Application of 15N Spectroscopy and Dynamic NMR to the Study of Ureas, Thioureas and their Lewis Acid Adducts

Article abstract of DOI:10.1002/mrc.1270130603

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