COMPUTING NUCLEAR-MAGNETIC-RESONANCE CHEMICAL SHIELDING IN LARGE SYSTEMS VIA MULTIPOLE SHIELDING POLARIZABILITIES

Whereas ab initio methods are feasible for computational evaluation of chemical shielding parameters in small molecules or molecular fragments, a simpler method is demonstrated for the evaluation of the relative effects of shielding due to electrostatic interactions, such as those which may arise from both inter- and intra-residue interactions in proteins and other biomolecules. The shielding of a small molecule or molecular fragment is expanded in a multipole power series, and terms of the series may be evaluated by contemporary ab initio methods. The convergence behavior of this expansion using fluorobenzene is shown as a prototypical molecule, and we conclude that the first three multipole polarizability terms are important, but that hyperpolarizability shielding corrections are negligible.