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Articles you may be interested inHigh-resolution electron spectroscopy, preferential metal-binding sites, and thermochemistry of lithium complexes of polycyclic aromatic hydrocarbonsThe McWeeny theory has been used to calculate the shielding parameters for polycyclic hydrocarbons up through four rings. These results are presented along with experimental values, some of which come from new analyses of lOO-Mc/sec NMR spectra of these compounds, and semiquantitative agreement is observed. The Jonathan-Gordon-Dailey semiempirical equation for spin-spin coupling constants is also subjected to further test by new data. Absence of inter-ring spin-spin coupling is reconfirmed.

The hyperfine structure Hamiltonian for hydrogen is derived by evaluating the magnetic field at the nucleus due to the electron’s spin current density. This approach clearly separates the treatment of states with l=0 (Fermi contact term) from those having l≠0.

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 density j=jring+jlocal. The term σ′ arises from (A2)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.