Signs of Hyperfine and Quadrupole Coupling Constants

Relative signs of hyperfine and quadrupole coupling constants. In an EPR spectrum with resolved hfs the relative signs of hf and quadrupole coupling constants may be determined from the dependence of the ENDOR intensities on the nuclear quantum number mi of the saturated EPR line . For two magnetically equivalent 1 = 1 nuclei, the signs can be obtained from similar, though more complex, intensity patterns .  

If the hfs of the nucleus under consideration is not resolved in the EPR spectrum, all nuclear spin states are simultaneously saturated and a sign determination using ENDOR line intensities is not possible. In this case the relative signs may sometimes be determined from second order hf contributions. This method has been applied by DuVarney and Spaeth to determine the sign of the K electric quadrupole moment using F centres in KCl. 

Sign determinations with the aid of more sophisticated ENDOR techniques will be discussed in the following section. 

In this section we will be concerned with various techniques which supplement the conventional ENDOR experiment. The principles of the different methods as well as their advantages, limitations and applications will be discussed. 

1 Orientation Selection in Powders, Frozen Solutions and Nematic Glasses 

The applicability of this selection technique depends on the anisotropy and the relative orientation of the various interaction tensors. Frequently occurring situations in transition metal complexes suitable for treatment by this technique are16,76)  

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