Barium atoms, reactions

In this paper we describe the use of laser-induced fluorescence as a molecular beam detector and apply it to the reactions of barium atoms with hydrogen halides,... 

Alkaline earth metal atoms have fairly low ionization potentials, as have alkali metal atoms (e.g., 5.21 and 5.14 eV for barium and sodium, respectively [89]). Hence the reactions of alkaline earth metal atoms with oxidizing molecules are also expected to be initiated by an electron transfer and should follow the harpoon mechanism. However, alkali metal atoms are monovalent species, whereas alkaline earth metal atoms have two valence electrons. Hence peculiarities are to be expected in the alkaline earth metal reaction dynamics, especially when doubly charged products such as BaO are to be formed [90]. The second valence electron also opens up the possibility of chemiluminescent reactions, which are largely absent in alkali metal atom reactions [91, 92]. The second electron causes the existence of low-lying excited states in the product. 

A long-range electron transfer is possible in this reaction, as in alkali metal atom reactions. However, the resulting electron-transfer complex Ba NO does not correlate to the ground-state products BaO which has the structure Ba +0. Moreover, the NOJ ion is stable and its dissociation into NO 4- 0 is endoergic. Hence the Ba "NOj complex may survive for many rotational periods despite the availability of a very exoergic reaction channel. This is expected to dissociate after the transfer of the second valence electron of barium, which is probably hindered by an energy barrier. 

The above results suggest that small barium aggregates are better reducing agents than atomic barium. This may be understood with the help of Figure 20, by considering that barium aggregates more easily transfer an electron in a redox reaction than barium atoms. Chevaleyre and co-workers measured the ionization potentials... 

H. F. Davis, A. G. Suits and Y. T. Lee, Reaction Dynamics of Ground State and Electronically Excited Barium Atoms., in Gas-Phase Metal Reactions, edited by A. Eontijn, pp. 319-347, North-Holland, Amsterdam, 1992. 

The second column from the left contains the alkaline earth metals, beryllium, magnesium, calcium, strontium, barium, and radium (Be, Mg, Ca, Sr, Ba, and Ra, respectively). Magnesium and calcium are present everywhere and are needed by our salty bodies and the salty bodies of our fellow creatures. Calcium is vital to bones, teeth, seashells, and exoskeletons. Calcium plays a critical role in the operation of our muscles as well as communication between cells. Because strontium is in this family, radioactive strontium, a fission product of certain atomic reactions, can be absorbed by the body and used as it would use calcium. Radium, another radioactive element, is also found in this family. 

One of the promising superconducting materials, YBa2Cu307-3 (YBCO or Y-123), requires barium in the correct stoichiometric ratio. However, polymerization results in the /8-diketonate precursors when barium is the metal coordination site. This reaction can be avoided by coordinating a third neutral ligand such as tetrahydrofuran, trimethylamine, or suitable polyethers to the barium atom. Tetraglyme, a methoxy-terminated polyether, and alkoxide ligands have also been used to increase the stability of /3-diketonates. ... 

Stabilization Mechanism. Zinc and cadmium salts react with defect sites on PVC to displace the labHe chloride atoms (32). This reaction ultimately leads to the formation of the respective chloride salts which can be very damaging to the polymer. The role of the calcium and/or barium carboxylate is to react with the newly formed zinc—chlorine or cadmium—chlorine bonds by exchanging ligands (33). In effect, this regenerates the active zinc or cadmium stabilizer and delays the formation of significant concentrations of strong Lewis acids. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Current views (100) on the mechanism of bromination by NBS invoke the formation of molecular bromine and bromine atoms in low concentration, which subsequently act as the brominating agent. The bromination reaction was studied in detail in this laboratory under a variety of conditions using 93 (R = Ms) as a model. The product 94 (R = Ms) was indeed formed (42%) when NBS was substituted by 1.1 equivalents of bromine which was added at a slow rate to the reaction mixture. The yield was 68% when benzoyl peroxide was used as a catalyst. Using NBS alone or in the presence of reagents such as barium carbonate, pyridine, or s-trinitrobenzene, the yield was 60-70%. 

Write a balanced chemical equation for (a) the hydrogenation of ethyne (acetylene, C2H2) to ethene (C2H4) by hydrogen (give the oxidation number of the carbon atoms in the reactant and product) (b) the shift reaction (sometimes called the water gas shift reaction, WGSR) (c) the reaction of barium hydride with water. 

As pointed out previously, controlled degradation reactions are very difficult with aliphatic or alicyclic hydrocarbons, and most of the relabeling work has been concentrated on aromatic reaction products. Procedures have been extensively described by Pines and co-workers (e.g., 97, 96, also 87, 89-98, 95, 98). For the present purpose, it suffices to note that the 14C contents of the methyl side-chains and the rings in aromatic reaction products are readily estimated by oxidation of the methyl to carboxyl, followed by decarboxylation, while ethyl side-chains may be oxidatively degraded one carbon atom at a time. Radiochemical assays may be made on CO2 either directly in a gas counter, or after conversion to barium carbonate, while other solid degradation intermediates (e.g., benzoic acid or the phthalic acids) may be either assayed directly as solids or burned to CO2. Liquids are best assayed after burning to CO2. 

Metals are extremely important not only for chemical reactions but also for the health and welfare of plants and animals. Some examples of metals required for good nutrition, even in trace amounts, are iron, copper, cobalt, potassium, sodium, and zinc. Other metals—for example, mercury, lead, cadmium, barium, beryllium, radium, and uranium—are very toxic. Some metals at the atomic and ionic levels are crucial for the oxidation process that metabolizes carbohydrates for all living cells. 

Kiliani-Fischer synthesis is a means of lengthening the carbon backbone of a carbohydrate. The process begins with the reaction of hydrogen cyanide (HCN) with an aldehyde to produce a cyanohydrin. Treatment of the cyanohydrin with barium hydroxide followed by acidification yields an aldose with an additional carbon atom, as shown in Figure 16-16. The formation of the cyanohydrin creates a new chiral center as a racemic mixture. 

In the course of studies on the reactivity30 of fluoro carbohydrates, the 3,4-isopropylidene acetals of 2-deoxy-2-fluorotetritols were obtained. These react with the Pnrdie reagents under mild conditions, to give the corresponding O-methyl derivatives. More-vigorous reaction-conditions cause expulsion of the fluorine atom, leading to 3,4-0-isopropylidene-l,2-di-0-methyl-DL-tetritols. Mild oxidation of 2-de oxy-2-fluoro-3,4-0 -isopropyl ide ne-DL-ery thritol with barium... 

In 1815, L. G. Gilbert1 noticed that thoroughly dried calcium oxide or hydroxide does not react chemically with dry chlorine, and similarly, in 1879, J. K. Weisherg showed that the same remark applies to dry barium or strontium oxide. The case is very different if moisture he present. With barium hydroxide two gram-atoms of chlorine are absorbed per gram-atom of the bivalent barium, and barium chloride and chlorate are produced so that if barium hypochlorite is formed as the first product of the reaction, it is immediately decomposed 3Ba(0Cl)2=Ba(C103)2 +2BaCl2 with calcium and strontium hydroxides, the reaction appears to he much... 

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