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

Abstract

Spin‐inversion mechanisms in O2 binding to a model heme complex, consisting of Fe(II)‐porphyrin and imidazole, were investigated using density‐functional theory calculations. First, we applied the recently proposed mixed‐spin Hamiltonian method to locate spin‐inversion structures between different total spin multiplicities. Nine spin‐inversion structures were successfully optimized for the singlet–triplet, singlet–quintet, triplet–quintet, and quintet–septet spin‐inversion processes. We found that the singlet–triplet spin‐inversion points are located around the potential energy surface region at short Fe–O distances, whereas the singlet–quintet and quintet–septet spin‐inversion points are located at longer Fe–O distances. This suggests that both narrow and broad crossing models play roles in O2 binding to the Fe‐porphyrin complex. To further understand spin‐inversion mechanisms, we performed on‐the‐fly Born‐Oppenheimer molecular dynamics calculations. The reaction coordinates, which are correlated to the spin‐inversion dynamics between different spin multiplicities, are also discussed.

Spin‐inversion mechanisms in O2 binding to a model heme compound: A perspective from nonadiabatic wave packet calculations

“…We use the standard 6‐311 + G* basis sets for Fe and O atoms, and the 6‐31G** basis sets for C, N, and H atoms, as in our previous work. [ 39 ] The B97D calculations are done at the grid points described by the three coordinates R , θ , and d , where the structure of the PorIm part is taken from the optimized structure of the isolated FePorIm in the lowest quintet state. Twenty‐seven points are used for R in the range of 1.5–6 Å. Eleven points are chosen for θ in the range of 80–150°, whereas 13 points in the range of −0.4–0.4 Å are used for d .…”

“…Similarly, we found that the motion along d was an important role in the quintet-septet and quintet-singlet spininversion processes. [39] The Hamiltonian of the three degrees of freedom model employed in this work is written (in atomic units) as…”

“…Extensive theoretical studies have been performed; however, most of the studies obtained static information about the features of the potential energy surfaces and their crossings. [ 12–36,39 ] Spin, nonadiabatic, and vibrational dynamics cannot be described independently due to strong SOCs in many molecular systems containing heavy elements. [ 40–47 ] The magnitudes of the SOCs of Fe(II) (100–300 cm −1 ) are roughly comparable to those of many low‐frequency vibrational modes of the oxyheme complex.…”

“…Recently, the mixed-spin Hamiltonian method, [37,38] where two potential energy surfaces with different spin multiplicities are mixed through a coupling parameter, has been employed to locate approximate spin-inversion structures between the two potential energy surfaces of the model O 2 -heme complex. [39] In this method, the approximate minimum energy crossing point structure, which is important for discussing the efficiency of the spin-inversion process, can be easily optimized as a transition state on the low-lying spin-coupled potential energy surface. A total of nine spin-inversion structures were successfully optimized for the singlet-triplet, singletquintet, triplet-quintet, and quintet-septet spin-inversion processes.…”

“…The IRC calculation showed that nuclear motions along the Fe-O-O angle and Fe out-of-plane distance (deviation of Fe from the porphyrin plane) are important along the spin-inversion IRC pathways. [39] In this work, we perform nonadiabatic quantum wave packet dynamics calculations to understand the spin-inversion mechanisms in O 2 binding to a model heme compound further from a dynamical perspective. Extensive theoretical studies have been performed; however, most of the studies obtained static information about the features of the potential energy surfaces and their crossings.…”