Solvent Effects in the Electron Spin Resonance Spectra of Some Phenoxyl, Nitroxide, and Anilino Radicals

The electron spin resonance spectra of 2,6-di-t-butyl-4-methylphenoxyl (I) and 2,4,6-tri-t butylphenoxyl (II) have been observed under high resolution, and it has been shown experimentally that the para methyl proton hyperfine splitting (aCH3H) in the radical (I) increases, while, inversely, the meta proton hyperfine interaction decreases, as the dipole moment of the solvent (μ) increases. The relationship between these two variables can be written as: a(μsolv)=a(μsolv=0)+K·μsolv, where μsolv is the dipole moment of the solvent and where K is the constant of proportionality. Further study of diphenyl nitric oxide (DPNO) (IV) and di-p-anisyl nitric oxide (DPANO) (V) radicals has shown this to be a general form of the solvent effect. However, in the case of various kinds of solvents with OH or NH groups, it has been found that aN deviates from the equation. This phenomenon has been explained qualitatively by the effect of hydrogen bonding. Therefore, the variation in the coupling constants has generally been explained by two effects, that is, the μ dependence described above and the electron delocalization arising from hydrogen bonding. Similar experiments performed with 2,4,6-tri-t-butylphenoxy (II) and 2,4,6-tri-t-butylanilino (III) radicals have shown that the variation in the hyperfine splitting constants is small because of the steric hindrance of tertiary butyl groups in these molecules.