Magnetic susceptibility concept

The spin-Hamiltonian concept, as proposed by Van Vleck [79], was introduced to EPR spectroscopy by Pryce [50, 74] and others [75, 80, 81]. H. H. Wickmann was the first to simulate paramagnetic Mossbauer spectra [82, 83], and E. Miinck and P. Debmnner published the first computer routine for magnetically split Mossbauer spectra [84] which then became the basis of other simulation packages [85]. Concise introductions to the related modem EPR techniques can be found in the book by Schweiger and Jeschke [86]. Magnetic susceptibility is covered in textbooks on molecular magnetism [87-89]. An introduction to MCD spectroscopy is provided by [90-92]. Various aspects of the analysis of applied-field Mossbauer spectra of paramagnetic systems have been covered by a number of articles and reviews in the past [93-100]. 

This chapter is intended to recall the principles of magnetism, the definition of magnetic induction and of magnetic induction in a vacuum which is referred to as magnetic field. Readers may not recollect that the molar magnetic susceptibility is expressed in cubic meters per mol Some properties of electron and nuclear spins are reviewed and finally some basic concepts of the magnetic resonance experiments are refreshed. In summary, this chapter should introduce the readers into the language used by the authors. 

It must be noted — and this does not in the least diminish Pople s merits — that Pauling had, about twenty years before, implicitly invoked the same concept to explain the unusually high magnetic susceptibility of the simplest of the arenes and we shall discuss further the general use we may make of the Pauling-Pople ring current concept for all the magnetic properties of matter. 

We now turn to a final analysis that uses a localized approach to magnetic parameters the concept of localized magnetic susceptibilities. The individual elements in the magnetic susceptibilities can be obtained... 

The concept of the closely related diamagnetic susceptibility exaltation was earlier discussed by Craig88 and Dauben et al,83 These groups suggested that the difference, the exaltation, in the observed molar magnetic susceptibility xm and that calculated from atomic and bond contribution Xm, provides a useful criterion for assessing aromaticity. Dauben et al.84 have recently provided an authorative survey of the subject and a comprehensive compilation of exaltation values for benzenoid, nonbenzenoid, and pseudo-aromatics, aromatic cations, keto aromatics and heteroaromatics. Examples of the last series are provided here in Table VI. Some uncertainties arise in the calculations of xm- for five-membered heteroaromatics. 

This model is less general than the one proposed by the Formal Graphs with the concept of coupling between energy varieties and with the generalization of the indnctance by means of an operator, as it will be shown. The magnetic susceptibility is introduced here as a coupling factor, which is a new concept in this domain. 

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