| Abstract
| - The magnetic deshielding caused by the amido group on CON−CHα protons of secondary amidescan easily be correlated with DFT-based structures at the B3LYP/6-31G* level of theory via a novelalgorithm that refines previous models, such as the classical McConnell equation. The shift is givenby δ = a + 2.16 cos2(α − 35)/d, where α denotes the virtual dihedral angle resulting from linkingthe carbonyl and the α-carbons and d is the distance (Å) between the shifted proton and the carbonyloxygen. Notably, in this equation a is a parameter that can be optimized for different solvents,namely, CDCl3, DMSO-d6, and D2O. For the development of these correlations, the preferentialconformation of amides is taken from the optimized structures in the gas phase obtained at theDFT level. The deshielding on anti and gauche protons in both rotamers of (Z)-acetamides and E/Zisomers of formamides has been evaluated. This methodology has proved to be highly reliable,allowing us to discard ab initio or DFT conformational arrangements when shifts calculated bythe above-mentioned equation differ from the experimental values. Thus, the anti dispositionbetween the CHα proton and the N−H bond appears to be the more stable conformation of simpleamides. For amides bearing only one proton at Cα, a local syn minimum can equally becharacterized. The rotational barriers around the CON−alkyl bond along with the pyramidalizationof the amido group have also been reassessed. As the conformation is taken away from anti orlocal syn minima, the nonplanarity of the amido group appears to increase.
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