AB-INITIO CALCULATIONS OF EXTERNAL CHARGE EFFECTS ON THE ISOTROPIC C-13, N-15 AND O-17 NUCLEAR SHIELDINGS OF AMIDES

C-13=O, (CH3)-C-13, N-15 and O-17 isotropic NMR shieldings of model am ides, acetamide and N-methylacetamide, in the presence of point charge s and of hydrogen fluoride simulating dipolar perturbations, have been calculated by the ab initio LORG method. Positive and negative charge s produce similar but opposite changes. The calculated effects are exa ggerated compared to experiment, but the trends are reproduced. The re sults are analyzed in terms of electric field effects and contribution s along the bond directions, assuming a cos theta angular dependence b etween the charge and the bond that is polarized, and a 1/r2 distance dependence. The parametrized expressions can form a basis for a protoc ol for analyzing changes in C-13, N-15 and possibly O-17 amide and pep tide shieldings caused by charged perturbers. Conversely, the location of an atom or group changing charge can be inferred from the chemical shift response of nuclei in its vicinity.