N-15 NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY - NATURAL-ABUNDANCE N-15 CHEMICAL-SHIFTS OF ALKYL-SUBSTITUTED AND ARYL-SUBSTITUTED UREAS

16N chemical shifts of urea and several alkyl-and arylureas have been determined at the natural-abundance level in DMF and Me2SO. Dilution has very little effect on the chemical shifts. N-Methylation at nitrogen induces systematic upfield shifts which contrast with expected downfield shifts. Alkyl substitution at positions β, γ, γ to the nitrogen induces shifts in the expected order based on aliphatic amines. Multiple regression analysis gives appropriate α, β, γ, and δ substituent parameters. The shifts of urea and the methylureas can be correlated with ionization potential differences between lone-pair molecular orbitals. Activation energy barriers for rotation around the C-N bond have been estimated using equations derived for substituted amides; the appropriateness of this method is discussed. 13C chemical shifts of the ureas have also been determined. © 1979, American Chemical Society. All rights reserved. © 1979, American Chemical Society. All rights reserved.