SOLVENT EFFECTS ON NITROGEN NMR SHIELDINGS IN THIAZOLE AND THIADIAZOLE SYSTEMS

High precision N-14 NMR measurements are reported for all six possible thiazole and thiadiazole molecules in a variety of solvents, Both sol vent polarity and hydrogen bonding effects on the nitrogen nuclear shi elding of the solutes are significant. Both types of effect produce an increase in the solute nitrogen shielding, Analysis of the experiment al data, and molecular orbital studies, indicate that an increase in t he 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 p yridine type nitrogen atoms in the corresponding diazole and triazole systems, The significant Shielding effects which result from solvent t o solute hydrogen bonding to ring nitrogen atoms are shown to be essen tially local in origin, Thus the shielding concerned is that of the ni trogen atom directly involved in the hydrogen bonding.