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 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 degrees C: kex = (1.30 +/- 0.05) x 10(6) s(-1); deltaH(not equal to) = 26.1 +/- 1.4 kJ/mol; deltaS(not equal to) = -40 +/- 5J 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 UO2(2+) 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 deltaU 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 comparisons between experimental data and theory. Theoretical calculations of the activation energy were not possible because of the excessive computing time required. On the basis of theoretical and experimental studies, we suggest that the water exchange in UO2(H2O)5(2+) follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)2- (and UO2(oxalate)F2(H2O)2- studied previously) are much slower than in the aqua ion, kex = 1.6 x 10(4) s(-1), an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO2(H2O)52+ and quenching of photo excited *UO2(H2O)5(2+) are very near the same, indicating similar mechanisms.

The screening for hypothyroidism in nursing home residents living in iodine-rich regions is justified by the high prevalence of unsuspected clinical hypothyroidism. The high prevalence of antibody positivity in old age is independent of the iodine supply, but iodine supply has a determining role in the development of autoimmune hypothyroidism in the aged. Most cases of subclinical hypothyroidism in iodine-rich regions are not of autoimmune origin. In old age, hypoechogenic texture of the thyroid gland is not predictive of thyroid dysfunction.

We investigate the migration mechanism of the carbon vacancy (V C) in silicon carbide (SiC) using a combination of theoretical and experimental methodologies. The V C , commonly present even in state-of-the-art epitaxial SiC material, is known to be a carrier lifetime killer and therefore strongly detrimental to device performance. The desire for V C removal has prompted extensive investigations involving its stability and reactivity. Despite suggestions from theory that V C migrates exclusively on the C sublattice via vacancy-atom exchange, experimental support for such a picture is still unavailable. Moreover, the existence of two inequivalent locations for the vacancy in 4H-SiC [hexagonal, V C (h), and pseudocubic, V C (k)] and their consequences for V C migration have not been considered so far. The first part of the paper presents a theoretical study of V C migration in 3C-and 4H-SiC. We employ a combination of nudged elastic band (NEB) and dimer methods to identify the migration mechanisms, transition state geometries, and respective energy barriers for V C migration. In 3C-SiC, V C is found to migrate with an activation energy of E A = 4.0 eV. In 4H-SiC, on the other hand, we anticipate that V C migration is both anisotropic and basal-plane selective. The consequence of these effects is a slower diffusivity along the axial direction, with a predicted activation energy of E A = 4.2 eV, and a striking preference for basal migration within the h plane with a barrier of E A = 3.7 eV, to the detriment of the k-basal plane. Both effects are rationalized in terms of coordination and bond angle changes near the transition state. In the second part, we provide experimental data that corroborates the above theoretical picture. Anisotropic migration of V C in 4H-SiC is demonstrated by deep level transient spectroscopy (DLTS) depth profiling of the Z 1/2 electron trap in annealed samples that were subject to ion implantation. Activation energies of E A = (4.4 ± 0.3) eV and E A = (3.6 ± 0.3) eV were found for V C migration along the c and a directions, respectively, in excellent agreement with the analogous theoretical values. The corresponding prefactors of D 0 = 0.54 cm 2 /s and 0.017 cm 2 /s are in line with a simple jump process, as expected for a primary vacancy point defect.

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The rate constants and the activation parameters for the exchange between water solvent and [U(H2O)10]4+ and [UF(H2O)9]3+, and a lower limit for the rate constant at room temperature for [Th(H2O)10]4+, were determined by 17O NMR spectroscopy in the temperature range 255−305 K. The experiments were made at different constant hydrogen ion concentrations, which varied between 0.16 and 0.8 mol kg-1. The Th(IV) system was investigated using Tb3+ as a shift reagent. The following kinetic parameters at 25 °C were obtained:  k ex = (5.4 ± 0.6) 106 s -1, ΔH ⧧ = 34 ± 3 kJ mol-1, ΔS ⧧ = −16 ± 10 J mol-1 K-1 for U4+(aq), k ex = (5.5 ± 0.7) 106 s -1, ΔH ⧧ = 36 ± 4 kJ mol-1, ΔS ⧧ = 3 ± 15 J mol-1 K-1 for UF3+(aq), and k ex > 5 107 s -1 for Th4+(aq), where the uncertainty is given at the 2σ-level. This is the first experimental information on the kinetic parameters for the exchange of water for any M4+ ion. There is no information on the rates and mechanisms of ligand substitutions involving other mono-dentate ligands, hence the mechanistic interpretation of the data is by necessity provisional. The kinetic data and the known ground-state geometry with a coordination number of 10 ± 1 for the Th(IV) and U(IV) complexes suggest a dissociatively activated interchange mechanism. There is no noticeable effect of coordination of one fluoride or one hydroxide to U(IV) on the water exchange rate. This is unusual, for other metal ions there is a strong labilizing of coordinated water when a second ligand is bonded, e.g., in complexes of aluminum and some d-transition elements. In previous studies of the rates and mechanisms of ligand exchange in uranium(VI) systems we found a strong decrease in the lability of coordinated water in some fluoride containing complexes.