Complex oxides thermodynamic properties

Eh are usually less than 10 M because of the extremely low solubilities of these solids. In the U(V) oxidation state, uranium occurs as the UOJ ion which forms relatively weak complexes (Grenthe et al. 1992). This species is only found at intermediate oxidation potentials and low pH s and is unstable relative to U(IV) and U(VI). In oxidized surface- and groundwater-uranium is transported as highly soluble uranyl ion (UOf ) and its complexes, the most important of which are the carbonate complexes. The thermodynamic properties of these minerals and aqueous species must be known if we are to understand the reactions that may control U concentrations in natural waters. 

Nevertheless, the system, composed of chain fragments of oxyfluoroniobate complexes, is thermodynamically less stable. Dipole properties of fragments of a certain length are re-orientated so as to be linked into typical infinite chains. There is no doubt that the fragment re-orientation and linking process initiates the partial reduction of niobium to Nb4+ and the oxidation of fluoride to elementary fluorine. The process scheme can be presented as follows ... 

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

Adequacy of Thermodynamic Data. Data on several important aluminosilicates appear to be insufficient for a detailed discussion of all equilibria. Information on the influence of solid solutions or coprecipitated phases on thermodynamic properties appears to be rather limited, as is that for metastable non-stoichiometric oxides (e.g., of manganese) and surface complexes. 

The interactions of diisopropylfluorophosphate (DFP) with model MgO and CaO surfaces have been investigated using density functional (DFT) and Mpller-Plesset second order perturbation techniques [67]. Geometries of considered complexes were fully optimized at the DFT level. The calculated interaction energies and the corresponding thermodynamic properties show that DFP is physisorbed on these two model oxide surfaces and the adsorption on the MgO surface is stronger. 

Arenes spontaneously form intermolecular 1 1 complexes with a wide variety of electrophiles, cations, acids, and oxidants that are all sufficiently electron-poor to be classified as electron acceptors. Spectral, structural, and thermodynamic properties of these donor/ acceptor associates are described within the context of the Mulliken charge-transfer (CT) formulation. The quantitative analyses of such CT complexes provide the mechanistic basis for understanding arene reactivity in different thermal and photochemical processes. 

Recent investigations have shed light on peculiarities of the NOS action mechanism the role of the H4B cofactor and CaM, and cooperativity in kinetic and thermodynamic properties of different components of the nitric oxide synthesis system. Stop flow experiments with eNOS (Abu-Soud et al., 2000) showed that calmodulin binding caused an increase in NADH-dependent flavin reduction from 0.13 to 86 s 1 at 10 °C. Under such conditions, in the presence of Arg, heme is reduced very slowly (0.005 s 1). Heme complex formation requires a relatively high concentration ofNO (>50 nM) and inhibits the entire process NADH oxidation and citrulline synthesis decreases 3-fold and Km increases 3-fold. NOS reactions were monitored at subzero temperatures in the presence of 50% ethylene glycol as an anti-freeze solvent (Bee et al., 1998). 

The goal of the present monograph is to generalize the works carried out in this research direction. The subject of investigations is the synthesis of complex oxides of the elements ofl-Vin groups of the Periodic Table from ordinary hydroxides. For numerous hydroxides, substantial differences in their properties are observed, in particular, the differences in acid-base characteristics due to the structure of electron shells of atoms and the nature of chemical bonds. This allows one to search the definite laws governing the formation of complex oxides, and to look for correlation between the structure of initial hydroxides, their thermodynamic parameters and kinetics of mechanochemical synthesis. 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

The fact that metal ions in complexes often have the ability to undergo oxidation and/or reduction, with the resultant complexes having distinctly different properties as a result of the change in the metal d-electron set, means that techniques able to probe these processes have developed (Table 7.4). In coordination chemistry, the technique of cyclic voltammetry (sometimes called electrochemical spectroscopy because of its capacity to rapidly probe behaviour in different oxidation states in simple solution experiments) is now commonly employed. Thermodynamic properties, such as reaction enthalpy and complex stability... 

Copper-dioxygen complexes are necessary intermediates in the biomimetic oxidations performed by type 3 Cu models. Several structural motifs for synthetic Cu 02 complexes exist and the main types are shown in Scheme 4. Their mechanism of formation, structural characteristics, spectroscopic and thermodynamic properties, and reactivity have been recently thoroughly reviewed (62,63). In general, the reaction between a Cu(I) complex and dioxygen initially forms a 1 1 Cu/02 adduct that, in the absence of steri-cally hindered ligands, rapidly evolves to the thermodynamically more stable 2 1 Cu/02 adduct. According to the usual formalism, these processes can be described by the following equilibria ... 

The enthalpy increment studies are all limited to about 2000 K. Above this temperature the rare earth oxides show a complex polymorphism (see Chapter 3 of this book), with the appearance of the hexagonal H-type and cubic X-type stractures [44]. No experimental data on the heat capacity of these phases are known, nor on the enthalpies of transition and fusion, which makes the description of the thermodynamic properties for these phases highly uncertain. 

In view of the fact that pharmacological effects of nitroprusside, [Fe(CN)5NO], a widely recognized hypotensive agent (61—65), have been attributed to the release of nitric oxide from its reduced form, i.e., [Fe(CN)5NO], the kinetic and thermodynamic properties of both nitrosyl complexes of pentacyanoferrate-(II) and -(III) have attracted considerable attention in the past two decades (66,67). In this context, the formation of [Fe(CN)5NO] and [Fe(CN)5NO] in the direct reactions of [Fe(CN)5(H20)] and [Fe(CN)5(Fl20)] with nitric oxide, respectively, was subjected to detailed kinetic and mechanistic investigations (68-70). As presented below, the results of these studies allowed to draw valuable conclusions concerning the validity of the mechanism of NO release from nitroprusside postulated in the Hterature. 

Ute] Utesheva, V., Vagina, X, Ermolaeva, L., Maslov, Yu.P., Maslov, P.G., Rymkevich, P.P., Perevozchikov, E.N., Korshunov, V.S., Formulas for the Temperature Dependence of Thermodynamic Properties of Certain Complex Crystalline Oxides , J. Appl. Chem. USSR, 12, 2619-2623 (1979), translated from Zh. Prikl Khim., 52(12), 2771-2776 (1979) (Calculation, Thermodyn., 9)... 

AH of reaction (17) is an estimate since PuCl4(s) has not been synthesized AfH° [PuCl4(s)] = — 964 kJ is an estimate (Fuger et al. 1983). The dramatic difference in these enthalpies [Pu(IV) is more stable than Pu(III) in this complex chloride by -95-( —4) = - 91 kJ or almost 1.0 V, in comparison with the binary chloride] shows how acid-base effects influence oxidation-reduction properties. As noted above, there are few known lanthanide (IV) complex halides and no thermodynamic data on even these few halides, so no quantitative comparison can be made. Nevertheless it does show how complexation effects by basic complexants make high f-element oxidation states attainable. Perhaps the most dramatic evidence of the enhancement of high oxidation state by a basic fluoride is the existence of the Nd(IV) and Dy (IV) compounds such as Cs2(Cs, Rb, K) (Nd, Dy)Cl2 these AjRF, double fluorides are the only known Nd (IV) and Dy(IV) compounds. 

In the assessment of the refining performance of uranium, systematic data has been reported for the chemical properties of uranium complex in various alkali chlorides such as LiCl-RbCl and LiCl-CsCl mixtures [3-5], Information on the coordination circumstance of solute ions is also important since it should be correlated with stability. The polarizing power of electrolyte cations controls the local structure around neodymium trivalent Nd " " as an example of f-elements and the degree of its distortion from octahedral symmetry is correlated with thermodynamic properties of NdClg " complex in molten alkali chlorides [6]. On the other hand, when F coexists with Cr in melts, it is well-known that the coordination circumstances of solute ions are drastically changed because of the formation of fluoro-complexes [7-9]. A small amount of F stabilizes the higher oxidation states of titanium and induces a negative shift in the standard potentials of the Ti(IV)ITi(ni) and Ti(III)ITi(II) couples [7, 8], The shift in redox potentials sometimes causes specific electrochemical behavior, for example, the addition of F to the LiCl-KCl eutectic leads to the disproportionation of americium Am into Am " and Am metal [9],... 

Although a number of binary uranium oxides with 0/U > 2.00 are known, along with many of their thermodynamic properties, there are no comparable properties of heavier actinide oxides with which to compare them. (The only transuranium binary oxide with O/An > 2.00 is NpjOs and none of its thermodynamic properties have been measured.) To compare thermodynamic properties of actinide oxides in high oxidation states, scientists have studied complex oxides. 

One class of complex oxides containing tetravalent f ions has been studied thermochemically, the perovskites BaMOa. Only the thermodynamic properties of two actinide(iv) perovskites have been determined, BaUOj and BaPuOj [104,355]. A general trend is that the perovskites are increasingly stabilized (with... 

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