Electron paramagnetic resonance spectroscopy of hetero[5]annulenes

Abstract

The EPR spectra of the radical cations of a variety of compounds containing the pyrrole moiety have been recorded in fluid solution. In all, the unpaired electron resides in an antisymmetric orbital with the nitrogen atom lying in the nodal plane. N-Arylpyrroles and p-phenylenebispyrroles show no hyperfine coupling to the aromatic group, and electron release into the pyrrole ring by 2,5-methyl groups is greater than by 2,5-ethyl groups. 2,2′,5,5′-Tetramethyl-1,1′-bipyrrolyl has an orthogonal structure, and the unpaired electron is delocalised over both rings. The EPR spectrum of the radical cation of 8b,8c-diazapyracyclene, which is isoelectronic with the radical anion of pyracyclene, has been observed, and it shows that, in the presence of mercury(II) trifluoroacetate, mercuriation occurs at the periphery of the molecule.