Tautomerism in a thioamide-nitroxide: solvent effects in terms of an electron spin resonance parameter for a 2-thiocarbonylimidazolidine 1-oxyl

Abstract

The tautomerism of 4,4,5,5-tetramethyl-2-thiocarbonylimidazolidine 1-oxyl (2) was studied by e.s.r. spectroscopy. In a range of 14 solvents, the proportion of ene-thiol (3a) correlated inversely with the hyperfine coupling constant a_{N – 1} of the nitroxide nitrogen atom of the ene-thiol. The coupling constant was low in solvents of low polarity and in hydroxylic solvents, high in electron-pair donating solvents. These effects have been interpreted in terms of hydrogen-bond acceptance by the imino-group of the ene-thiol, hydrogen-bond donation by the thiol group, and resulting changes in the CN bond order as revealed by a_{N – 1}.