A complete mechanistic study of the solution dynamics of [Eu(DOTAM)(H2O)]3+ is performed through 1H and 17O NMR variable pressure and temperature studies. An unambiguous understanding of the water exchange was possible thanks to the first 17O NMR observation of the bound water signal on both the M- and m-isomers (M: k(M)/250 = 0.113 ± 0.013 x 103 s-1, k(M)/298 = 8.3 ± 0.3 s-1, ΔH(+) = 53.1 ± 2 kJ mol-1, ΔS(+) = +8.4 ± 5 J K-1 mol-1, AV(+) = +4.9 ± 1 cm3 mol-1; m: k(M)/250 = 8.9 ± 0.6 s-1, k(M)/269 = 327 ± 60 x 103 s-1, Δ(+) = 44.2 ± 2 kJ mol-1, ΔS(+) = +8.8 ± 7 J K-1 mol-1). The water exchange on m is about 50 times faster than on M, and even though the equilibrium constant K = [M]/[m] equals 4.5, the contribution of m to the overall exchange rate is 90%. These results can be transferred to an aqueous solution since they agree with the overall exchange rate obtained by 17O NMR for an aqueous solution of [Gd(DOTAM)(H2O)]3+. 2D-EXSY and variable temperature and pressure 1H NMR experiments reveal that the interconversion between the M and m isomers happens mainly through a rotation of the amide arms in an interchange activated mechanism. (Interconversion rates measured by magnetization transfer: M → m: k(a)/250 = 60 ± 10 s-1; m → M: K(a)/250 = 260 ± 50 s-1. Activation volumes: M → m: ΔV(+) = -0.5 ± 0.3 cm3 mol-1, m → M: ΔV(+) = -0.5 ± 0.7 cm3 mol-1 and reaction volume ΔV°= 0 ± 1 cm3 mol-1). In light of a simultaneous fitting of the water exchange and interconversion NMR data (M → m: ΔH(+) = 39.1 ± 1.9 kJ mol-1, ΔS(+) = -52.1 ± 7.4 J K-1mol-1, k250 = 68 ± 5 s-1), as well as an interpretation of the activation and reaction volumes, we deduce a correlation between the two processes. A nonhydrated complex is proposed as a common intermediate for both the water exchange and the arm rotation processes, but only one M → m interconversion happens while two to three water exchanges take place.
"First O-17 NMR observation of coordinated water on both isomers of [Eu(DOTAM)(H2O)](3+): A direct access to water exchange and its role in the isomerization"
AIME, Silvio;
2000-01-01
Abstract
A complete mechanistic study of the solution dynamics of [Eu(DOTAM)(H2O)]3+ is performed through 1H and 17O NMR variable pressure and temperature studies. An unambiguous understanding of the water exchange was possible thanks to the first 17O NMR observation of the bound water signal on both the M- and m-isomers (M: k(M)/250 = 0.113 ± 0.013 x 103 s-1, k(M)/298 = 8.3 ± 0.3 s-1, ΔH(+) = 53.1 ± 2 kJ mol-1, ΔS(+) = +8.4 ± 5 J K-1 mol-1, AV(+) = +4.9 ± 1 cm3 mol-1; m: k(M)/250 = 8.9 ± 0.6 s-1, k(M)/269 = 327 ± 60 x 103 s-1, Δ(+) = 44.2 ± 2 kJ mol-1, ΔS(+) = +8.8 ± 7 J K-1 mol-1). The water exchange on m is about 50 times faster than on M, and even though the equilibrium constant K = [M]/[m] equals 4.5, the contribution of m to the overall exchange rate is 90%. These results can be transferred to an aqueous solution since they agree with the overall exchange rate obtained by 17O NMR for an aqueous solution of [Gd(DOTAM)(H2O)]3+. 2D-EXSY and variable temperature and pressure 1H NMR experiments reveal that the interconversion between the M and m isomers happens mainly through a rotation of the amide arms in an interchange activated mechanism. (Interconversion rates measured by magnetization transfer: M → m: k(a)/250 = 60 ± 10 s-1; m → M: K(a)/250 = 260 ± 50 s-1. Activation volumes: M → m: ΔV(+) = -0.5 ± 0.3 cm3 mol-1, m → M: ΔV(+) = -0.5 ± 0.7 cm3 mol-1 and reaction volume ΔV°= 0 ± 1 cm3 mol-1). In light of a simultaneous fitting of the water exchange and interconversion NMR data (M → m: ΔH(+) = 39.1 ± 1.9 kJ mol-1, ΔS(+) = -52.1 ± 7.4 J K-1mol-1, k250 = 68 ± 5 s-1), as well as an interpretation of the activation and reaction volumes, we deduce a correlation between the two processes. A nonhydrated complex is proposed as a common intermediate for both the water exchange and the arm rotation processes, but only one M → m interconversion happens while two to three water exchanges take place.
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