Understanding the Ring Current Effects on Magnetic Shielding of Hydrogen and Carbon Nuclei in Naphthalene and Anthracene

The local response to an external magnetic field normal to the molecular plane of naphthalene and anthracenewas investigated via current density and magnetic shielding density maps. The Biot-Savart law shows that the deshieldingcaused by π-ring currents in naphthalene is stronger for α- than for β-protons due to geometrical factors. The shieldingtensor of the carbon nuclei in both molecules is strongly anisotropic and its out-of-plane component determines the up-fieldchemical shift of 13C in nuclear magnetic resonance spectra. The π-ring currents flowing beyond the C-skeleton in frontof a probe carbon nucleus, and on remote parts of the molecular perimeter, yield positive contributions to the out-of-planecomponent of carbon shielding as big as ≈10–15% of the total values. Near Hartree-Fock estimates of magnetizabilityand magnetic shielding at the nuclei fully consistent with the current model are reported.