Conformational isomers and rotational barriers in the ketyl radicals of thiophen and thienothiophens

Abstract

The conformational isomers of the ketyl radicals (I)–(III) generated from di-2-thienyl ketone, bis-2-thieno[3,2-b] thienyl ketone, and bis-2-thieno[2,3-b]thienyl ketone, have been detected by e.s.r. spectroscopy. The two preferred conformations are cis–trans and trans–trans. Study of the line shape variations with temperature of the e.s.r. spectra allowed the determination of the activation parameters for the rotation of the aromatic rings. The activation energies follow the same order as the electron-delocalizing power of the nuclei, the values being 31·30, 37·95, 35·73, and 26·57 kJ mol^–1 for (I), (II), (III) and for benzophenone ketyl, respectively.