Electron paramagnetic resonance studies of electron attachment to thiophene, bi(thienyl) and some halogen-substituted thiophenes

Abstract

Exposure of dilute solutions of 2, 2′-bithienyl in 2-methyltetrahydrofuran glasses at 77 K to ionizing radiation gave the corresponding radical-anion, as indicated by the EPR spectra. Similar treatment of thiophene solutions failed to give the expected it radical-anion, although electron capture clearly occurred: for CD₃OH and CD₃OD solutions the primary products were identified as the H- and D-atom-addition products, formed by protonation of the anions.

Under similar conditions, 2-bromothiophene gave clear EPR signals with features characteristic of the C₍₂₎–˙Br^–σ* radical-anion (with a spin density of ∼ 37% on bromine). A σ* anion, with the unpaired electron localized in one of the C–Cl σ* orbitals, was detected following electron-addition to 2,5-dichlorothiophene, whereas for 2-chlorothiophene the corresponding σ* radical-anion, if formed, evidently decomposes rapidly to the 2-thienyl radical, whose EPR spectrum is reported. Only for 2,2′-bithienyl was the radical-anion's EPR spectrum detected in fluid solution following electron attachment.

These findings and the results of MO calculations are discussed in terms of the structures of the radicalanions and the alternative routes available for reaction following electron-addition.