1H NMR spectra part 31: 1H chemical shifts of amides in DMSO solvent

The 1H chemical shifts of 48 amides in DMSO solvent are assigned and presented. The solvent shifts (DMSO-CDCl3) are large (1-2ppm) for the NH protons but smaller and negative (-0.1 to -0.2ppm) for close range protons. A selection of the observed solvent shifts is compared with calculated shifts from the present model and from GIAO calculations. Those for the NH protons agree with both calculations, but other solvent shifts such as (CHO) are not well reproduced by the GIAO calculations.The 1H chemical shifts of the amides in DMSO were analysed using a functional approach for near (3 bonds removed) protons and the electric field, magnetic anisotropy and steric effect of the amide group for more distant protons. The chemical shifts of the NH protons of acetanilide and benzamide vary linearly with the density on the N and C atoms, respectively. The C=O anisotropy and steric effect are in general little changed from the values in CDCl3. The effects of substituents F, Cl, Me on the NH proton shifts are reproduced. The electric field coefficient for the protons in DMSO is 90% of that in CDCl3. There is no steric effect of the C=O oxygen on the NH proton in an NH...O=C hydrogen bond. The observed deshielding is due to the electric field effect. The calculated chemical shifts agree well with the observed shifts (RMS error of 0.106ppm for the data set of 257 entries). Copyright (c) 2014 John Wiley & Sons, Ltd.