A DFT and multi-configurational perturbation theory study on O2 binding to a model heme compound via the spin-change barrier

Abstract

Dioxygen binding to a model heme compound via intersystem crossing (ISC) was investigated with a multi-state multi-configurational self-consistent field method with second-order perturbation theory (MS-CASPT2) and density functional theory (DFT) calculations. In elongated Fe–O distances, the energy levels of the S₀ and T₁ states are separated, which decreases the probability of intersystem crossing in these structures. At the DFT(B97D) level of calculation, the Fe–O distances of the S₀ and T₁ states were 1.91 and 2.92 Å, respectively. The minimum energy intersystem crossing point (MEISCP) was located as a transition state at a Fe–O distance of 2.17 Å with an energy barrier of 1.0 kcal mol⁻¹ from the T₁ minimum. The result was verified with MS-CASPT2 calculations including the spin–orbit interaction which also showed the intersystem crossing point at a Fe–O distance of 2.05 Å. An energy decomposition analysis on the reaction coordinate showed the important contribution of the ring-shrinking mode of the porphyrin ring, indicating that the reaction coordinates which control the relative energy level of the spin-states play a key role in intersystem crossing.