Aqua ions, exchange time with solvent water

Aqua ions, exchange time with solvent water, 231-233 Aromatic p-hydroxyoxime complexes, solvent extraction of metals, 387... 

The smaller contribution to solvent proton relaxation due to the slow exchanging regime also allows detection of second and outer sphere contributions (62). In fact outer-sphere and/or second sphere protons contribute less than 5% of proton relaxivity for the highest temperature profile, and to about 30% for the lowest temperature profile. The fact that they affect differently the profiles acquired at different temperature influences the best-fit values of all parameters with respect to the values obtained without including outer and second sphere contributions, and not only the value of the first sphere proton-metal ion distance (as it usually happens for the other metal aqua ions). A simultaneous fit of longitudinal and transverse relaxation rates provides the values of the distance of the 12 water protons from the metal ion (2.71 A), of the transient ZFS (0.11 cm ), of the correlation time for electron relaxation (about 2 x 10 s at room temperature), of the reorienta-tional time (about 70 x 10 s at room temperature), of the lifetime (about 7 x 10 s at room temperature), of the constant of contact interaction (2.1 MHz). A second coordination sphere was considered with 26 fast exchanging water protons at 4.5 A from the metal ion (99), and the distance of closest approach was fixed in the range between 5.5 and 6.5 A. 

Although there are aqua ions still to be identified, many have been characterized as already described. On the other hand, information about the number of water molecules in the second-coordination sphere of metal ions and their residence times is scarce, and the only experimentally determined lifetime of a water molecule exchanging between the 12 H20 of the second-coordination sphere and bulk water is 1.28 x 10-los (Ih o = 7.8x 109s 1) at 25°C for [Cr(H20)6]3+whieh compares with 1.44 x 10-los from molecular dynamics calculations.24 63 Similar calculations show Nd3+, Sm3+, and Yb3+ to have 17.61, 17.13, and 16.74 water molecules in the second-coordination sphere, with residence times of 1.3 x 10-11 s, 1.2 x 10—11 s, and 1.8 x 10 11 s, respectively.211 These studies are consistent with the exchange of water between the second-coordination sphere and the bulk solvent being close to diffusion controlled, as has generally been assumed in mechanistic models for the substitution of water in the first-coordination sphere. 

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