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 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.