The spin 3/2 state and quantum spin mixtures in haem proteins

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Abstract
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 3delectrons, yielding spin states ofS= 1/2, 3/2 and 5/2. The first and third are extremely familiar, and often lie close enough in energy so that athermalmixture of low-spinS= 1/2 and high-spinS= 5/2 states exists in an ensemble of molecules. Selection rules for common perturbations (ΔS= ∘, ± 1 for the spin-orbit interaction and ΔS= ∘ for the electronic Zeeman interaction) ensure thatquantummixtures, in which the wave function is a true combination ofS= 1/2 andS= 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.

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