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Ab initio IGLO study of the syn / anti dependence of the 13 C NMR chemical shifts in simple amides
Author(s) -
Jiao Ding,
Barfield Michael,
Hruby Victor J.
Publication year - 1993
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260310116
Subject(s) - chemistry , chemical shift , amide , ab initio , computational chemistry , molecule , carbon 13 nmr , crystallography , stereochemistry , organic chemistry
The syn / anti dependences of the alkyl 13 C 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 13 C 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 13 C chemical shift results and the experimental data. The syn / anti dependence of 13 C 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 13 C chemical shifts in amides arises almost entirely from the PBC of the syn and anti NC bonds, and is completely analogous to the electronic origin for the cis / trans γ‐effect in hydrocarbons.