Definitive Benchmark Study of Ring Current Effects on Amide Proton Chemical Shifts

摘要

The ring current effect on chemical shifts of amide protons (Δδ_(RC)) is computed at the B3LYP/6-311++G(d,p)// B3LYP/aug-cc-pVTZ level of theory for 932 geometries of dimers of N-methylacetamide and aromatic amino acid side chains extracted from 21 different proteins. These Δδ_(RC) values are scaled by 1.074, based on MP2/cc-pVQZ//B3LYP/aug-cc-pVTZ chemical shift calculations on four representative formamide/benzene dimers, and are judged to be accurate to within 0.1 ppm based on CCSD(T)/CBS//B3LYP/aug-cc-pVTZ calculations on formamide. The 932 scaled Δδ_(RC) values are used to benchmark three empirical ring current models, including the Haigh-Mallion model used in the SPARTA, SHIFTX, and SHIFTS chemical shift prediction codes. Though the RMSDs for these three models are below 0.1 ppm, deviations up to 0.29 ppm are found, but these can be decreased to below0.1 ppm by changing a single parameter. The simple point-dipole model is found to perform just as well as the more complicated Haigh-Mallion and Johnson-Bovey models.