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The magnetic state of iron in haem proteins has long been recognized as a convenient indicator of chemical coordination as well as more subtle biochemical properties. In the trivalent case in particular, there are three magnetically distinct configurations of the five 3 d electrons, yielding spin states of S = 1/2, 3/2 and 5/2. The first and third are extremely familiar, and often lie close enough in energy so that a thermal mixture of low-spin S = 1/2 and high-spin S = 5/2 states exists in an ensemble of molecules. Selection rules for common perturbations (Δ S = ∘, ± 1 for the spin-orbit interaction and Δ S = ∘ for the electronic Zeeman interaction) ensure that quantum mixtures, in which the wave function is a true combination of S = 1/2 and S = 5/2 components, are not observed. The mid-spin, S = 3/2, state, and allowed 5/2–3/2 and 1/2–3/2 quantum mixtures including it, are much less well known. These have only rarely been invoked in recent years as an explanation for experimental haem protein magnetic data.
Maltempo et al. (Sat,) studied this question.