NMR Spectra of 1-Vinylimidazole, Transformed by Paramagnetic Complexes

Abstract

The NMR spectra of 1-vinylimidazole, transformed by the additives of paramagnetic complexes of manganese, cobalt, nickel and copper chlorides with this azole, have been studied. It is shown that the coordination atom in a solution has octahedral surrounding, four ligand molecules being located in the equatorial position. The nature of hyperfine coupling in the above complexes is evaluated by comparison of the experimental constants of hyperfine coupling with theoretically calculated spin densities of unpaired electrons on s-orbitals in the vinylpyrrole radical, which is isostructural to the molecule of 1-vinylimidazole. It is found that in the complexes of cobalt, nickel and copper, the molecules of 1-vinylimidazole are coordinated to the central ion by the pyridine N-3 atom. Therefore, paramagnetic shifts observed for the specified complexes are mainly caused by unpaired spin density delocalized in $\sigma$- system of the ligand molecule. Notably, in the paramagnetic complex of manganese, hyperfine coupling is transferred both through $\sigma$- and $\pi$- systems of the 1-vinylimidazole molecule. Localization of the unpaired spin density is due to the fact that molecules of this azole can also be coordinated by the pyrrole N-1 atom at a certain range of temperatures. In its turn, the specified possibility is explained by non-planar structure of a heterocycle in 1-vinylimidazole.