Magnetically dilute systems

At the end of this section on the relationship between the electronic properties and the stereochemistry of complexes of the copper(II) ion, it is worth summarizing the most useful physical techniques which offer a criterion for the presence of a polynuclear copper(II) complex rather than a mononuclear complex. These are (i) magnetic susceptibility measurements down to near absolute zero, for the determination of O or / values (ii) ESR spectra of magnetically dilute systems, in the solid state or in solution, to obtain hyperfine data and (iii) cyclic voltammetry to show evidence for a one-step reduction process in a Cu2 species. 

In a magnetically-dilute system, the distribution of paramagnetic centres over the available Zeeman states is determined by Boltzmann statistics, and the magnetic state adopted at any instant by one centre is in no way influenced by its neighbours. Deviations from such behaviour may be classified as ferromagnetic or antiferromagnetic. 

It is important to observe the applicability limits of the Curie-Weiss law and the implied physical meaning of the Weiss temperature It can only be applied to magnetic materials containing spin-only magnetic centers, and it only applies to magnetically condensed systems (as opposed to magnetically dilute systems) at sufficiently high temperatures fer>/ex). 

However, in a magnetically dilute system it is possible in principle to slow the spin-spin relaxation to the point where hyperfine structure is seen, and in this case the matrix elements involving S+ and S (i.e. a mixing of different electronic states) cause an unequal spacing of the hyperfine levels. 

EPR spectroscopic measurements can be performed at high, room or low (>4K) temperature. Samples may be solid (single crystal or powder) or liquid (fluid or frozen solution, a glass ). In this brief introduction to EPR spectroscopy, we shall be concerned only with magnetically dilute systems in which the unpaired electrons are involved in intramolecular (not intermolecular) interactions. Moreover,... 

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