The structure of the second coordination sphere of transition metal complexes. V. Nuclear magnetic resonance T1 measurements

Abstract

Proton spin–lattice relaxation times have been measured for several solute and solvent molecules in the presence of paramagnetic chromium(III) acetylacetonate. In each case the T1, values obtained are considerably shorter than those measured in the absence of the paramagnetic complex. Two models are necessary to describe the results. In the first T1 is determined by the diffusional correlation time of the solute molecule. This corresponds to a case in which there is no detectable second coordination sphere and leads to the prediction that T1 will be independent of solute concentration. Acetone, chloroform, and methylene chloride show this behaviour. The second model involves a discrete second coordination sphere and leads to the prediction that above a certain concentration T1 will vary linearly with solute concentration. Methanol shows this behaviour. Calculations show good agreement with the observed T1's for a model with a coordination number of 8, a Cr-to-CH3 separation of 7 Å and an equilibrium constant for displacing solvent (chloroform) of around 10. It is suggested that these results lead to a criterion for second coordination sphere formation compatible with the second coordination sphere requirements for an "interchange" type of ligand exchange mechanism.