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Characterization of Amides and Ureas by Proton Magnetic Resonance Spectra of Hexamethylphosphoramide Solutions
Volume 28, Number 2 (April 1974) Page 197-199
Arsenault, M.A.; Freeman, C.D.; Hooper, D.L.
The broad bands observed for amide and urea NH protons are familiar features of the proton magnetic resonance (PMR) spectra of natural products, yet little use of these signals is made in the determination of molecular structure or conformation. Two reasons for this are evident: chemical shifts for these (and other NH protons) are strongly concentration-, solvent-, and temperature-dependent, while the broad bands rarely show any of the expected fine structure resulting from spin-spin coupling to adjacent protons. The primary cause of the observed broadening is the efficient relaxation of NMR signals brought about by the electric quadrupole moment of the 14N nucleus; however, there must certainly be a contribution from intermolecular hydrogen bonding of NH protons, and there may be a contribution from intermolecular proton exchange. When intermolecular hydrogen bonding and proton exchange are hindered, for example, by formation of an intramolecular hydrogen bond, the NH signals of amines are narrowed and the fine structure may be observed. In a solvent which forms strong hydrogen bonds to an amide, intermolecular exchange may similarly be so hindered that fine structure can be observed and the multiplicity of NH signals used to characterize amides in the same way that the fine structure of OH signals in the spectra of dimethylsulfoxide (DMSO) solutions is used to characterize alcohols