NMR Investigation of Relaxation and “Chemical-Exchange Spin Decoupling” in Dilute Solutions of Co2+ with Various “P–O” Containing Solvents

The concentration and temperature dependence of the proton relaxation times of complexes of Co2+ with dimethyl methylphosphonate, dimethyl hydrogen phosphite, and methyl phosphate have been examined. As the concentration of the paramagnetic ion Co2+ is increased, the phosphorus nucleus may be spin decoupled from the proton. This phenomenon has been called “Chemical-exchange spin decoupling.” The equations of Swift and Connick are employed to describe the solvent exchange process. The approach of McConnell is shown to apply to the two nuclear spin orientations of phosphorus as a kinetic process in a double resonance experiment. Combining these equations accounts for the dependence of the observed relaxation times on the concentration of paramagnetic ion.