Magnetism molecular origins

In the Hamiltonian conventionally used for derivations of molecular magnetic properties, the applied fields are represented by electromagnetic vector and scalar potentials [1,20] and if desired, canonical transformations are invoked to change the magnetic gauge origin and/or to introduce electric and magnetic fields explicitly into the Hamiltonian, see e.g. refs. [1,20,21]. Here we take as our point of departure the multipolar Hamiltonian derived in ref. [22] without recourse to vector and scalar potentials. 

The quantum-mechanical picture of hyperfine structures presented by the spin-spin nuclear magnetic resonance (NMR) and electron-spin resonance (ESR) spectra involves a variety of spin Hamiltonian parameters of molecular origin whose magnitude determines that of the coupling constants. In such an analysis, the most characteristic term arises from the Fermi -or contact -operator ... 

Aucar et al demonstrated, by means of a four-component relativistic calculation, that the origin of the diamagnetic contribution to any magnetic molecular property is due to contributions from positronic spinors in calculating the response of the system. Several approximations for the calculation of the DSO term were also investigated. As example, the DSO term for the chalcogen hydrides, XH2 (X = O, S, Se and Te), were calculated. 

Structural investigations, using the molecular origin of the spectra to provide information which is often in support of nuclear magnetic resonance (NMR) or mass spectrometry studies. 

The pH-dependence of the magnetic moment of a ferri-haem undecapeptide, produced by peptic digestion of horse heart cytochrome c, has been measured using an n.m.r. method. Below pH 3 the magnetic moment is consistent with the presence of hydroxo-bridged dimers of high-spin iron(iii), while above pH 7 the iron(iii) is low-spin the spin crossover conforms to a simple titration curve with a pK of 6.3 and its possible molecular origin is discussed. 

The little atomic magnets are of course quantum mechanical, but Weiss s original theory of paramagnetism and ferromagnetism (1907) [7] predated even the Boln atom. He assumed that in addition to the external magnetic field Bq, there was an additional internal molecular field B. proportional to the overall magnetization M of the sample. 

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