QUESTION OF THE RELATIONSHIP OF N-15 CHEMICAL-SHIFTS TO BARRIERS TO C-N INTERNAL-ROTATION - DYNAMIC NUCLEAR MAGNETIC-RESONANCE OF UREA AND ANILINE DERIVATIVES

Dynamic 1H and 13C NMR effects resulting from restricted internal rotation about C-N bonds are observed in tetramethylurea (1), tetramethylthiourea (2), TV-methylaniline (3), and p-nitro-VV-methylaniline (4) in the temperature range 0 to -150°C. The free-energy barriers for C(=X)-N internal rotation in 1 and 2 are both 6.3 kcal/mol and do not agree with barriers recently predicted for these compounds from 15N chemical shift data. The barrier found for 1 by means of line-shape measurements agrees with a previously determined value obtained by a T1p technique. A reason for the failure of the predictions based on chemical shifts is presented. The barrier to C-N internal rotation in 3 is 6.1 kcal/mol, and is in reasonable agreement with barriers predicted from 15N chemical shifts and from a Hammett relationship. The barrier to internal rotation in 4 is distinctly solvent dependent, being larger in acetone-d6 (11 kcal/mol) than in CD2Cl2 (10.2 kcal/mol). The barriers predicted for 4 on the bases of 15N chemical shifts and of the Hammett relationship are in fair agreement with the experimental value. © 1979, American Chemical Society. All rights reserved.