Advances ENDOR Techniques

2) The anisotropy of the g and the hfs tensor of the central ion are of the same order of magnitude, but the principal axes of each tensor with the largest anisotropy coincide. Examples are planar cobalt and copper complexes. 

Furthermore, the method of orientation selection can only be applied to systems with an electron spin-spin cross relaxation time Tx much larger than the electron spin-lattice relaxation time Tle77. In this case, energy exchange between the spin packets of the polycrystalline EPR spectrum by spin-spin interaction cannot take place. If on the other hand Tx Tle, the spin packets are coupled by cross relaxation, and a powder-like ENDOR signal will be observed77. Since T 1 is normally the dominant relaxation rate in transition metal complexes, the orientation selection technique could widely be applied in polycrystalline and frozen solution samples of such systems (Sect. 6). 

Proper single crystal-like ENDOR spectra can best be obtained by saturating the low-and high-field flanks of an EPR spectrum. At these field positions the resolution of the ENDOR spectrum is increased and distortions of the ENDOR lines are minimized78- 79). 

In many planar metal complexes it is not possible to record an ENDOR spectrum which only contains contributions from Bo orientations in the complex plane. This is due to the fact that in the powder EPR spectrum the high- or low-field turning points may arise from extra absorption peakssl which do not correspond to directions of the principal axes. ENDOR spectra observed near the in-plane region of such a powder EPR spectrum are due to molecules oriented along a large number of B0 directions (in- and out-of-plane), so that the orientation selection technique is no longer effective. 

For cylindrical or disk-like shaped metal complexes the orientation selection may be improved using nematic glasses as host compounds. It is well established that the molecules of a liquid crystal in the nematic phase temperature region are oriented in a 

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Contents Introduction. - ENDOR-Instrumentation. - Analysis of ENDOR Spectra. - Advances ENDOR Techniques. - Interpretation of Hyperfine and Quadrupole Data. - Discussion of the Literature. - Concluding Remarks. - Appendix A Abbreviations Used in this Paper. - Appendix B Second Order ENDOR Frequencies. - Appendix C Relations Between Nuclear Quadrupole Coupling Constants in Different Expressions of Hq (Sect.5.2). - References. - Subject Index. 

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