A model for estimating 59Co nmr chemical shifts and line widths and its application to cobalt dioxygen complexes

Abstract

A simple point charge model has been developed for calculating the chemical shifts and line widths obtained from the 59Co nmr spectra of Co(III) complexes. The chemical shift calculations assume the predominance of the paramagnetic term in the Ramsey equation and the relationship of this term to the energies of the d–d electronic transitions. The line width calculations assume the predominance of quadrupolar broadening and use a point charge model to calculate electric field gradients in a manner previously used to interpret Mössbauer spectra. The model is tested by application to some fifty known Co(III) complexes. Most of the 59Co spectra have been remeasured for this purpose. One chemical shift parameter and one line width parameter are required for each different type of ligand. These parameters are related to each other and to the crystal field splitting parameters of the ligands. The calculations can be readily applied to complexes of any symmetry and differentiate between different isomers of the same complex. The calculated chemical shifts cover a range of more than 14 000 ppm and the standard deviation between the calculated and experimental values is 108 ppm. Relative chemical shifts between complexes with similar ligands are calculated with considerably higher accuracy. The agreement between calculated and experimental line widths is only qualitative but the line width calculations provide valuable additional information for making assignments. The model is applied to the assignment of different isomers of cobalt dioxygen compounds and to the identification of products from oxygenation reactions.