Oxygen polyanion

The hydrated insular groups may polymerize in various ways by splitting out water this process may be accompanied by the breaking of boron-oxygen bonds within the polyanion framework. 

Anions with 8, and probably 16-18, Mo atoms also appear to be formed, before increasing acidity suffices to precipitate the hydrous oxide. It is clear from the above equation that the condensation of M0O4 polyhedra to produce these large polyanions requires large quantities of strong acid as the supemumary oxygen atoms are removed in the form of water molecules. Careful... 

Adding potassium hydroxide, KOH, to a melt containing KF and a 0.1 mol fraction of K2TaF7 leads to the appearance of an additional band at 900 cm 1, as shown in Fig. 79 [342]. This band corresponds to TaO bond vibrations in TaOF63 complex ions. Interpretation of IR spectra obtained from more concentrated melts is less clear (Fig. 80). The observed absorption in the range of 900-700 cm 1 indicates the formation of oxyfluoride polyanions with oxygen bridges. ..OTaO. The appearance of a fine band structure could be related to very low concentrations of some isolated components. These isolated conditions prevent resonance interaction between components and thus also prevent expansion of the bands by a mechanism of resonance [362]. 

Evaluated using the simple relation r(nm) = 0.164n (with n the number of oxygen atoms in the polyanion) [30]. 

In order to give a concrete expression for Feff(x), a simple model has been assumed (see Fig. 6). In this model, it is postulated that an electron in the HOMO (essentially the 2p orbital) of the surface oxygen atom (Osu g) of the polyanion partially transfers to the LUMO of water (the 4ai molecular orbital). Then Feff(x) can be expressed as... 

The coefficients Co, nnd C2 (denoted as mq, ai, and aj in Ref. 33) are influenced by various molecular properties of the solvent and an ion, including their electron-donating or accepting abilities. Hence, these coefficients are specific to the ion. Nevertheless, they may be considered as common to a family of ions such as the polyanions whose surface atoms, directly interacting with solvents, are oxygens. This is the case for hydrated cations or anions whose surfaces are composed of some water molecules that interact with outer water molecules in the W phase or with organic solvents in the O phase. 

The veiy large Mo36 polyanion is crystallized at very low pH (Z —1.8 to 2) (116). The structure of the potassium salt Kg[Mo360112(H20)16] 36H20 has been determined by X-ray crystallography. It actually consists of two 18-molybdate subunits which combine via four common oxygen atoms (116b). 

Figure 6,4 Ternary plot of singly bonded oxygen 0 , silicon Si and bridging oxygen in each kind of polyanion. Shaded areas compositional ranges 6.31 and 6.33. Reprinted from Ottonello (1983), with kind permission of Theophrastus Publishing and Proprietary Co.

SOD is a major oxygen free radical scavenging enzyme, which may therefore have beneficial effects in liver fibrosis. Through mannosylation or coupling to the polyanion DIVEMA, SOD was made more liver specific. Both conjugates showed superior inhibition of intrahep-atic ROS production in fibrotic rats as compared to unmodified SOD. DIVEMA-SOD, however, exhibited the most potent inhibitory effects [112]. Although their mode of action is most probably extracellular free radical scavenging, Man-SOD and DIVEMA-SOD are likely to... 

The structure of ulexite [26] is based on the isolated pentaborate trianion shown in Fig. 21. This polyborate anion is associated with one crystallograph-ically unique 6-coordinate Na" cation and one unique 8-coordinate Ca " cation. The cations form chains by sharing coordinated oxygen, and these chains link the polyanions together into parallel sheets. These sheets are associated by coordination with interstitial Ca " cations and hydrogen-bonding. 

In addition, such generic terms as metal-oxygen cluster ion, metal-oxide molecule, etc., are used for polyanions (and polyacids). Since the traditional heteropoly- and isopoly- are unsatisfactory terms to express a variety of polyacids and polyanions, the terms polyoxometalates and polyoxoacids have been used recently (3). The terminology is still changing, thus reflecting the rapid expansion of the chemistry. 

Results from TG and DTA show the presence of two types of water in heteropoly compounds, i.e., water of crystallization and constitutional water molecules (59). Loss of the former usually occurs at temperatures below 473 K. At temperatures exceeding 543 K for H3PMoI204o or 623 K for H3PW12O40, the constitutional water molecules (acidic protons bound to the oxygen of the polyanion) are lost. Data obtained by in situ XRD, 31P NMR, and thermoanalysis show that thermolysis of H3PM012O40 proceeds in two steps, as shown by Eq. (6) (60). 

The presence of Lewis acidity [type (2)] or Bransted acidity [type (1)] is revealed by the IR spectrum of pyridine sorbed on AI1.5PW12O40 (138, 139). IR spectra of sorbed of NH3 show that Cui.5PW]204o has Lewis acidity as well as Bransted acidity (140). Ghosh and Moffat (141) measured the acidities of several salts of H3PW12O40 with Hammett indicators. The acid strength increases with an increase in the calculated charge on the peripheral oxygen atom of the polyanion Zr > Al > Zn > Mg > Ca > Na (141). 

The first step is H2 - 2H+ + 2e. Here, the number of electrons introduced per anion may be variable. In the second step, the protons formed in the first step react with the oxygen of the polyanion to form water. Accordingly, in TPR experiments with H2, the H2 uptake takes place but no H2O is observed at low temperatures (I - II), while H20 appears in the gas phase at higher temperatures (II - III) (259, 261). Further reduction produces several irreversibly reduced species. 

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