Structure and ion exchange properties of tunnel type titanium silicates

A large number of titanium silicates are known to exist in the mineral realm and recently a considerable literature has developed concerned with the synthesis and properties of these and related compounds. This paper describes two such families that have tunnel structures filled with cations that may easily be exchanged. A sodium titanosilicate of ideal formula, Na₂Ti₂O₃(SiO₄)₂·2H₂O, is tetragonal and built up of Ti₄O₄ cubane-like structures linked together by silicate groups in the a and b directions in the form of a square. In the c-axis direction, the cubane groups are linked by oxo-groups to form a framework enclosing tunnels parallel to the c-axis direction. The several ion exchange sites were identified based upon X-ray diffraction studies and the reason for the great affinity of this compound for Cs⁺ elucidated. The second family of compounds have the general composition M₃H(TiO)₄(SiO₄)₃·4H₂O (M=alkali metal cation) and have the pharmacosiderite structure. The sodium or potassium phases are selective for Sr²⁺ and Cs⁺. These compounds are cubic and have similar Ti₄O₄ cubane-like groups. In this case, the connectivity via silicate groups extends along all three crystallographic axes equally. This change in crystal system has a profound effect upon the ion exchange behavior. This effect, as well as the effect of germanate substitutions for silicate, will be described.     

Exchange coupling mediated through-bonds and through-space in conformationally constrained polyradical scaffolds: calix[4]arene nitroxide tetraradicals and diradical

Calix[4]arenes constrained to the 1,3-alternate conformation and functionalized at the upper rim with four and two tert-butylnitroxides have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and (1)H NMR) spectroscopy, and magnetic studies. The 1,3-alternate nitroxide tetraradical and diradical provide unique polyradical scaffolds for dissection of the through-bond and through-space intramolecular exchange couplings. In addition, detailed magnetic studies of the previously reported calix[4]arene nitroxide tetraradical, which possesses cone conformation in solution, reveal conformational dependence of exchange coupling. Through-bond coupling between the adjacent nitroxide radicals is mediated by the nitroxide-m-phenylene-CH(2)-m-phenylene-nitroxide coupling pathway, and through-space coupling is found between the diagonal nitroxide radicals at the conformationally constrained N...N distance of 5-6 A. Magnetic studies of the calix[4]arene polyradical scaffolds in frozen solutions show that the through-bond exchange coupling in the 1,3-alternate calix[4]arene tetraradical is antiferromagnetic, while that in cone calix[4]arene tetraradical is ferromagnetic. The through-space exchange couplings are antiferromagnetic in both cone and 1,3-alternate calix[4]arene tetraradical, as well as in the 1,3-alternate calix[4]arene diradical. The exchange coupling constants (|J/k|) are of the order of 1 K.     

Towards a better understanding of magnetic interactions within m-phenylene alpha-nitronyl nitroxide and imino nitroxide based radicals, part III: Magnetic exchange in a series of triradicals and tetraradicals based on the phenyl acetylene and biphenyl coupling units

The present work completes and extends our previous reports on the determination of the magnetic ground state and on the strength of the through bond exchange coupling within series of biradicals. This knowledge was subsequently exploited for the analysis of the magnetic interactions in their crystals. We report here the studies of series of triradicals incorporating alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN) as terminal radical fragments connected through a m-phenylene coupling unit in one case and a phenyl acetylene unit in other case. Tetraradical derivatives have also been studied. The studies of isolated molecules (EPR in solution and DFT calculations) allow the assessment of the magnetic interactions through the magnetic coupling unit. All triradical derivatives are found to exhibit a quartet ground state, whereas a singlet ground state is determined for the tetraradical. This last result reinforces previous findings that the singlet ground state is favoured in related biradicals involving similar m-phenylene couplers. Moreover, the through bond magnetic exchange coupling for the ortho-meta connectivity could be demonstrated as being ferromagnetic, thus ascertaining our previous hypotheses. The magnetic properties of the triradicals and tetraradicals in their solid state have been rationalized by using a previously proposed methodology, allowing to identify the most relevant magnetic pathways.     

Structure and magnetic exchange in heterometallic 3d-3d transition metal triethanolamine clusters

Synthetic methods are described that have resulted in the formation of seven heterometallic complexes, all of which contain partially deprotonated forms of the ligand triethanolamine (teaH(3)). These compounds are [Mn(III)(4)Co(III)(2)Co(II)(2)O(2)(teaH(2))(2)(teaH)(0.82)(dea)(3.18)(O(2)CMe)(2)(OMe)(2)](BF(4))(2)(O(2)CMe)(2)·3.18MeOH·H(2)O (1), [Mn(II)(2)Mn(III)(2)Co(III)(2)(teaH)(4)(OMe)(2)(acac)(4)](NO(3))(2)·2MeOH (2), [Mn(III)(2)Ni(II)(4)(teaH)(4)(O(2)CMe)(6)]·2MeCN (3), [Mn(III)(2)Co(II)(2)(teaH)(2)(sal)(2)(acac)(2)(MeOH)(2)]·2MeOH (4), [Mn(II)(2)Fe(III)(2)(teaH)(2)(paa)(4)](NO(3))(2)·2MeOH·CH(2)Cl(2) (5), [Mn(II)Mn(III)(2)Co(III)(2)O(teaH)(2)(dea)(Iso)(OMe)(F)(2)(Phen)(2)](BF(4))(NO(3))·3MeOH (6) and [Mn(II)(2)Mn(III)Co(III)(2)(OH)(teaH)(3)(teaH(2))(acac)(3)](NO(3))(2)·3CH(2)Cl(2) (7). All of the compounds contain manganese, combined with 3d transition metal ions such as Fe, Co and Ni. The crystal structures are described and examples of 'rods', tetranuclear 'butterfly' and 'triangular' Mn(3) cluster motifs, flanked in some cases by diamagnetic cobalt(III) centres, are presented. Detailed DC and AC magnetic susceptibility and magnetization studies, combined with spin Hamiltonian analysis, have yielded J values and identified the spin ground states. In most cases, the energies of the low-lying excited states have also been obtained. The features of note include the 'inverse butterfly' spin arrangement in 2, 4 and 5. A S = 5/2 ground state occurs, for the first time, in the Mn(III)(2)Mn(II) triangular moiety within 6, the many other reported [Mn(3)O](6+) examples having S = ? or 3/2 ground states. Compound 7 provides the first example of a Mn(II)(2)Mn(III) triangle, here within a pentanuclear Mn(3)Co(2) cluster.     

Structure and magnetism of an exchange coupled system: An NMR approach

A binuclear copper(II) complex [Cu₂L(OH)](C1O₄)₂·2H₂O has been synthesized and characterized by X-ray crystallography and¹H-NMR studies. The crystal structure shows that the bridging angles between Cu(l)-O(l)-Cu(2) and Cu(l)-O(2)-Cu(2) are 98.9(2)° and 102.2(2)° respectively. The Cu(l)-Cu(2) distance is 3.0097(12)?. This indicates that the interaction between the two copper atoms is antiferromagnetic in nature. The geometry around Cu(l) is distorted square-pyramid with one water molecule occupying the axial fifth position, whereas, the geometry around Cu(2) is distorted square-planar with weak interactions of one of the perchlorate anion. There are eight molecules present in the unit cell. There is an interdimer interaction between the dimers. The temperature-dependent¹H-NMR chemical shift studies have been performed on six different protons of this complex which reveals that the exchange coupling constant (− 2J) is same for all protons (208 ± 1 cm⁻¹). However, the hyperfine coupling constant (A′) was found to be different in magnitude as well as in sign. We also report solvent dependent NMR properties.     

Structure and bonding in solution of dioxouranium(VI) oxalate complexes: isomers and intramolecular ligand exchange

Structural isomers of [UO(2)(oxalate)(3)](4-), [UO(2)(oxalate)F(3)](3-), [UO(2)(oxalate)(2)F](3-), and [UO(2)(oxalate)(2)(H(2)O)](2-) have been studied by using EXAFS and quantum chemical ab initio methods. Theoretical structures and their relative energies were determined in the gas phase and in water using the CPCM model. The most stable isomers according to the quantum chemical calculations have geometries consistent with the EXAFS data, and the difference between measured and calculated bond distances is generally less than 0.05 A. The complex [UO(2)(oxalate)(3)](4-) contains two oxalate ligands forming five-membered chelate rings, while the third is bonded end-on to a single carboxylate oxygen. The most stable isomer of the other two complexes also contains the same type of chelate-bonded oxalate ligands. The activation energy for ring opening in [UO(2)(oxalate)F(3)](3-), deltaU++ = 63 kJ/mol, is in fair agreement with the experimental activation enthalpy, deltaH++ = 45 +/- 5 kJ/mol, for different [UO(2)(picolinate)F(3)](2-) complexes, indicating similar ring-opening mechanisms. No direct experimental information is available on intramolecular exchange in [UO(3)(oxalate)(3)](4-). The theoretical results indicate that it takes place via the tris-chelated intermediate with an activation energy of deltaU++ = 38 kJ/mol; the other pathways involve multiple steps and have much higher activation energies. The geometries and energies of dioxouranium(VI) complexes in the gas phase and solvent models differ slightly, with differences in bond distance and energy of typically less than 0.06 A and 10 kJ/mol, respectively. However, there might be a significant difference in the distance between uranium and the leaving/entering group in the transition state, resulting in a systematic error when the gas-phase geometry is used to estimate the activation energy in solution. This systematic error is about 10 kJ/mol and tends to cancel when comparing different pathways.     

Modification of properties of ion exchange membranes V. Structure of cationic polyelectrolyte on the surface of cation exchange membranes

Summary Electrodialytic transport properties of cation exchange membrane are changed by adhesion of polyethylene imine on the membrane surface. The effect of conformation of polyethylene imine molecules on the transport properties of the membrane, thickness of adherent layer and mechanism of adhesion were discussed. Observed were relative transport numbers of calcium ions to sodium ions,P _Na ^Ca , current efficiency, electric resistance of the membrane during electrodialysis and pH-shift of anolyte after electrodialysis. P _Na ^Ca decreased remarkably, when the conformation of polyethylene imine was compact by adding salt to the solution, adjusting pH of the solution to a high value where polyethylene imine was scarcely protonated and adding water-soluble neutral polymers and multivalent anions. It was confirmed that the conformation of polyethylene imine on the membrane surface controlled the degree of the change of the transport properties of the membrane. The adherent amount of polyethylene imine on the cation exchange membrane was calculated assuming that a monolayer of polyethylene imine molecules with hydrodynamic radii was formed. This differed markedly from the amount observed. The reason for the difference was discussed. There was no observation of salting out by sodium chloride and calcium chloride. It was concluded that the polyethylene imine molecules on the cation exchange membrane are mainly bound by ion-exchange.

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Zusammenfassung Die elektrodialytischen Transporteigenschaften von Kationenaustauschmembranen werden mit Polyäthylenimin (PEI) als kationischer Polyelektrolyt verändert. Sie werden durch die Konformation des PEI in der Lösung beeinflußt.Als Transporteigenschaften werden die relativen Transportzahlen der Ca-Ionen im Vergleich zu Na-Ionen, der elektrische Widerstand der Membran während der Elektrodialyse and die pH-Änderung nach der Elektrodialyse im Anodenraum untersucht.

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With 13 figures and 3 tables     

Structure of the optimized effective Kohn—Sham exchange potential and its gradient approximations

An analysis of the structure of the optimized effective Kohn-Sham exchange potential v_x and its gradient approximations is presented. The potential is decomposed into the Slater potential v_s and the response of v_s to density variations, v_resp. The latter exhibits peaks that reflect the atomic shell structure. Kohn—Sham exchange potentials derived from current gradient approaches for the exchange energy are shown to be quite reasonable for the Slater potential, but they fail to approximate the response part, which leads to poor overall potentials. Improved potentials are constructed by a direct fit of v_x with a gradient-dependent Padé approximant form. The potentials obtained possess proper asymptotic and scaling properties and reproduce the shell structure of the exact v_x. © 1996 John Wiley & Sons, Inc.     

Ionic exchange and statistical thermodynamics I. Equilibria in simple exchange systems

• 1.1. The conditions for ionic exchange equilibria are derived by an application of classical thermodynamics. Only thermodynamically defined terms are used.
• 2.2. The appropriate ionic concentration terms are derived by a modification of a statistical theory of localized monolayers due to Fowler and Guggenheim.
• 3.3. The theory is first applied to a uni-univalent exchange. The resultant equation is of the simple mass action type.
• 4.4. Unsymmetrical exchanges are treated by combining a method first presented by Guggenheim with an analysis of the problem of mutual ionic interaction in a monolayer of mixed valence types. Equilibrium exchange equations of a mass action type are developed.
• 5.5. Vanselow's well known equation is an approximate form of the equations derived in this paper.
     

Determination of exchange interaction parameters in heterospin exchange clusters

An approach to calculation of the intracluster exchange parameters of molecular heterospin systems is proposed. This approach allows one to estimate the model errors of the parameters to be determined. The efficiency of this technique is demonstrated by analyzing the experimental data for the Cu(II), Ni(II), and Co(II) molecular complexes with stable nitroxides.     

Structure and magnetism in Fe-Gd based dinuclear and chain systems. The interplay of weak exchange coupling and zero field splitting effects

The synthesis and characterization of two Fe-Gd systems based on bpca(-) (Hbpca = bis(2-pyridilcarbonyl)amine) as bridging ligand is presented, taking the systems as a case study for structure-property correlations. Compound 1, [Fe(LS)(II)(μ-bpca)(2)Gd(NO(3))(2)(H(2)O)]NO(3)·2CH(3)NO(2), is a zigzag polymer, incorporating the diamagnetic low spin Fe(LS)(II) ion. The magnetism of 1 is entirely determined by the weak zero field splitting (ZFS) effect on the Gd(III) ion. Compound 2 is a Fe(III)-Gd(III) dinuclear compound, [Fe(LS)(III)(bpca)(μ-bpca)Gd(NO(3))(4)]·4CH(3)NO(2)·CH(3)OH, its magnetism being interpreted as due to the antiferromagnetic coupling between the S(Fe) = ? and S(Gd) = 7/2 spins, interplayed with the local ZFS on the lanthanide center. In both systems, the d-f assembly is determined by the bridging capabilities of the ambidentate bpca(-) ligand, which binds the d ion by a tridentate moiety with nitrogen donors and the f center by the diketonate side. We propose a spin delocalization and polarization mechanism that rationalizes the factors leading to the antiferromagnetic d-f coupling. Although conceived for compound 2, the scheme can be proposed as a general mechanism. The rationalization of the weak ZFS effects on Gd(III) by multiconfiguration and spin-orbit ab initio calculations allowed us to determine the details of the small but still significant anisotropy of Gd(III) ion in the coordination sites of compounds 1 and 2. The outlined methodologies and generalized conclusions shed new light on the field of gadolinium coordination magnetochemistry.     

Hindered rotation and exchange in trimethylsilylanilides

A series of trimethylsilylanilides were prepared and characterized as amide—imidate mixtures. Low temperature NMR spectra reveal both hindered rotation and exchange processes for the formanilides and benzanilides and exchange for the acetanilides. Isomer ratios and free energies of activation are reported and used to postulate a mechanism for the exchange process.     

Metal chelate exchange in the organic phase-II Extraction and exchange constants of dithizonates and diethyldithiocarbamates

Dithizonates and diethyldithiocarbamates of Ag, Tl(I), Cu(II), Zn, Cd, Hg(II), Pb, Fe(II), Co(II), Ni, Pd(II), In(III), As(III), Sb(III), Bi, Se(IV) and Te(IV) have been prepared and their reactions in carbon tetrachloride have been studied spectrophotometrically. From the exchange constants determined, the extraction constants of metal diethyldithiocarbamates have been calculated. Where formation of mixed chelates has been observed, corresponding exchange constants have been determined. Finally, the influence of organic solvents (CCl(4), CHCl(3), C(6)H(6) and C(6)H(5)Cl) on the exchange reaction of zinc diethyldithiocarbamate with dithizone has been investigated.     

Metal chelate exchange in the organic phase-III Extraction and exchange constants of dithizonates and oxinates

Reactions between dithizonates and oxinates of Ag, Tl(I), Cu(II), Zn, Cd, Hg(II), Pb, Co(II), Ni, Pd, In, Ga and Bi in chloroform have been studied spectrophotometrically. From the exchange constants determined, the extraction constants of metal dithizonates or metal oxinates have been calculated. By this method extraction constants of many metal chelates can be calculated which are difficult to determine by studies of distribution between aqueous and chloroform phases.     

Homogenous redox catalysis competition between electron exchange and proton exchange

Whether proton exchange and nucleophilic substitution reactions can become competitive with electron exchange reactions in homogeneous redox catalysis processes due to the reduction of bromoesters has been evaluated.Electrochemical data have been correlated with the results of the analyses carried out in solution during, and at the end of, the catalytic reduction processes. Thus, it has been possible to obtain the values of γ (γ = c_RX/c_O, where c_RX is the substrate concentration and c_O that of the catalyst corresponding to pure catalysis conditions whereby only the electron-exchange reaction is observed.The hypothesis that the catalyst (9,10-diphenylanthracene) is dihydrogenated by the substrate according to DISP₂ mechanism is in agreement with the experiments.     

Study of kinetics,equilibrium and isotope exchange in ion exchange systems. II

The kinetics of isotope exchange in the²³⁸U(VI)-²³³U(VI)-strongly acidic cation exchanger Ostion KS system was studied in the temperature range 275–307K and for total uranium concentration 2.94·10⁻⁴–1.75·10⁻² mol·l⁻¹ in UO₂(NO₃)₂ solution. The experimental results were evaluated by means of the “two-film mass-transfer model” and by the use of Fick's diffusion equations which have been proved more suitable for the system studied than McKay's equation. The influence of the temperature was evaluated using the Arrhenius equation. The diffusion character of the process follows also from the value of the activation energy (15.12 kJ·mol⁻¹). In comparison with the UO ₂ ²⁺ ↔H⁺ ion exchange⁶ the isotope exchange studied is faster and less dependent on temperature (the activation energy is substantially lower).