WATER EXCHANGE ON [GD(H2O)(8)](3+) AND [GD(PDTA)(H2O)(2)](-) IN AQUEOUS-SOLUTION - A VARIABLE-PRESSURE, VARIABLE-TEMPERATURE AND VARIABLE-MAGNETIC FIELD O-17 NMR-STUDY

O-17 NMR longitudinal and tranverse relaxation rates and chemical shifts were measured at variable temperature at three magnetic fields (1.4, 4.7 and 9.4 T) for aqueous solutions of the complexes [Gd(H2O)(8)](3+) and [Gd(PDTA)(H2O)(2)](-). The transverse relaxation rates end chemical shifts were analysed in the light of recent EPR line width measurements to obtain the parameters for water exchange kinetics: k(ex)(298) = (8.30 +/- 0.95) x 10(8) and (1.02+/-0.10) x 10(8) s(-1), Delta H-double dagger =14.9+/-1.3 and 11.0+/-1.4 kJ mol(-1) and Delta S-double dagger =-24.1+/-4.1 and -54.6+/-4.6 J K-1 mol(-1) for [Gd(H2O)(8)](3+) and [Gd(PDTA)(H2O)(2)](-), respectively. The longitudinal relaxation rates were used to obtain the parameters for the rotation time of the complexes: tau(c)(298) = (2.9+/-0.2) x 10(-11) and (7.9+/-0.3) x 10-(11) s and E(c)=15.1+/-1.5 and 19.2+/-1.1 kJ mol(-1) for [Gd(H2O)(8)](3+) and [Gd(PDTA)(H2O)(2)](-), respectively. The O-17 NMR transverse relaxation rates were measured at variable pressure, and were used to determine the activation volumes for the water exchange process: Delta V-0(double dagger) =-3.3+/-0.2 and -1.5+/-0.5 cm(3) mol(-1) for [Gd(H2O)(8)](3+) and [Gd(PDTA)(H2O)(2)](-), respectively. It can be concluded that water exchange occurs via an associative limiting A mechanism for [Gd(H2O)(8)](3+) (without excluding I-a) and as associative interchange, I-a, mechanism for [Gd(PDTA)(H2O)(2)](-). These water exchange kinetic and mechanistic results are compared with those for the heavy Ln(3+) aqua ions and the Gd3+ complexes with DTPA(5-) and DOTA(4-).