Radical cations of substituted pyridines formed by radiolysis

Exposure of dilute solutions of various substituted pyridines in trichlorofluoromethane to 60Co γ-rays at 77 K gave the corresponding cation radicals, characterised by their e.s.r. spectra. As was first established by Shida and coworkers, the pyridine cation has a σ-structure rather than π, the unpaired electron being in the in-plane s-p hybrid 'lone-pair' orbital on nitrogen. Photoelectron-spectroscopic studies and theory suggest that the orbital energies of the highest filled π-orbital and the N(σ)-orbital are very similar, so we have attempted to produce π-cations by using suitably substituted derivatives. Surprisingly mono- and most di-methylated derivatives gave only the N(σ)-cations, as did the o- and m-CHO derivatives. However, the 2,5-dimethyl derivative gave only the π-cation. Also the 2-chloro and 2-bromo derivatives gave π-cations, whilst the m-chloro derivative probably gave both N(σ)- and π-cations. The π-cation was also obtained from the o-amino derivative. Aspects of structure for these cations are discussed. On annealing more concentrated solutions, bimolecular reactions gave 2-pyridyl radicals rather than σ*-dimers. (author)