Bond , 16-25 with carbon CONTENTS

The B mas and static NMR spectra of a series of boron carbides show a broad major resonance at about 1.3 to - 4.6 ppm, the peak position varying almost linearly with carbon content (Figure 7.10A). This resonance has been assigned to boron in the B-rich icosahedral units which are bonded together both directly and via three-atom chains (Kirkpatrick etal. 1991). A small additional shoulder on the major resonance of the static B spectra (Figure 7.10B) which increases in intensity with decreasing C content and can be simulated as a second-order quadrupolar lineshape has been assigned to the boron site in the centre of the various possible C-B-C chains (Kirkpatrick et al. 1991). 

Fujimoto, Migita and Kasuya (218) have shown that the ultimate elongation changes little with carbon content, but at fixed strain rate the temperature at which eb (max) is realized increases with black concentration (compare with Fig. 19). The authors suggest that bond rupture occurs mainly in the rubber matrix and scarcely at all at the surface of the carbon black. The greatest proportion of surface bond rupture is thought to occur at eb (max). 

Gurau et al. published a comprehensive study of binary, ternary and quaternary eatalysts for methanol oxidation consisting of Pt, Ru, Ir and Os [122], The theoretieal foundation for this work has been the metal-carbon and metal-oxygen bond strength. Pt and Ir are forming strong bonds with carbon, while Ru and Os are both oxophilic. Furthermore, it was found that flie best eatalysts are Arose that maximize the content of Ru, Os and Ir in the fee Pt phase based on solubility limits. XPS spectra showed that the catalyst surfaces were enriched with Ru, Os and Ir present either as oxide or hydrous oxide species [122]. 

Esters can participate m hydrogen bonds with substances that contain hydroxyl groups (water alcohols carboxylic acids) This confers some measure of water solubil ity on low molecular weight esters methyl acetate for example dissolves m water to the extent of 33 g/100 mL Water solubility decreases as the carbon content of the ester increases Fats and oils the glycerol esters of long chain carboxylic acids are practically insoluble m water... 

Silica-based stationary phases with a chemically bonded ligand on the surface can be characterized by the carbon content (grams of carbon per 100 g of packing) and by the bonding density (micromols of ligand bonded/square meter of initial silica surface area). 

Palladium catalysts resemble closely the platinum catalysts. Palladium oxide (PdO) is prepared from palladium chloride and sodium nitrate by fusion at 575-600° [29,30]. Elemental palladium is obtained by reduction of palladium chloride with sodium borohydride [27, 31], Supported palladium catalysts are prepared with the contents of 5% or 10% of palladium on charcoal, calcium carbonate and barium sulfate [32], Sometimes a special support can increase the selectivity of palladium. Palladium on strontium carbonate (2%) was successfully used for reduction of just y, (5-double bond in a system of oc, / , y, (5-unsaturated ketone [ii]. 

Sulfur-selenium phases can be prepared by cooling molten mixtures of the elements either slowly or by quenching followed by extraction with carbon disulfide, carbon tetrachloride or benzene. The crystals are obtained upon evaporation or cooling of the resulting solutions. Their colour deepens from yellow to ruby red with increasing selenium content In the older literature there has been some confusion whether to consider these phases as mixed crystals of discrete Sg and SCg molecules or as binary compounds containing SeS bonds. 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

The polar groups are, on the other hand, responsible for an induced polar selectivity. Analytes able to form hydrogen bonds like phenols are retarded more strongly with polar-embedded stationary phases than with the corresponding classical RP of an identical carbon content. This is demonstrated in Figure 2.5 for the separation of polyphenolic compounds present in red wine. The retention time of the polyphenolic compound kaempferol with the shielded phase is more than three times longer than with the corresponding RP column of an identical carbon content. The polar... 

Stationary phases with a high density of bonded alkyl groups can differentiate between two molecules of identical size where one is planar and the other twisted out of plane. This shape selectivity has been described by Sander and Wise [53] for polymeric stationary phases, where in the preparation, water has been added on purpose and trichloro alkyl silanes have been used. The selectivity for the retention of tetrabenzonaphthalene (TEN) and benzo[a]pyrene (BaP) was taken as a measure to differentiate between polymeric and standard RP columns. With standard ( monomeric ) RP columns, the twisted TBN elutes after the planar BaP, which on the other hand is more strongly retarded as TBN on polymeric stationary phases. In these cases the relative retention of TBN/ BaP is smaller than 1, whereas with monomeric phases the value is >1.5. The separation of the standards on three different phases is shown in Figure 2.9. These stationary phases have superior selectivity for the separation of polyaromatic hydrocarbons in environmental analysis. Tanaka et al. [54] introduced the relative retention of triphenylene (planar) and o-terphenyl (twisted), which are more easily available, as tracers for shape selectivity. However, shape selectivity is not restricted to polymeric phases, monomeric ones can also exhibit shape selectivity when a high carbon content is achieved (e.g., with RP30) and silica with a pore diameter >15 nm is used [55]. Also, stationary phases with bonded cholestane moieties can exhibit shape selectivity. 

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