PPP (Peel) calculations on iron porphyrins and iron phthalocyanine. Charge distribution and charge transfer

Abstract

Semiempirical molecular orbital calculations within a modified PPP method, the Peel method, have been performed in order to study the electronic structure and properties of porphyrins and phthalcxyanines. It is found that the calculations can reproduce the results from more sophisticated ab initio calculations. It is found that the electronic reactions may take place at the metal atom and in the axial direction. The role of the porphine plane seems to be that of an electron buffer, keeping the electronic population in the porphyrin plane constant. A mechanism is suggested where electron transfer reactions can take place by changes in the molecular geometry, as for example by vibrations.

Noninnocence of the ligand atoms in iron‐porphine: Chemical consequences of the delocalized electron spin

“…To model the effect of axial coordination the complex [HPFe(Im) 2 ] + was used, in which iron is held in an octahedral (O h ) ligand‐field created by the four nitrogen donor atoms of porphine and the nitrogen atoms of two imidazole molecules. This complex should be a quite realistic model of the situation in many biological environments, in which iron is normally axially coordinated by, for example, histidine, carboxylic amino acids, or water molecules. The relatively strong axial coordination of imidazole increased the energetic splitting of the orbitals in the valence space, resulting in low‐spin ground states for both [PFe(Im) 2 ] + ( S = 1/2) and [HPFe(Im) 2 ] + ( S = 0) .…”

“…To model the effect of axial coordination the complex [HPFe (Im) 2 ] 1 was used, in which iron is held in an octahedral (O h ) ligand-field created by the four nitrogen donor atoms of porphine and the nitrogen atoms of two imidazole molecules. This complex should be a quite realistic model of the situation in many biological environments, [47,48] atom should be more endergonic and slower in the octahedral complex than for the square planar complex. Also, since a more endergonic reaction corresponds to an even smaller equilibrium constant for hydrogen atom transfer, the homo-coupling discussed above will be retarded due to the lower concentration of the C 6 H · 7 radical.…”

Spiro-Conjugated Molecular Junctions: Between Jahn–Teller Distortion and Destructive Quantum Interference

“…There, it is possible to diagonalize the molecular Hamiltonian in such a way that each of the E states is localized on either moiety and therefore coupled only to a single (left or right) electrode. (This can also be done in other systems with D 2 d symmetry, for instance, the one studied in ref .) This corresponds to a complete destructive (interorbital) QI at the spiro-link.…”