Theory of nuclear electric shielding in molecules (original) (raw)

In order to rationalize the effects of an external uniform electric field on a perturbed molecule, we introduce the concept of electric shielding as a tensor which determines the effective field at an arbitrary point in the molecular space. The fundamental properties of the nuclear shielding tensor are analyzed in the case of heteronuclear diatomics HF, HC1, and HBr. It is shown that this quantity is essential in accounting for the deformation of the molecular geometry induced by the external field. Uncoupled and coupled Hartree-Fock perturbation theory is applied in order to obtain quantitative estimates of the shieldings and their derivatives with respect to the interatomic distance in HF, HC1, HBr, H,O, NH"and CH4 molecules. Accurate linear combination of atomic orbitals wave functions are prepared for the diatomics, and their quality is tested by evaluating a series of firstand second-order electric properties, giving fair agreement with corresponding experimental data. The reliability of the computed shielding is examined to obtain information on the vibrational contribution to the property and the role of the electronic correlation.