Fine Structure or Zero-Field Splitting

For a total spin of 5 = 1 or higher, an additional interaction appears that gives rise to a fine structure or zero-field splitting in the ESR spectrum. Fig. 1.7. 

The value of the fine structure constant D depends on the distance between the radicals of the pair. This property is employed for distance measurements by pulsed ESR in biochemical applications. 

Fine structure or zero-field splitting also occurs for ttiplet state molecules. Also the previously mentioned ions of transition metal ions can have 5 1 and therefore fine structure appears also there. It arises, however, by a different mechanism, spin orbit interaction. 

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El. 4 Consider the influence of the microwave frequency on the spectrum in Fig. 1.7 showing fine structure or zero-field splitting (z/s). 

The pros and cons of these suggestions have been discussed in depth [357, 360, 368], Experimental evidence for high-spin states usually comes from the EPR fine structure and from magnetic susceptibility measurements. EPR is most often used for well defined radicals also in dilute solution. In the solid state the spin-spin interaction is usually referred to as zero-field splitting and represented by a traceless tensor D. The susceptibility data, on the other hand, are predictive for the bulk magnetization of the whole sample and typically measured for the powdered or crystalline material. 

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