Stereoelectronic properties of photosynthetic and related systems. 1. Ab initio quantum mechanical ground state characterization of free base porphine, chlorin, and ethyl pheophorbide a

Abstract

Ab initio SCF calculations on the ground state of free base porphine, chlorin, and ethyl pheophorbide a have been carried out using the molecular fragment procedure. Molecular orbital energies and ordering, and the correlations of specific orbitals among the molecules studied, have been examined in detail. Ionization potentials have been estimated, and the first ionization potentials are 6.8, 6.4, and 6.4 eV for porphine, chlorin, and ethyl pheophorbide a, respectively. The calculations show the expected approximate separation of the HOMO, HOMO--1, LUMO, and LUMO + 1 from the remainder of the MO manifold in keeping with the ''four orbital'' model, and isodensity contour plots of occupied and unoccupied molecular orbitals indicate a striking similarity in the ''shapes'' of these orbitals in all three molecules. Charges and bond orders have been examined. In porphine and chlorin, the bonding picture includes an extended ..pi.. system whose path of conjugation involves the atoms of the interior of the macrocycle, including the nitrogens and methine carbons. Also, relatively localized ..pi.. bonds are found between the exterior carbon atoms of the pyrrole moieties. In ethyl pheophorbide a, the ..pi.. bonds of the keto carbonyl of ring V and the vinyl group of ring I aremore » mostly localized, but the path of conjugation within the macrocycle is somewhat less clear. Finally, molecular electrostatic isopotential maps have been constructed and an analysis of the long-range electrostatic field and its relationship to intermolecular interactions is discussed.« less