Abstract
Variable-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 153...
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