Ring Currents and Proton Shielding in Polycyclic Hydrocarbons

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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.

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