Ab initio IGLO study of the syn/anti dependence of the ¹³C NMR chemical shifts in simple amides

The syn/anti dependences of the alkyl ¹³C chemical shifts provide a useful probe of stereochemistry around the amide bonds in peptides and proteins. To investigate these dependences the ab initio IGLO method was used to obtain the ¹³C chemical shielding for the following series of amide molecules as models for the peptide bond: syn‐ and anti‐N‐methylformamide (NMF), cis‐ and trans‐N‐methylacetamide (NMA), N,N‐dimethylformamide (DMF) and N,N‐dimethylacetamide (DMA). Molecular geometries for shielding calculations are optimized ones at the HF/6–31G* level. Good correlations were observed between the calculated ¹³C chemical shift results and the experimental data. The syn/anti dependence of ¹³C chemical shifts around amide bonds is perdicted either with double‐ζ or triple‐ζ quality basis sets. Based on an analysis of the paramagnetic bond contributions (PBC) to the chemical shieldings, it is concluded that the syn/anti dependence of the ¹³C chemical shifts in amides arises almost entirely from the PBC of the syn and anti NC bonds, and is completely analogous to the electronic origin for the cis/trans γ‐effect in hydrocarbons.