Oxygen binding to iron-porphyrin: A density functional study using both LSD and LSD+GC schemes

Equilibrium structures of iron–porphyrin (FeP) and its O2 complex, computed with Car–Parrinello molecular dynamics, are in excellent agreement with experimental data on synthetic heme models. Geometry optimization can affect significantly the relative energies among spin states of these systems. The FeO bond, best described as FeIIIO2−, increases its strength from 9 to 15 kcal/mol upon binding an imidazole axial ligand (Im). Our results are consistent with an open-shell singlet as the ground state of FeP(Im)(O2), in competition with a low-lying closed-shell singlet state and, as the FeOO angle increases, with a triplet state. The conformation in which the imidazole plane and the OO axis projection lie in the same NFeN porphyrin quadrant is found to be stable, although easy rotation of the O2 molecule around the FeO axis at room temperature is predicted. Comparison of the performance between the local spin density approximation (LSD) and the gradient-corrected scheme (LSD+GC) is provided.