The electron paramagnetic resonance experiment

Structural Methods in Molecular Inorganic Chemistry, First Edition. David W. H. Rankin, Norbert W. Mitzel and Carole A. Morrison. 2013 John Wiley Sons, Ltd. Published 2013 by John Wiley Sons, Ltd. 

Energy levels and EPR transitions for a single unpaired electron in an external magnetic field, (a) with no coupling and (b) with isotropic hyperfine coupling to one nucleus with spin 1/2. 

The simple picture of two energy levels must be modified if the electron can interact with a neighboring nuclear magnetic dipole. The energy levels then become 

Absolute intensities are not easy to measure reliably, but relative intensities of signals are in principle proportional to the relative total numbers of unpaired electrons in the systems. Integrated EPR absorption 

Pulsed EPR is also possible. It works in more or less the same way as Fourier-transform NMR (Section 4.3.3). The net magnetization vector of the electron spins is aligned in a constant magnetic field, and this alignment is then perturbed by a short and intense microwave pulse. The microwave signals generated by the sample s magnetization are then measured. Fourier transformation of this yields an EPR spectrum. Such methods allow direct measurement of spin dynamics (e.g. relaxation), and are most commonly used to measure weak hyperfine interactions. 

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The electron paramagnetic resonance experiments on the yttrium ceramics, on the other hand, are ambiguous. In the study by Murrieta et al. [55], the EPR signal of YBa2Cu307 sample was interpreted as a superposition of two different lines attributed to the Cul and Cu2 sites. In some other studies of yttrium ceramics, the EPR signal was not detected or was attributed to an impurity phase [56]. Thus, Ilirther more refined EPR experiments are needed to confirm the location of the unpaired spin in the cluster. 

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