Solvent effects on nitrogen NMR shieldings in thiazole and thiadiazole systems

Abstract

High precision ¹⁴N NMR measurements are reported for all six possible thiazole and thiadiazole molecules in a variety of solvents. Both solvent polarity and hydrogen bonding effects on the nitrogen nuclear shielding of the solutes are significant. Both types of effect produce an increase in the solute nitrogen shielding. Analysis of the experimental data, and molecular orbital studies, indicate that an increase in the polarity of the solvent favours the delocalization of the lone pair electrons from the sulfur atoms into the conjugated rings. This leads to an increase in electronic charge at the nitrogen atom(s) concerned. This effect is more pronounced than analogous effects observed for pyridine type nitrogen atoms in the corresponding diazole and triazole systems. The significant shielding effects which result from solvent to solute hydrogen bonding to ring nitrogen atoms are shown to be essentially local in origin. Thus the shielding concerned is that of the nitrogen atom directly involved in the hydrogen bonding.