Theoretical study of the dimers of ferriporphyrin analogues (MC34H31N4O4)2 and their positive ions with 3d-metal atoms M

Abstract

The electronic and geometric structures and the energetic characteristics of isolated dimeric molecules and positive ions of ferriporphyrin analogues (MC34H31O4N4) 2 0,+ and corresponding monomers MC34H31O4N 4 0,+ with 3d-metal atoms M = Sc-Ni in states with different multiplicities were calculated by the density functional theory B3LYP method with the Gen-1 = 6-31G*(Fe) + 6-31G(C,H,N,O) basis set. Their energies were refined with the use of the extended basis set Gen-2 = 6-311+G*(Fe) + 6-31G*(C,H,N,O). The computation results were compared with the analogous calculated data on the dimers of heme analogues (MC34H32O4N4) 2 0,+ and MC34H32O4N 4 0,+ with the same M atoms. The behavior of the above-mentioned properties was analyzed in going along the 3d series, upon ionization and a change in multiplicity, and in other related series. Trends were traced in the calculated energies D 1 of dissociation of the ferriporphyrin analogue dimers into monomers, (MC34H31O4N4) 2 0,+ → MC34H31O4N 4 0,+ MC34H31O4N 4 0 , and in the energies D 2 of removal of two hydrogen atoms from heme analogue dimers resulting in ferriporphyrin analogues, (MC34H32O4N4) 2 0,+ → (MC34H31O4N4) 2 0,+ + 2H. It was shown that, in going along the 3d series, the D 1 energy rapidly decreases from ∼210 kcal/mol for M = Sc to a few tens of kcal/mol for M = Ni, with a small minimum for Mn and a small maximum for Fe, as the energy of the pair of broken covalent (polar) bonds M-O decreases. Conversely, the D 2 energy increases rapidly and rather monotonically from ∼45 kcal/mol for M = Sc to ∼135 kcal/mol for M = Fe, as the difference between the energies of the pair of forming covalent bonds M-O and the pair of broken donor-acceptor bonds M ← O decreases. For the positive ions, the D 1 and D 2 values are, as a rule, 5–10 kcal/mol higher than those for the parent neutral analogues. The character of the spin density distribution in states with different multiplicities was considered.