Title : Use of high-resolution NMR spectra transformed by paramagnetic complexes for studying molecular structure.
Abstract:
This paper provides an overview of the features specific to the nuclear magnetic resonance (NMR) of paramagnetic molecules. These features can be attributed to the hyperfine or electronic coupling between unpaired electrons, which are localised on the coordinating ion, and ????) resonant nuclei. That leads both to the paramagnetic broadening and to the paramagnetic shifts (contact and pseudo-contact ones) of resonance lines in the NMR spectra. A contact shift is observed when the probability of an unpaired electron location in the place of a resonant nucleus differs from zero. Therefore, these shifts constitute a source of information on the nature of the metal-ligand bond as well as on the ligand electronic structure. Pseudo-contact shifts characterise the spatial structure of the molecule, thus being important for solving various structural problems. Thus, the study of the NMR phenomenon in paramagnetic systems makes it possible to obtain important information about the structure of multielectron systems. Paramagnetic shifts are much larger than chemical shifts in the spectra of diamagnetic compounds and, in addition, have a characteristic temperature dependence. For paramagnetic substances, various relaxation effects are possible, which significantly expands the possibilities of NMR as a research method. In particular, the study of temperature dependence Paramagnetic shifts and amplification of signals make it possible to find the thermodynamic parameters of the process of complexation in solution, to study the stoichiometry of complexes, as well as the mechanisms of league exchange. This paper covers pioneering works describing the specifics of the NMR spectra transformed by adding paramagnetic complexes of iron-group elements on the example of cobalt and nickel complexes, as well as com- plexes of rare-earth elements on the example of europium (Hinckley C.C., Sanders J.K.M. and Williams D.H., Molin Yu.N. and Zaev E.E. with colleagues [1-5]). We present main features of the paramagnetic additive’s method, allowing resolution of difficulties associated with large para- magnetic broadening of resonance lines in high-resolution NMR spectra. Of iron-group elements, a paramagnetic ion Co2+ is shown to be an effective shift reagent. In some cases, a Ni2+ ion may also be used for this purpose.
