Ab initio calculations on first-row transition metal porphyrins Part 2. Ground state spin multiplicities, calculated ionisation potentials and electron affinities and their relation to catalytic activity

Abstract

This article continues the discussion of ab initio calculations on chromium(III)-chloro-, manganese(III)-chloro-, iron(III)-chloro-, cobalt(III)-chloro-, cobalt(II)-, nickel(II)-, copper(II)- and zinc(II)-porphyrin. Restricted Hartree Fock (RHF) and Singles and Doubles Configuration 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 donate the first electron, even though there are singly occupied orbitals present in the system. Acceptance of the first electron takes place in an orbital dominated by metal Atomic Orbital components. The partially occupied d-like orbitals present in the porphyrins take little part in this process and tend to preserve their identities. The energy differences of the charged porphyrins may give an indication of their ionisation potentials and electron affinities. A relation is apparent between the spin multiplicity of the ground states of the porphyrins discussed here and the activity of the porphyrin in the catalytic decomposition of cyclohexyl peroxide: all porphyrins with a quartet or higher spin ground state may be found among the active catalysts.