EXPLOITING LINESHAPE SINGULARITIES IN ESEEM OF ORIENTATIONALLY DISORDERED-SYSTEMS

Experimental strategies are presented to obtain the magnetic interaction parameters of I = 1/2 nuclei weakly coupled to paramagnetic species (S = 1/2) in orientationally disordered materials: By performing ESEEM experiments as a function of the microwave frequency it is possible to induce lineshape singularities either in tie basic modulation features due to the primary nuclear spin transitions or in the sum and difference "combination" lineshapes which are generated using 2-pulse and 4-pulse ESEEM techniques. The position of these lineshape singularies can be easily interpreted in terms of hyperfine parameters using analytical expressions. The analysis of the basic and combination lineshape singularities can also be extended to two-dimensional 4-pulse ESEEM (HYSCORE) experiments. In this case the combination lines are replaced by mountain ridge shaped correlation features in the two-dimensional frequency domain. Like the combination lines the correlation feature can be classified as sum (Zeeman dominated) and difference (Hyperfine dominated) features. Furthermore, it is demonstrated that two-dimensional techniques are very power-full in disentangling broad and complicated spectral features due to overlapping basic lineshapes or to different interacting nuclei.