Spin-line models assumptions

This procedure is applicable if the relaxation between the spin states is fast (t<1 X 10 s) and thus the quadrupole doublets of the two spin states collapse into one. It is found that, in the intermediate temperature range, the widths of the two lines are significantly enlarged. This shows that the assumption of fast relaxation strictly does not apply. In spite of this, the areas of the lines ean be well reproduced within the Debye model employing the same Debye temperature for both spin states, p 123 K. 

A general model for electronic relaxation of the Gd3+ S = 7/2 ion in various complexes in solution was presented by Rast el al. [86]. Contrary to the usual assumption, the electronic relaxation in their model is not only due to the effects of the transient zero field splitting, but is also strongly influenced by the static crystal field effect which is modulated by the random Brownian rotation of the complex. Experimental peak-to-peak widths of three gadolinium complexes could be well interpreted as a function of temperature and frequency using three static and one transient crystal field parameters. Moreover, their interpretation of experimental data did not require the addition of any field independent contribution to the line width like the spin-rotation mechanism. 

While slow tumbling spectra are best simulated to extract rotational correlation times, fast tumbling spectra can be quantified via relative fine widths T2,r or amplitudes hi of the three fines using Kivelson fine width theory. As three lines define two independent amplitude ratios, the imderlying assumption of isotropic Brownian rotational diffusion can be tested. For spin labels in polymers or sirrfactant spin probes nitroxide motion is usually not isotropic In favorable cases, more detailed models of the motion can be obtained by the microscopic order macroscopic disorder (MOMD) or slowly relaxing local stracture (SRLS) models. ... 

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