Nuclear Magnetic Resonance Spectroscopy of Paramagnetic Species

Abstract

This chapter focuses on the nuclear magnetic resonance spectroscopy (NMR) of paramagnetic species. Isotropic shifts in the NMR spectra of paramagnetic species may arise from either or both of two different mechanisms, one of which produces contact interactions while the other gives rise to dipolar or pseudo-contact interactions between unpaired electronic and nuclear spins. To obtain realistic estimates of the spin density distribution and geometry of paramagnetic species a theoretical description of the induced shifts is required, which is not founded on these assumptions. NMR contact shifts and hyperfine splittings in ESR spectra provide a description of the distribution of the unpaired electrons in paramagnetic species. The most prolific growth area during the last four years, in the field of NMR studies on paramagnetic species, has been that dealing with lanthanide shift reagents. The presence of covalent bonds between a metal and its ligands provides a pathway for the delocalization of unpaired electrons throughout the molecule. The ensuing contact shifts in the NMR spectra of the nuclei of the ligands may reveal the mode of spin delocalization. The NMR spectra of molecules with very small structural differences are often almost identical. The presence of a paramagnetic centre tends to amplify slight spectroscopic differences arising from subtle changes in structure. A study of the NMR spectra of paramagnetic species can lead to information on various forms of equilibria and isomerism as well as to other structural data. The formation of paramagnetic metal complexes can be studied by means of the shifts and line broadenings observed in the NMR spectra of the ligand molecules.