The free radical 2,3,4,5-tetraphenylpyrryl in dilute, oxygen-free solution gives an electron spin resonance rich in hyperfine structure. The principal features of the spectrum are seven peaks in a binomial intensity distribution. These are attributed to approximately equal isotropic hyperfine interactions with the six ortho and para protons of the 2- and 5-phenyl groups. A molecular-orbital computation of spin densities in the pyrryl ring is qualitatively consistent with this result. The radical concentration may be enhanced by ultraviolet radiation but subsequently decays toward its original value with second-order kinetics. It is postulated that: (i) the radical dimerizes spontaneously until an equilibrium between radical and dimer is established; (ii) the dimer may undergo photodissociation back to the radical.

This paper describes a detailed experimental and theoretical study of the hyperfine structure of diphenylpicrylhydrazyl (hydrazyl) and picrylaminocarbazyl (carbazyl). Both isotropic and anisotropic hyperfine interactions involving the α and β nitrogen atoms are considered. Experimentally, electron spin resonance spectra are observed for dilute solutions of each radical in both fluid and solid media. Concentrations of the order of 0.001 M are employed so as to minimize dipolar and exchange interactions among radicals. The fluid spectra yield, in straightforward fashion, the absolute values of the isotropic coupling constants | A1 | and | A2 |. These splitting parameters are chosen such that a synthesized line shape composed of Gaussians gives optimal reproduction of the observed resonance profile. For the solid solutions, Duco cement (with hydrazyl) and Lucite (with carbazyl) provide glassy trapping matrices. It is assumed that the radicals are frozen in an ensemble of random orientations in these solids. Appropriate theoretical line shapes, considering interactions with two magnetic nuclei, are derived, and values assigned to the anisotropic parameters B1 and B2 from comparison of experimental and theoretical resonance patterns. The isotropic constants are assumed to be unchanged in the transition from liquid to solid solution. It is found moreover that the inherent Gaussian half-width σ is approximately the same in both states. The parameters assigned through our analysis are Hydrazyl—A1=9.35±0.20 gauss; A2=7.85±0.20 gauss; B1=6.6±0.5 gauss; B2=5.8±0.5 gauss; σ=3.36±0.20 gauss. Carbazyl—A1=10.2±0.5 gauss; A2=5.8±0.5 gauss; B1=7.7±1.0 gauss; B2=4.1±1.0 gauss; σ=3.4±0.5 gauss. Resonance of the hydrazyl-Duco solution during incipient solidification was also observed. In this intermediate medium the anisotropic hyperfine interactions are only partially averaged out by molecular motions. It was found possible to characterize the rigidity of the medium by a parameter λ (0≤λ≤1) such that each anisotropic parameter is exhibited in the spectrum as if it were λ times its full value. Various implications of our results on the structure of hydrazyl and carbazyl are considered. One conclusion drawn is that both radicals are largely planar in configuration.

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