Transition metals contaminants

Eithei oxidation state of a transition metal (Fe, Mn, V, Cu, Co, etc) can activate decomposition of the hydiopeioxide. Thus a small amount of tiansition-metal ion can decompose a laige amount of hydiopeioxide. Trace transition-metal contamination of hydroperoxides is known to cause violent decompositions. Because of this fact, transition-metal promoters should never be premixed with the hydroperoxide. Trace contamination of hydroperoxides (and ketone peroxides) with transition metals or their salts must be avoided. 

Aluminium is much cheaper than transition metals, and aluminium oxide is non-toxic. Aluminium residues in a polymer would probably not be harmful. Thus, a catalyst based on aluminium could be extremely attractive, even if it were significantly less active than a transition metal catalyst. This has probably contributed to the continued interest in (potential) aluminium polymerization catalysts. However, such studies are difficult, as even traces of transition metal contamination may lead to erroneous conclusions. According to calculations, insertion barriers at aluminium are typically >10 kcal/mol higher than at transition metal catalysts, corresponding to a reactivity difference of 10, so... 

Several methods of metal removal from process solutions exist. Some of these approaches include crystallization, distillation and extraction. However, these methods can be very time-consuming and often result in loss of some valuable product. To overcome these shortcomings, Degussa has developed Deloxan Metal Scavengers as an attractive option to solve problems associated with transition metal contamination of process solutions. [3, 4, 5] Two resins are commercially available. Deloxan THP 11 (1) is a thiourea functionalized, macroporous, organofunctional polysiloxane while Deloxan MP (2) is mercapto functionalized. 

Contamination Studies. Schmidt and Pearce (22,). in a classic study, using NAA evaluated sources of contamination of silicon during device processing. Their work demonstrated clearly that high-temperature oxidation steps are particularly likely to add transition-metal contaminants. Mechanical polishing compounds, boats and handling tools, and components of ion-beam equipment were also identified as being potentially troublesome. 

Transition-metal contamination of ion exchangers can be removed by washing the column with an EDTA-containing eluent. 

Plutonium metal is often purified by electrolytic refining the plutonium sample is immersed in a molten chloride salt under an inert atmosphere, comprising the anode in an electrolytic cell. Liquid Pu metal is collected on the surface of a tungsten cathode and drips off into a collector. Transition-metal contaminants remain in the residue of the anode, and rare earths and other actinides concentrate in the molten salt. The yield of purified Pu metal can be as high as 97%. Zone melting is also used to purify metallic plutonium the plutonium is fabricated into a bar along which a high-temperature zone is passed. As the melt zone is moved... 

Many of these impurities have strong absorption features in the visible and NIR. To monitor the transition metal contamination in the glass the visible spectrum was taken along the length of each tube. Since spectral features were being monitored, polished surfaces were not necessary. With a meter long path the sensitivity for Cr(III) was found... 

Figure 2-27. A LAB (Lightness, a, b) colorspace plot using data from 380 to 780 nmfora 0.75 ppm level impurities of several transition metal contaminants in silica. The chroma, which is the length of the vector to a data point, is proportional, at these levels, to the concentration.

Nearly every technical difficulty known is routinely encountered in transition metal calculations. Calculations on open-shell compounds encounter problems due to spin contamination and experience more problems with SCF convergence. For the heavier transition metals, relativistic effects are significant. Many transition metals compounds require correlation even to obtain results that are qualitatively correct. Compounds with low-lying excited states are difficult to converge and require additional work to ensure that the desired states are being computed. Metals also present additional problems in parameterizing semi-empirical and molecular mechanics methods. 

Many transition metal systems are open-shell systems. Due to the presence of low-energy excited states, it is very common to experience problems with spin contamination of unrestricted wave functions. Quite often, spin projection and annihilation techniques are not sufficient to correct the large amount of spin contamination. Because of this, restricted open-shell calculations are more reliable than unrestricted calculations for metal system. Spin contamination is discussed in Chapter 27. 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

Basic oxides of metals such as Co, Mn, Fe, and Cu catalyze the decomposition of chlorate by lowering the decomposition temperature. Consequendy, less fuel is needed and the reaction continues at a lower temperature. Cobalt metal, which forms the basic oxide in situ, lowers the decomposition of pure sodium chlorate from 478 to 280°C while serving as fuel (6,7). Composition of a cobalt-fueled system, compared with an iron-fueled system, is 90 wt % NaClO, 4 wt % Co, and 6 wt % glass fiber vs 86% NaClO, 4% Fe, 6% glass fiber, and 4% BaO. Initiation of the former is at 270°C, compared to 370°C for the iron-fueled candle. Cobalt hydroxide produces a more pronounced lowering of the decomposition temperature than the metal alone, although the water produced by decomposition of the hydroxide to form the oxide is thought to increase chlorine contaminate levels. Alkaline earths and transition-metal ferrates also have catalytic activity and improve chlorine retention (8). 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

The ultraviolet cutoff or the absorption edge for pure vitreous siUca is 8.1 eV or 153 nm (171). This uv cutoff is influenced by the impurity level and stoichiometry of the material. Several impurities, such as the transition metals (Fe, Cu, Ti, etc) and alkaU metal ions (Na, Li, K), degrade the ultraviolet performance, shifting the uv cutoff to longer wavelengths. Ferric ions (Fe " ) cause absorption or result in network defects under reducing conditions. This contaminant at only a few ppm can be detected as an absorption at 230 nm and below (176). 

More effort has probably been devoted to study of the corrosion and passivation properties of Fe-Cr-Ni alloys, e.g. stainless steel and other transition-metal alloys, than to any other metallic system [2.42, 2.44, 2.55, 2.56]. The type of spectral information obtainable from an Fe-Cr alloy of technical origin, carrying an oxide and contaminant film after corrosion, is shown schematically in Fig. 2.13 [2.57]. 

In 1992/1994, Grubbs et al. [29] and MacDiarmid et al. [30] described an improved precursor route to high molecular weight, structurally regular PPP 1, by transition metal-catalyzed polymerization, of the cyclohexa-1,3-diene derivative 14 to a stereoregular precursor polymer 16. The final step of the reaction sequence is the thermal, acid-catalyzed elimination of acetic acid, to convert 16 into PPP 1. They obtained unsupported PPP films of a definite structure, which were, however, badly contaminated with large amounts of polyphosphoric acid. 

A potentially interesting development is the microwave-assisted transition-metal-free Sonogashira-type coupling reaction (Eq. 4.10). The reactions were performed in water without the use of copper(I) or a transition metal-phosphine complex. A variety of different aryl and hetero-aryl halides were reactive in water.25a The amount of palladium or copper present in the reaction system was determined to be less than 1 ppm by AAS-MS technique. However, in view of the recent reassessment of a similarly claimed transition-metal-free Suzuki-type coupling reaction, the possibility of a sub-ppm level of palladium contaminants found in commercially available sodium carbonate needs to be ruled out by a more sensitive analytical method.25 ... 

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