Nuclear and Electronic Relaxation of Eu²⁺₍aq):  An Extremely Labile Aqua Ion¹

Variable-temperature (at 4.7 and 9.4 T) and variable-pressure (9.4 T) 17 O nuclear relaxation rates were measured for the Eu(II) aqua ion. Variable-temperature 1 H 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 k ex 298 = 4.4 × 10 9 s - 1 for the water exchange rate with activation parameters Δ H ⧧ = 15.7 kJ mol - 1 and Δ S ⧧ = −7.0 J K - 1 mol - 1, Δ V ⧧ = −11.3 cm 3 mol - 1 for the volume of activation for exchange, and τ R 298 = 16.3 ps for the rotational correlation time. Water exchange at Eu 2+ (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 151 Eu and 153 Eu and the coupling constants are determined (37.3 and 16.4 G for 151 Eu and 153 Eu, respectively). The electronic relaxation times, T 1e and T 2e, 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.