Determination of metal-proton distances and electronic relaxation times in lanthanide complexes by nuclear magnetic resonance spectroscopy

The longitudinal and transverse relaxation times of the proton resonances of three lanthanide(III) complexes of 1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″-tetraacetate have been measured. This allows determination of the metal-proton distances and the electronic relaxation times of the paramagnetic metal ions by exploiting the Curie-spin mechanism contribution to the transverse relaxation rates, once the molecular reorientational time τ_r is known by an independent experiment. Values of the parameters calculated from data obtained at a single magnetic field (9.4 T) were in good agreement with those found from the evaluation of the magnetic field-induced line broadening of the proton resonances as measured at 2.1, 4.7 and 9.4 T. A chemical exchange contribution to the linewidths is excluded on the basis of experiments performed at different temperatures.