Water Exchange Kinetics Research Articles

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

Abstract17O 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(H2O)8]3+ and [Gd(PDTA)(H2O)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) × 108 and (1.02 ± 0.10) × 108 s−1, ΔH‡ = 14.9 ± 1.3 and 11.0 ± 1.4 kJ mol−1 and ΔS‡ = −24.1 ± 4.1 and −54.6 ± 4.6 J K−1 mol−1 for [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)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 Ec = 15.1 ± 1.5 and 19.2 ± 1.1 kJ mol−1 for [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]−, respectively. The 17O NMR transverse relaxation rates were measured at variable pressure, and were used to determine the activation volumes for the water exchange process: ΔV0‡ = −3.3 ± 0.2 and −1.5 ± 0.5 cm3 mol−1 for [Gd(H2O)8]3 + and [Gd(PDTA)(H2O)2]−, respectively. It can be concluded that water exchange occurs via an associative limiting A mechanism for [Gd(H2O)8]3 + (without excluding Ia) and an associative interchange, Ia, mechanism for [Gd(PDTA)(H2O)2]−. These water exchange kinetic and mechanistic results are compared with those for the heavy Ln3+ aqua ions and the Gd3+ complexes with DTPA5− and DOTA4−.

Correlation between the preparation conditions and the mesoporous texture of titania gels

The titania gels studied have been prepared by hydrolysis of titanium alkoxides, using a micellar solution obtained from a non-ionic surfactant dissolved in a non-polar solvent such as decane. The kinetic of the hydrolysis was controlled by the water exchange kinetics between reversed micelles. Two parameters, n and m, have been used to control the kinetic of gel growth and the texture of the final product. The parameter n is the water/surfactant molar ratio and m is the alkoxide molar concentration. Correlation between the variation of these two parameters and the mesoporous textural evolution have been investigated, using thermoporometry. Thermoporometry is a textural characterization method for porous materials, performed in a liquid medium. It appears to be a very appropriate method to characterize the mesoporous texture of wet gels. Although monolithic titania gels can be obtained in a large range of n and m values, thermoporometry results showed that kinetic parameters have to be precisely adjusted in order to achieve a homogeneous porous texture.

Triaqua(benzene)ruthenium(II) and triaqua(benzene)osmium(II): synthesis, molecular structure, and water-exchange kinetics

Solid salts of M(/eta/-C/sub 6/H/sub 6/)(H/sub 2/O)/sub 3//sup 2+/ (M = Ru, Os) are obtained by reacting (MCl/sub 2/(/eta/-C/sub 6/H/sub 6/))/sub 2/ with Ag/sup +/ in aqueous solution or by the reaction of Ru(H/sub 2/O)/sub 6//sup 2+/ with cyclohexadiene in ethanol. (Ru(/eta/-C/sub 6/H/sub 6/)(H/sub 2/O)/sub 3/)SO/sub 4/ crystallizes in the orthorhombic space group Pbca with a = 12.892 (2) /angstrom/, b = 12.441 (1) /angstrom/, and c = 12.183 (2) /angstrom/ (T = 125 K), and Z = 8. The structure was refined to 1.9% for 2168 reflections with F/sub 0/ > 3/sigma/(F/sub 0/). The relative arrangement of the benzene ring and the three water molecules is approximately staggered, the torsional angle being 19.2 (4)/degree/. After correction for thermal motion, average distances are Ru-C = 2.164 (4), C-C = 1.419 (5), and Ru-O = 2.117 (11) /angstrom/. The Ru-center of the benzene plane distance is 1.631 /angstrom. Structural results obtained at 295 K agree with those at 125 K. Water-exchange rates at variable temperature and pressure were determined by line width measurements of 17/O NMR spectra at 4.7 T. For Ru(/eta/-C/sub 6/H/sub 6/)(H/sub 2/O)/sub 3//sup 2+/ and Os(/eta/-C/sub 6/H/sub 6)(H2/O)/sub 3//sup 2+/ the following results are obtained: k(298 K), 11.5more » /plus minus/ 3.1 and 11.8 /plus minus/ 2.0 s/sup /minus/1/; /Delta/H*, 75.9 /plus minus/ 3.8 and 65.5 /plus minus/ 2.2 kJ mol/sup /minus/1/; /Delta/S*, +29.9 /plus minus/ 10.6 and /minus/4.8 /plus minus/ 6.1 J K/sup /minus/1/ mol/sup /minus/1/; /Delta/V*, +1.5 /plus minus/ 0.4 and +2.9 /plus minus/ 0.6 cm/sup 3/ mol/sup /minus/1/. The reaction proceeds via an interchange mechanism (I) where the bond-breaking contribution has only a slightly larger weight than the bond-making one. The kinetic behavior indicates a strong trans-labilizing influence of the aromatic ligand. 30 refs., 3 figs., 7 tabs.« less

Water exchange kinetics on lanthanide(III) ions: a variable temperature and pressure 17O NMR study

Water exchange kinetics on lanthanide(III) ions: a variable temperature and pressure 17O NMR study

Sodium ions in hydrated faujasites as studied by 23Na and 1H NMR

Longitudinal ( T 1) and transverse ( T 2) relaxation times and the absolute intensity of 23Na NMR have been studied as a function of temperature, Si/Al ratio and ion exchange. The NMR spectrum of the quadrupole nucleus 23Na of spin I= 3 2 consists of a central line and satellites. Depending on the strength of the quadrupole interaction of the 23Na ions, a whole spectrum, only the central line, or even nothing may be observed experimentally. But the observed 23Na-NMR spectrum can be assigned unambiguously by pulse methods because the NMR signal of a whole spectrum and of a central line depend quite differently on the r.f.-pulse length. By measuring the intensity of the signal as a function of the r.f.-pulse length, it can be shown that a whole spectrum of only a fraction of the Na ions is observed in hydrated NaX and NaY faujasites. In NaY with seven Na ions per cage only four are observed by NMR. It can be concluded from selective ion exchange, and from 1H NMR of water-exchange kinetics between the supercages and the β-cages, that only and all the Na ions in site S2 are detected by NMR. There is extreme motional narrowing above 230 K. The fluctuations of the electrical-field gradient are caused by the diffusion of the sorbed water and not by the diffusion of the Na ions themselves. The quadrupole coupling constant QCC is estimated from T 1. The value of QCC = 3 MHz agrees well with model calculations. In NaX, 2.4 Na per cage are observed. The same value is found in NaY at higher temperatures. No reasonable explanation can be given for this fact.

Osmoregulation of the grass shrimp Palaemonetes pugio exposed to polychlorinated biphenyls (PCBs). I. Effecton chloride and osmotic concentrations and chloride-and water-exchange kinetics

Grass shrimp, Palaemonetes pugio, were capable of hypo- and hyper-osmotic regulation of body fluids. Hemolymph chloride and osmotic concentrations were maintained at relatively stable levels over a wide salinity range. Following an abrupt transfer from intermediate (14 and 17‰) to high (31 and 35‰) or low (1 and 2‰) salinities, hemolymph chloride levels exhibited initial overshoot and undershoot, respectively, of new steady-state levels. Osmotic concentrations exhibited an initial undershoot at low, but not overshoot at high salinity. Chloride space in salinity-acclimated shrimp was relatively stable at salinities from 1 to 35‰. Changes in chloride space following salinity transfer paralleled those of hemolymph chloride levels, and are discussed in the light of alterations in intracellular sodium concentrations reported earlier. Rate constants for chloride turnover indicated independent exchanges of sodium and chloride ions. Water-turnover measurements showed that permeability of P. pugio was greatest at the isosmotic salinity (17‰) and reduced at salinities which were associated with active osmoregulation. Exposure to sublethal and 96-h LC50 levels of Aroclor® 1254 did not seriously alter hemolymph chloride and osmotic concentrations, chloride space or chloride-exchange kinetics in adult shrimp. Disruption of hemolymph chloride regulation in juvenile shrimp was associated with large mortalities not observed in adults. Shrimp exposed to Aroclor 1254 at 17‰ S exhibited reduced water permeability similar to levels previously observed in controls at high and low salinities in response to osmotic or ionic gradients. Exposure to PCBs did not result in further reduction in permeability at the latter salinities.

Water-exchange kinetics in labile aquo and substituted aquo transition metal ions by means of 17o nmr studies

Water-exchange kinetics in labile aquo and substituted aquo transition metal ions by means of 17o nmr studies