Ring Currents and Proton Shielding in Polycyclic Hydrocarbons

Abstract

The ring‐current contribution to the shielding constant, σring, is shown to depend on (A·p)Av. In an LCAO perturbation approach σring is then determined as a sum of two terms, each term depending on the same three‐center integral. The derivation further indicates that an appropriate partition of the shielding constant in an arbitrary molecule would be σ=σring+σlocal+σ′, corresponding to a current densityj=j ring+j local. The term σ′ arises from (A 2)Av. Numerical calculation of the ratio σring/σring (benzene) for the two naphthalene protons gives values 1.39 and 1.21. Estimates of these ratios experimentally are 1.28 and 1.0.