Rotational barriers in thienyl triethylsilyloxy nitroxides

Abstract

Triethylsilyloxy nitroxide radicals of thiophen and thienothiophens have been produced from the corresponding nitro-derivatives. Their e.s.r. spectra showed the presence of rotational isomers at low temperature for the thiophen radicals and even at room temperature for the thienothiophen analogues. In the former case the kinetics of the interconversion could be followed by examining variations in the line shape with temperature. The activation parameters have been determined and the rotational barrier is larger for the 2-thienyl than for the 3-thienyl silyloxy nitroxide. For the purpose of comparison the parameters related to the internal rotation of phenyl triethylsilyloxy nitroxide have been obtained.