Electron Spin Resonance of Tetraphenylpyrryl Radical

Abstract

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.