Application of ¹⁵N spectroscopy and dynamic NMR to the study of ureas, thioureas and their lewis acid adducts

Rotational barriers and 15 N chemical shifts have been measured in a number of ureas and thioureas. As anticipated on the basis of the 15 N shifts, several previously unobserved rotational barriers could be detected by using lanthanide reagents or a high field spectrometer. Nearly constant effects on both the rotational activation energy and the 15 N shift are produced on going from ureas to the corresponding thioureas, and correlations are found between the ΔG‡ and δ 15 N values. The results are discussed in terms of lone pair delocalization, and anomalies with respect to the general behaviour are tentatively explained in the light of the effect of steric torsion in crowded structures on the 15 N shifts and rotation barriers.