Nitrogen NMR shieldings of thiourea systems as a function of solvent polarity and hydrogen bond effects

Abstract

Tetramethylthiourea, 1, and thiourea, 2, have been studied in a wide variety of solvents by high-precision 14N NMR measurements. The chosen solvents exhibit a wide range of hydrogen-bonding and polarity/polarisability properties. The observed nitrogen shielding variations of the solutes, due to solvent change, are significant and are attributed to solvent polarity, solute to solvent and solvent to solute hydrogen-bonding effects. Nitrogen deshielding of 1 and 2 is produced by all these interactions, due to an enhanced delocalisation of the nitrogen lone-pair electron into the π systems of 1 and 2. The nitrogen NMR shieldings of 1 and 2 are calculated by the CHF–GIAO ab initio molecular orbital procedure using a 6-31++G** basis set. The calculations are for isolated molecules, and their results satisfactorily reproduce the position of the thiourea nitrogen resonance obtained in a dilute solution in cyclohexane with respect to that of urea systems and nitromethane.