Understanding the mechanism stabilizing intermediate spin states in Fe(II)-Porphyrin

TL;DR

This study uses advanced quantum chemical methods to reveal how electron delocalization stabilizes triplet spin states in Fe(II)-porphyrins, providing insights into the fundamental mechanisms behind spin state stabilization in heme proteins.

Contribution

It introduces the use of the novel Stochastic-CASSCF method to elucidate the electron delocalization mechanisms stabilizing specific spin states in Fe(II)-porphyrins, advancing understanding of spin chemistry.

Findings

Electron delocalization via charge-transfer stabilizes triplet states.

Constraining out-of-plane orbitals reverses spin ordering.

Delocalization correlates with Kekulé resonance structures.

Abstract

Spin fluctuations in Fe(II)-porphyrins are at the heart of heme-proteins functionality. Despite significant progress in porphyrin chemistry, the mechanisms that rule spin state stabilisation remain elusive. Here, it is demonstrated by using multiconfigurational quantum chemical approaches, including the novel Stochastic-CASSCF method, that electron delocalization between the metal centre and the pi system of the macrocycle differentially stabilises the triplet spin states over the quintet. This delocalisation takes place via charge-transfer excitations, involving the out-of-plane iron d orbitals, key linking orbitals between metal and macrocycle. Through a correlated breathing mechanism, the 3d electrons can make transitions towards the pi orbitals of the macrocycle. This guarantees a strong coupling between the on-site radial correlation on the metal and electron delocalization. Opposite-spin 3d electrons of the triplet can effectively reduce electron repulsion in this manner. Constraining the out-of-plane orbitals from breathing hinders delocalization and reverses the spin ordering. Our results find a qualitative analogue in Kekul\'e resonance structures involving also the metal centre.