroxide against that in the hydroperoxide for a series of nitric oxide radicals a straight line is obtained and a tentative theoretical treatment is given to account for this based on the equation of Karplus and Fraenkel. Similar results are shown to hold for the para-substituted nitrobenzene anions.