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 multi-state multiconfigurational self-consistent field 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. Minimum energy intersystem crossing point (MEISCP) was located as a transition state at Fe-O distance of 2.17 Å with the energy barrier of 1.0 kcal/mol from T1 minimum. The result was verified with MS-CASPT2 calculations including spin-orbit interaction which also showed the intersystem crossing point at 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 the intersystem crossing.

Spin‐inversion mechanisms in O2 binding to a model heme complex revisited by density function theory calculations

“…7 and 9, respectively. Another important point seen in Figure is that the energy gap between the singlet and triplet minima is smaller than the previously reported values . This behavior is also due to the small basis sets used as well as the chosen B97D functional.…”

“…To further understand the correlation between the motion along the d ‐coordinate and spin‐inversion process, we artificially added an initial kinetic energy to the Fe atom along this coordinate. Again, it should be emphasized that the importance of the Fe out‐of‐plane distance in O 2 binding has been frequently highlighted in previous static DFT calculations . A recent nonadiabatic quantum wave packet simulation study on photolysis of the CO‐bound heme also showed that the Fe out‐of‐plane motion plays an essential role in the spin‐crossover dynamics …”

“…Our computational model consisted of a Fe(II)‐porphyrin complex (FeC 20 N 4 H 12 ) with a proximal imidazole (C 3 N 2 H 4 ), which is called FePorIm hereinafter. This model compound is the most extensively used in theoretical studies . Throughout this study, we used density functional theory (DFT) calculations with the B97D exchange–correlation functional because the B97D functional including empirical dispersion corrections showed good performance in calculations of 3d transition metal compounds .…”

“…These two spin states were called T 1 and T 2 in Ref. . In the former T 1 configuration, O 2 has an original triplet character even in the O 2 ‐FePorIm complex, and thus, the O 2 moiety and the FePorIm moiety have opposite spin characters (i.e., α‐ and β‐spin, respectively).…”

“…Hasegawa and coworkers recently succeeded in optimizing the crossing point structure between the singlet and triplet potential energy surfaces in full dimensions. [15] However, systematic crossing point searches for other spin states have not yet been performed.…”