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

Abstract 17 O 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(H 2 O) 8 ] 3+ and [Gd(PDTA)(H 2 O) 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) × 10 8 and (1.02 ± 0.10) × 10 8 s −1 , Δ H ‡ = 14.9 ± 1.3 and 11.0 ± 1.4 kJ mol −1 and Δ S ‡ = −24.1 ± 4.1 and −54.6 ± 4.6 J K −1 mol −1 for [Gd(H 2 O) 8 ] 3+ and [Gd(PDTA)(H 2 O) 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 E c = 15.1 ± 1.5 and 19.2 ± 1.1 kJ mol −1 for [Gd(H 2 O) 8 ] 3+ and [Gd(PDTA)(H 2 O) 2 ] − , respectively. The 17 O NMR transverse relaxation rates were measured at variable pressure, and were used to determine the activation volumes for the water exchange process: Δ V 0 ‡ = −3.3 ± 0.2 and −1.5 ± 0.5 cm 3 mol −1 for [Gd(H 2 O) 8 ] 3 + and [Gd(PDTA)(H 2 O) 2 ] − , respectively. It can be concluded that water exchange occurs via an associative limiting A mechanism for [Gd(H 2 O) 8 ] 3 + (without excluding I a ) and an associative interchange, I a , mechanism for [Gd(PDTA)(H 2 O) 2 ] − . These water exchange kinetic and mechanistic results are compared with those for the heavy Ln 3+ aqua ions and the Gd 3+ complexes with DTPA 5− and DOTA 4− .