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Theory of NMR shifts and endor spectroscopy in paramagnetic complexes

A theory of NMR and ENDOR spectroscopy in paramagnetic complexes is presented. The relevant matrix elements of both the contact and pseudo-contact interaction within the eigenstates of the magnetic electrons are treated exactly. It is shown, that the interactions depend upon the azimuthal angle ? of the resonant nucleus, even for high point symmetries of the paramagnetic center. In the case of NMR shift measurements, where the spin-lattice relaxation time is short, the interaction of the nuclear spin with the electron's spin and orbital motion can be described by a temperature-dependent induced magnetic field which is not parallel to the applied field. The results differ even in lowest order of the multipole expansion from previous theories. The pseudo-contact shift for rare-earth complexes is correlated to susceptibility in a modified form. In iron-series complexes, such a correlation is generally not possible. Here, the familiar point-dipole approximation used in earlier theories is valid only, if the orbital angular momentum is quenched or if the resonant nucleus lies on the p-fold rotation axis and p ≥ 2.