An electron spin resonance study of the radical cations of pyrroles, furans, and thiophenes in liquid solution

Photolysis of alkylpyrroles in trifluoroacetic acid containing mercury(II) trifluoroacetate, alkylfurans in trifluoroacetic acid, or alkylthiophenes in sulphuric acid, induces oxidation to the corresponding radical cations. The e.s.r. spectra show that the electronic configuration is similar in all three species, the unpaired electron occupying the ψA MO in which the heteroatom lies in a nodal plane. Photolysis of 2,6-dimethyl- and 2,6-diethyl-thiophene in trifluoroacetic acid containing mercury(II) trifluoroacetate, on the other hand, gave rise to spectra with a high g value (2.0062), showing hyperfine coupling to two non-equivalent pairs of alkyl groups in an unsymmetrical dimer.