Electron spin resonance investigation of sulphur-33 and nitrogen-15 substituted dithiazol-2-yl and dithiazolidin-2-yl free radicals

Abstract

Thermal cyclo-addition reactions of tetrasulphur dinitride with certain alkynes generate a new series of stable free radicals possessing the 1,3,2-dithiazolyl ring. The radicals have simple e.s.r. spectra and are potentially useful spin probes or labels. The sharpness of the spectral lines has enabled us to observe natural abundance lines from nitrogen-15, sulphur-33 and carbon-13. The presence of two equivalent sulphur atoms in the heterocyclic ring has been demonstrated by isotopic double labelling with nitrogen-15 and sulphur-33. An isotopic triple labelling experiment with the latter two isotopes and with deuterium was carried out with the related 1,3,2-dithiazolidinyl radicals. Temperature coefficients of coupling constants were measured for both classes of radicals and INDO calculations showed that nitrogen out-of-plane wagging is responsible for all the temperature coefficients.

Molecular orbital calculations were performed for the 1,3,2-dithiazolyl and 1,3,2-dithiazolidinyl structures using semi-empirical INDO and ab inito Gaussian 76 computer programs: the optimised structural parameters from both calculations are in good agreement. The INDO calculations gave signs and magnitudes of isotropic coupling constants in overall agreement with those observed experimentally.