AB-INITIO CALCULATIONS ON FIRST-ROW TRANSITION-METAL PORPHYRINS .2. GROUND-STATE SPIN MULTIPLICITIES, CALCULATED IONIZATION-POTENTIALS AND ELECTRON-AFFINITIES AND THEIR RELATION TO CATALYTIC ACTIVITY

This article continues the discussion of ab initio calculations on chr omium(III)-chloro-, manganese(III)-chloro-, iron(III)-chloro-, cobalt( III)-chloro-, cobalt(II)-, nickel(II)-, copper(II)- and zinc(II)-porph yrin. Restricted Hartree Fock (RHF) and Singles and Doubles Configurat ion Interaction (SDCI) results were obtained with the quantum chemical program system GAMESS-UK. The SDCI calculations were used to confirm the high spin multiplicities found with RHF for the electronic ground states of the metal porphyrins. The metal, nitrogen and chlorine atoms were described with the Split Valence 3-21G basis set; the carbon and hydrogen atoms with STO3G. The largest SDCI calculation dealt with 14 .6 million Configuration State Functions. The electron distribution of singly charged porphyrins (cations and anions) show that the Highest Occupied Molecular Orbitals (HOMOs) of each porphyrin are delocalised, doubly occupied orbitals from the porphyrin ring moiety. The HOMOs do nate the first electron, even though there are singly occupied orbital s present in the system. Acceptance of the first electron takes place in an orbital dominated by metal Atomic Orbital components. The partia lly occupied d-like orbitals present in the porphyrins take little par t in this process and tend to preserve their identities. The energy di fferences of the charged porphyrins may give an indication of their io nisation potentials and electron affinities.A relation is apparent bet ween the spin multiplicity of the ground states of the porphyrins disc ussed here and the activity of the porphyrin in the catalytic decompos ition of cyclohexyl peroxide: all porphyrins with a quartet or higher spin ground state may be found among the active catalysts.