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

✍ Scribed by Károly Micskei; D. Hugh Powell; Lothar Helm; Ernö Brücher; André E. Merbach

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 956 KB
• Volume: 31
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260311111

✦ Synopsis

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−^.