HYDROGEN-BONDING AND THE STRUCTURE OF SUBSTITUTED UREAS - SOLID-STATE NMR, VIBRATIONAL SPECTROSCOPY, AND SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDIES

Crystalline N1,N1-dimethyl-N2-arylureas with a para X-substituent (X = Cl, Br, and NO2) and an ortho hydroxyl group in the phenyl ring (with respect to the urea moiety) were studied by C-13 CP/MAS NMR, IR, and Raman spectroscopies and by single-crystal X-ray diffraction (XRD). Details of single-crystal X-ray determination of the structure of N1,N1-dimethyl-N2-(2-hydroxy-4-nitrophenyl)urea are given. Solid-state NMR effects related to hydrogen bonding and to molecular conformations were observed. In the NO2 derivative, the carbonyl and hydroxyl groups are in pseudotrans orientation to one another, while in the Cl derivative both pseudocis and pseudotrans orientations occur. For the pseudocis orientation, the hydrogen bond is intramolecular and completes a seven-membered ring, while for the pseudotrans orientation it is intermolecular. The NO2 derivative conformation is largely determined by a transquinoidal resonance between the NO2 group and the urea moiety, mediated by the aromatic ring. The study demonstrates the power of XRD data analysis on the basis of the comparison of experimental and literature results as well as the advantages of using physical methods in tandem for solving complex organic structures.