Water exchange on [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]‐ in aqueous solution: A variable‐pressure, ‐temperature and ‐magnetic field 17O NMR study

Abstract

Abstract17O 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 and chemical shifts were analysed in the light of recent EPR line width measurements to obtain the parameters for water exchange kinetics: k = (8.30 ± 0.95) × 108 and (1.02 ± 0.10) × 108 s−1, ΔH‡ = 14.9 ± 1.3 and 11.0 ± 1.4 kJ mol−1 and ΔS‡ = −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: τ = (2.9 ± 0.2) × 10−11 and (7.9 ± 0.3) × 10−11 s and Ec = 15.1 ± 1.5 and 19.2 ± 1.1 kJ mol−1 for [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]−, respectively. The 17O NMR transverse relaxation rates were measured at variable pressure, and were used to determine the activation volumes for the water exchange process: ΔV0‡ = −3.3 ± 0.2 and −1.5 ± 0.5 cm3 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 Ia) and an associative interchange, Ia, mechanism for [Gd(PDTA)(H2O)2]−. These water exchange kinetic and mechanistic results are compared with those for the heavy Ln3+ aqua ions and the Gd3+ complexes with DTPA5− and DOTA4−.