Pom Structure

FT-IR study revealed that no damage of the Co-POM structure occurred when its TBA-salts dissolved in MeCN were used for the immobilization. As one can judge from Figure 2, the IR spectra after subtraction of the peaks due to NH2-X exhibited the principal stretching modes of the Keggin Co-POM imit (956, 888, 818, 752, 720 cm" ). 

The MIL-supported POM eatalysts were eharaeterized by elemental analysis, XRD, N2 adsorption, and FT-IR-speetroseopy, whieh indieated the preservation of both MIL and POM structures after immobilization. The eomposite M-POM/MIL-101 materials demonstrated fairly good catalytie performanee in a-pinene allylie oxidation (81-84 % verbenol/verbenone seleetivity at 15-25 % substrate eonversion) and earyophyllene epoxidation (100 % selectivity at 88 % conversion) with green oxidants - H2O2 (Ti-POM) and O2 (Co-POM) [126]. 

Members of another class of powerful direct mutagens, four nitroazabenzo[a]pyrene derivatives, were identified by Sera and co-workers (1994) in the basic fraction of extracts of diesel exhaust and in ambient POM. Structures and direct mutagenic activities on strains TA98 and YG1024 (-S9 mix) of the 1- and 3-nitro-6-azabenzo[a]pyrenes and the 1- and 3-nitro-6-azabenzo[a]pyrene-7V-oxides are shown in Table 10.21 (Sera et al., 1992 Fukuhara et al., 1992). They have been measured in ambient air in Fukuoka, Japan, at concentrations of 1.1, 1.2, 0.8, and 0.3 ng/g, respec-... 

Lacunary spedes can act as structural motifs with numerous metal cations, leading to mono-, di- and trinuclear transition-metal-substituted POMs according to the number of vacant sites. The metal incorporation involves the reaction of aquated first-row and occasionally second-row d-block metal ions, M(H20)6z +, with a lacunary POM derived from the Keggin, Wells-Dawson and other POM families. In addition, lacunary spedes assemble into large POM structures, either directly or with incorporation of metal ion linkers. 

Nie, Z. H., Fava, D., Rubinstein, M., Kumacheva, E., Supramolecular assembly of gold nanorods end-terminated with polymer Pom-Poms Effect of pom-pom structure on the association modes. J. Am. Chem. Soc. 2008, 130, 3683-3689. 

It has been known for well over 150 years that other positive-valent elements can be incorporated into POM structures as "heteroatoms" (hence the term "heteropoly acids"). Most elements of the Periodic Table have been incorporated, some such as phosphorus yielding dozens of compositions and structures. At the present time the field of POMs appears to be limitless. 

Kumar and Landry217 used 31P SS NMR for the examination of the physical properties of the material obtained by immobilisation of the [PV2MoioC>4o] POM anion on the mesoporous MCM-41. The results obtained showed that the POM structure is intact after impregnation. The texture and structure of the support as a function of POM contents were studied, too. 

Figure 1 Representative POM structures in polyhedral notation. Top left the common Keggin structure top right the hexametalate structure bottom left the Wells-Dawson structure bottom right the Pope-Jeannin-Preyssler structure. In polyhedral notation, the center of each M06 unit is a metal nucleus and each...

FIGURE 11.40 Catalytic oxidation of a phenolic lignin model compound with acidic oxygen in the presence of a POM structure. 

In a previous work we found that the addition of small amounts of Sb3+ to the POM-based catalyst makes possible the occurrance of a redox reaction between Mo in the primary POM structure and antimony, with development of a reduced compound which is stable even under oxidizing conditions [19], The objective of the work reported here was twofold 1) to evaluate the catalytic performance of this Sb-doped POM catalyst in the title reaction, and 2) to analyze the effect of the pH of precipitation of the POM on its chemical-physical features and catalytic performance, as a possible parameter to control the reactivity of these compounds. 

A similar approach was used to construct a unique triblock copolymer having dendritic termini connected by linear segment. The synthesis of the dendritic-fe/oc/c-linear-fetocA -dendritic PS, referred to as the pom-pom structure due to its dumbbell-like shape, results from a convergent anionic polymerization procedure using dichlorodimethyl-silane as a coupling agent [119]. 

Third, molecular branched architecture can be tuned to ensure maximum concentration of mechanical tension at a specific chemical bond. For example, star-like [134] and pom-pom structures [125] as well as short bottlebrushes [135] may be used to introduce site-specific activation at the branching center. Figure 10b displays the concentration of tension in the middle of the backbone resulting in its mid-chain scission. In a similar fashion (Fig. 10c), molecular stars with a spoke wheel core and bottlebrush arms exhibit preferential dissociation of the arms followed by the scission of covalent bonds in the bottlebrush backbones... 

Polyoxometalates are able to act as ligands for different redox-active transition metals (M ) such as iron, manganese, cobalt and ruthenium or d metals (such as zirconium, hafnium, titanium). The coordination of a heterometal by a polyoxometalate framework takes place either through the superficial coordination of a metal cation by electrostatic interaction, or by incorporation of the transition metal within the POM structure resulting in the formation of transition metals substituted polyoxometalates (TMSPs), which generally exhibit a higher stability. 

Fig. 7.6 Side (top) and axial (bottom) view of the oxygen (red) and hydrogen (white) atoms spatial distribution function (SDF) of the encapsulated water molecules at different temperatures 300 K (left), 200 K (center) and 100 K (right). The POM structure is omitted for clarity (Color figure online)...

Stockmayer theory can be extended to compute s for more complicated long-chain branched polymer structures, such as the pom-pom structure depicted in Figure 7.15 [39,40]. Defining the ratio of the molecular weight of the linear connector molecule to that of the total polymer as (1, and the number of arms per branch point as/ ... 

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