Calculation of the spin density and spin-spin dipolar interaction in the triplet state of metal porphins

Abstract

The spin density and zero-field splitting (ZFS) of the lowest triplet state of a light metal porphin are determined by means of a Pariser-Parr-Pople ground state SCF π-electron MO-calculation with CI. Only the p_z -orbitals of the carbon and nitrogen atoms are considered. The semi-empirical values of the integrals involving nitrogen orbitals are varied between the values commonly used for pyrrole and pyridine-type nitrogen. The two lower triplet states (corresponding to the excitations ³(e_g ←a _2u ) and ³(e_g ←a _{1 u} ) in Gouterman's four orbital model) are considered, both for a b _1g and a b _2g the spin densities determined by McLachlan's extended Hartree-Fock method lead to a perfect explanation of the hyperfine structure observed for zinc porphin and magnesium porphin in n-octane.

The ZFS parameters are calculated by a semi-empirical method; satisfactory agreement with experiment is obtained only if the CI expansion is truncated at the point where the weight of the individual configurations becomes less than 0·005. Additional calculations of the ZFS for the lowest triplet states of free base porphin and coronene are reported to test the applicability of the method.