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Tl-205 longitudinal relaxation rate measurements were performed on several thallium(III) complexes with the composition Tl(OH)(n)(H2O)(6-n)((3-n)+) (n = 1,2), Tl(Cl)(n)(H2O)(m-n)((3-n)+), Tl(Br)(n)(H2O)(m-n)((3-n)+) (m = 6 for n = 1-2, m = 5 for n = 3, m = 4 for n = 4), Tl(CN)n(H2O)(m-n)((3-n)+) (m = 6 for n = 1-2, m = 4 for n = 3-4) in aqueous solution, at different magnetic fields and temperatures. C-13 and D-2 isotopic labelling and 114 decoupling experiments showed that the contribution of the dipolar relaxation path is negligible. The less symmetric lower complexes (n < 4) had faster relaxation rate dominantly via chemical shift anisotropy contribution which depended on the applied magnetic field: T, values are between 20 and 100 ms at 9.4 T and the shift anisotropy is &UDelta;σ = 1000-2000 ppm. The tetrahedral complexes, n = 4, relax slower; their T-1 is longer than 1 s and the spin-rotation mechanism is probably the dominant relaxation path as showed by a temperature dependence study. In the case of the TICl4- complex, presumably a trace amount of TICl52- causes a large CSA contribution, 300 ppm. Since the geometry and the bond length for the complexes in solution are known from EXAFS data, it was possible to establish a correlation between the CSA parameter and the symmetry of the complexes. The relaxation behaviour of the Tl-bromo complexes is not in accordance with any known relaxation mechanism., QC 20100525

Dynamics of cyanide exchange between Tl((CN)-C-13)(4)(-) and (CN-)-C-13 was studied by means Tl-205 and C-13 NMR. The rate law consists of two parts: w = k(CN)[Tl(CN)(4)(-)][CN-] + k(OH)[Tl(CN)(4)(-)][CN-][OH-] with k(CN) = 9.7(+/-0.4) x 10(6) M(-1)s(-1) and k(OH) = 5.4(+/-0.4) x 10(10) M(-2)s(-1). It was shown that the exchange between the two cyano species can be studied, not only by C-13 NMR, but also by Tl-205 NMR using the heteronuclear carbon-thallium scalar coupling as an indicator, despite the fact that only one Tl site is present in the system. The reaction mechanism is discussed in terms of an associative interchange mechanism, I-A. Penetration of the incoming cyanide ion into the coordination sphere of Tl(CN)(4)(-) Tl(CN)(4)(OH)(-) is suggested to be the rate-determining step for the two parallel reactions paths. Possible reasons for the strong labilizing effect of the cyanide ligand is discussed in the light of thermodynamic and structural data. The possibility of detection of an exceptional exchange reaction, namely cyanide exchange between two Tl(CN)(4)(-) entities by a direct encounter was theoretically settled, but found to be too slow to be detected., QC 20100525

A careful reinvestigation by high-field F-19 NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The F-19 NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlFi(3-i)+(aq) with i less than or equal to 5. There was no need to invoke AlFi(OH)(j)((3-i-j)+) mixed complexes in the model under our experimental conditions (pH less than or equal to 6.5), nor was any evidence obtained for the formation of AlF63-(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the F-19 signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log K-i values are 6.42, 5.31, 3.99, 2.50, and 0.84 (for species i = 1-5) and 6.35, 5.25, and 4.11 (for species i = 1-3), respectively. Disappearance of the F-19 NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF22+ + *F- reversible arrow AIF*F2+ + F- (k(02) = (1.8 +/- 0.3) x 10(6) M-1 s(-1)), AlF30 + *F- reversible arrow (AlF2F0)-F-* + F- (k(03) = (3.9 +/- 0.9) x 10(6) M-1 s(-1)), and AlF30 + H*F reversible arrow AlF2*F-0 + HF (k(03)(H) = (6.6 +/- 0.5) x 10(4) M-1 s(-1)). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T-1 NMR time scale, while some of the reactions of AlF30(aq) were fast, causing large line broadening. The ligand exchange appears, QC 20100525

This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO2(H2O)(5)(2+) and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)(5)(2+), UO2(H2O)(4)(2+), and UO2(H2O)(6)(2+) and the thermodynamics of their reactions with water. In the second part we made an experimental study of the rate of water exchange in uranyl complexes and investigated how this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using O-17 NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 degrees C: k(ex) = (1.30 +/- 0.05) x 10(6) s(-1); Delta H double dagger = 26.(1) +/- 1.(4) kJ/mol; Delta S double dagger = -40 +/- 5 J/(K mol). Additional mechanistic indicators were obtained from the known coordination geometry of U(VI) complexes with unidentate ligands and from the theoretical calculations. A survey of the literature shows that there are no known isolated complexes of UO22+ with unidentate ligands which have a coordination number larger than 5. This was corroborated by quantum chemical calculations which showed that the energy gains by binding an additional water to UO2(H2O)(4)(2+) and UO2(H2O)(5)(2+) are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in Delta U for the reactions UO2(H2O)(5)(2+) --> UO2(H2O)(4)(2+) + H2O and UO2(H2O)(5)(2+) + H2O --> UO2(H2O)(6)(2+) indicates that the thermodynamics favors the second (associative) reaction in gas phase at 0 K, while the thermodynamics of water transfer between the first and second coordination spheres, UO2(H2O)(5)(2+) --> UO2(H2O)(4)(H2O)(2+) and UO2(H2O)(5)(H2O)(2+) --> UO2(H2O)(6)(2+), favors the first (dissociative) reaction. The energy difference between the associative and dissociative reactions is small, and solvation has to be included in ab initio models in order to allow quantitative compariso, QC 20100525