Interpretation of Hyperfine and Quadrupole Data

Electron-nuclear dipole-dipole coupling. The electron-nuclear dipole-dipole coupling (5.1 a) of the ligand nucleus N is described by 

(Ay )] and (Ay D)3 denote the one- and three-center contributions, respectively. In the two-center contribution (Ajj )2 1, both atomic orbitals are centered on the same nucleus ( N), whereas in (Ay )2 2 one of the atomic orbitals is centered on nucleus N127 . Contributions (Ay )i, (Ajj )2 2 and (AiIj D)2,i are also called local , nonlocal and distant , respectively1281. 

In the ENDOR literature, the tensor ADD has been treated at several levels of approximation. For ligand nuclei other than protons, the largest coupling generally arises from the one-centre contribution 

For protons (ADD) = 0, so that the dipolar coupling is determined by the two- and three-center contributions. Recently, Keijzers and Snaathorst1271 have shown that the three-center contributions (ADD)3 should not, in general, be neglected in the computation of anisotropic proton hf coupling constants. In most ENDOR work, however, only the two-center contribution (ADD)2 , ( distant term) has been considered. 

For small distances R(k) (S2 A) between the nuclei N and k the two-center integrals in (ADD)z, can be approximated by expanding (5.1a) in powers of r(k)/R(k), where r(k) denotes the distance of the electron from the nucleus k125). For R(k) S 2.5 A, the unpaired electron can be considered to be concentrated at the nuclei k, so that the distant contribution may be approximated by the classical electron-nuclear point-dipole formula 

---

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. 

---