Nuclear and Electronic Relaxation of Eu 2+ ( a q) : An Extremely Labile Aqua Ion 1

International audienceVariable-temperature (at 4.7 and 9.4 T) and variable-pressure (9.4 T) 17O nuclear relaxation rates were measured for the Eu(II) aqua ion. Variable-temperature 1H nuclear magnetic relaxation dispersion (NMRD) profiles were recorded. In addition, EPR spectra of the Eu(II) aqua ion are reported as a function of temperature at 0.34, 2.7, 5.4, and 8.1 T. The simultaneous fit of the nuclear and electronic relaxation rate data results in kex298 = 4.4 × 109 s-1 for the water exchange rate with activation parameters ΔH⧧ = 15.7 kJ mol-1 and ΔS⧧ = −7.0 J K-1 mol-1, ΔV⧧ = −11.3 cm3 mol-1 for the volume of activation for exchange, and τR298 = 16.3 ps for the rotational correlation time. Water exchange at Eu2+(aq) occurs via an associative mechanism and has the highest rate ever measured at an aqua ion by magnetic resonance. The high-field EPR spectra, reported here for the first time for divalent Eu, show hyperfine coupling to 151Eu and 153Eu and the coupling constants are determined (37.3 and 16.4 G for 151Eu and 153Eu, respectively). The electronic relaxation times, T1e and T2e, are longer than for the isoelectronic Gd(III) aqua ion. The implications of these results for ligand exchange at Ca(II) and for magnetic resonance imaging contrast agents are discussed