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A number of ferrous and ferric compounds of 57Fe enriched rat haemoglobin have been studied by observing nuclear resonant absorption by the iron of the haem group. The ferrous compounds, reduced haemoglobin and haemoglobin carbon monoxide, show only quadrupole splittings at all temperatures, with values characteristic of high and low spin materials respectively. The ferrous compound haemoglobin nitric oxide exhibits a broad spectrum with an effective field value of about 200 kilogauss at all temperatures. This is interpreted in terms of a very strong covalent bond between the π orbitals of the NO molecule and the t2g orbitals of the Fe ion; this bonding is so strong that the distinction between ferrous and ferric assignments is blurred. The observed quadrupole Mössbauer absorption spectra of oxygenated haemoglobin suggest a similar intermediate assignment.
Lang et al. (1966) studied this question.