Ring magnetic susceptibilities in conjugated hydrocarbons

In the context of an SCF π-electron theory an exact expression is derived which yields xπ ⊥, the total π-electron contribution to the magnetic susceptibility perpendicular to the plane of a polycyclic conjugated hydrocarbon. Once given the first-order correction to the bond-order matrix of the molecule in question (which, in the present calculation, is obtained via a coupled Hartree-Fock procedure), this expression makes no further appeal to the London integral approximation. The terms which occur in the expression for xπ ⊥ due to the ‘integrated current densities’ correspond to those which would have arisen if the London approximation had been invoked, the remaining terms being, in general, non-zero if this approximation is not made. The relative ‘integrated current densities’ associated with the various constituent rings in 12 molecules agree well with the corresponding ‘ring current’ intensities estimated via the London-Pople-McWeeny method. It is shown that for polycyclic conjugated systems which are ...