MAGNESIUM PORPHYRIN RADICAL CATIONS - A THEORETICAL-STUDY OF SUBSTITUENT EFFECTS ON THE GROUND-STATE

Semiempirical (INDO) calculations have been performed to investigate the effect of substituents on the nature of the pi cation radical states for a series of meso- and beta-substituted four-coordinated Mg porphyrins. In particular, the effect of these substituents in modulating whether the ground state is a 2A1u or 2A2u pi cation radical was determined. The beta substituents were found to preserve the 2A1u ground state of the unsubstituted porphine with a modest (+/- 3 kcal/mol) effect on the relative energies of the 2A1u and 2A2u states. In contrast, the effect of the meso substituents was more pronounced, leading to the 2A2u ground state in some instances. The calculated state orderings are in good agreement with known experimental results. A balance between the pi effects and sigma effects of the substituents explains the modulation of the state energy orderings in these substituted Mg(II) porphyrins.