A series of diaryl nitric oxide radicals have been studied by the use of the electron spin resonance (ESR) technique.Several methyl derivatives of diphenyl nitric oxide were synthesized and their ESR spectra investigated.It has been shown that the hyperfine coupling constants of 14N, especially those of protons, are much affected in the case of the ortho-methylated compounds as a result of steric hindrance, which is discussed using the Huckel molecular-orbital theory and the valence-bond theory.In conclusion, two benzene rings of diphenyl nitric oxide derivatives are equivalently distorted for the symmetrically-substituted compounds and unequivalently for the unsymmetrically-substituted compounds. Furthermore, this tendency is strengthened by ortho-substitution.Many investigators have previously studied various kinds of neutral free radicals by means of the ESR method. Some of these radicals have been those of alkyl or aryl derivatives of nitric oxide. Since they are not as stable as diphenyl picryl hydrazyl (DPPH), however, there have been few results of the investigation, and the experiments have been carried out by the reaction of corresponding amines or hydroxylamines with the appropriate oxidants in the various systems.1) However, many new stable radicals of nitric oxide derivatives have recently been found.2)Some of the present authors have previously reported on ESR studies of diphenyl nitric oxide (DPNO) in a solid state3) and in a liquid solution,4) and of di p-anisyl nitric oxide, a dimethoxy derivatives of DPNO, in a liquid state.5) Similarly, the ESR spectra of methyl-substituted DPNO have been reported.6) However, there have been no detailed ESR studies of methyl-substituted DPNO.We have now observed the proton hyperfine spectra of p-, m-, o-methyl diphenyl nitric oxide (tolyl phenyl nitric oxide), p, p'-, m, m'-, o, o'-dimethyl diphenyl nitric oxide (ditolyl nitric oxide), p, o'-dimethyl diphenyl nitric oxide, p, o, otrimethyl diphenyl nitric oxide (mesityl phenyl nitric oxide), and p, o, o, p'-tetramethyl diphenyl nitric oxide (mesityl tolyl nitric oxide) in liquid solutions and have studied the effect of methyl substitution on the spin density distribution on nitrogen and ring-carbon atoms.The steric influences of substituents have been reported in many regions of ultraviolet spectroscopy, infrared spectroscopy, and electron diffraction. The steric effect has also been observed in ESR studies of several ortho-substituted radicals.7-9) DPNO derivatives which contain methyl groups in orthopositions have a similar effect and a remarkable change in spectra is also observed in them; this change has been attributed to the forced internal rotation of the phenyl ring resulting from the steric repulsion between two ortho-groups. We will also discuss our findings in terms of the steric repulsion energy and the resonance energy from the view point of molecular-orbital calculation and the valence-bond theory. Finally, the solvent effect will be discussed. Experimental DPNO and its derivatives were synthesize...
