Two ground states of heme: spin-forbidden equilibrium.

Abstract

We investigate the possibility of a spin-forbidden equilibrium between the lowest triplet and quintet spin states of Fe(II)-porphine, Fe(II)-tetraphenylporphine, and Fe(II)-tetra(α,α,α,α-orthopivalamide)phenylporphine. The electronic structures of these complexes were studied using density functional theory, complete active space second-order perturbation theory, and multiconfigurational pair-density functional theory. These methods predict small triplet-quintet energy gaps and low minimum energy crossing point (MECP) barriers for all three complexes, pointing to fast spin-forbidden transitions between the two spin states. Furthermore, a strong spin-orbit coupling of the two spin states at MECP suggests that the electronic structures of these complexes are better described via a single spin-mixed state with a global minimum located between the triplet and quintet minima. This finding is supported by the Gibbs free energy calculations accounting for the zero-point vibrational energy and entropic contributions.

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