Electron binding energies of free base porphyrin and magnesium-porphyrin: A sequential Born-Oppenheimer molecular dynamics/quantum mechanics approach

The electronic properties of free base porphyrin (H2P) and magnesium-porphyrin (MgP) were investigated through a sequential Born-Oppenheimer molecular dynamics/quantum mechanics approach. The quantum mechanics calculations for the electronic density of states were performed with Green's function or electron propagator theory and also with a recent series of approximations for the exchange-correlation functional proposed by Truhlar and collaborators. The distortions of the porphyrin structures were analysed through normal-coordinate structural decomposition. The role played by thermal effects on structural distortions of H2P and MgP and their relationship with electron binding energies are discussed. Our results indicate that distortions of the porphyrin macrocycle induced by thermal effects do not influence in a significant way electron binding energies for the valence states of H2P and MgP. However, some correlation has been observed between nonplanar breathing distortions and electron binding energies of Mg(1s), Mg(2s), and Mg(2p) states, which can be blue-shifted by ∼0.8 eV.