998 resultados para Bond cycles


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The debate over the possible extension of transparency regulation in Europe to include sovereign bonds has opened up a number of other issues in need of serious consideration. One such issue is the appropriateness of the entire infrastructure supporting the trading of European sovereign bonds. In recent years sovereign issuers have supported the development of an electronic inter-dealer market but have remained unconcerned with the opacity of dealer-to-customer trading. The degree of segmentation in this market is high relative to what exists in nearly all other financial markets. This paper explores why European sovereign bond markets have developed in such a segmented way and considers how this structure could be altered to improve transparency without adversely affecting liquidity, efficiency or the benefits enjoyed by primary dealers and issuers. It is suggested that the structure of the market could be improved greatly if the largest and most active investors were permitted access to the inter-dealer electronic trading platforms. This would solve a number of market imperfections and increase the proportion of market activity that is conducted in a transparent way. The paper argues that sovereign issuers in Europe have the means to provide incentives that would influence dealers to support reduced segmentation. Some practical examples of how this could be achieved are provided and the potential benefits are outlined.

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Unusual TFA catalyzed cleavage reaction is reported for peptide containing pipecolic acid residues. Although the use of TFA under standard cleavage conditions is sufficiently mild to prevent degradation of the desired products. the amide bond between consecutive pipecolic acid residues is unexpectedly hydrolyzed by standard TFA treatment. The hydrolysis is proposed to proceed via an oxazolinium ion intermediate, This mechanism is supported by H/D exchange as observed by ESI-MS and NMR experiments.

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Phosphonates are organophosphorus molecules that contain the highly stable C-P bond, rather than the more common, and more labile, C-O-P phosphate ester bond. They have ancient origins but their biosynthesis is widespread among more primitive organisms and their importance in the contemporary biosphere is increasingly recognized; for example phosphonate-P is believed to play a particularly significant role in the productivity of the oceans. The microbial degradation of phosphonates was originally thought to occur only under conditions of phosphate limitation, mediated exclusively by the poorly characterized C-P lyase multienzyme system, under Pho regulon control. However, more recent studies have demonstrated the Pho-independent mineralization by environmental bacteria of three of the most widely distributed biogenic phosphonates: 2-aminoethylphosphonic acid (ciliatine), phosphonoacetic acid, and 2-amino-3-phosphonopropionic acid (phosphonoalanine). The three phosphonohydrolases responsible have unique specificities and are members of separate enzyme superfamilies; their expression is regulated by distinct members of the LysR family of bacterial transcriptional regulators, for each of which the phosphonate substrate of the respective degradative operon serves as coinducer. Previously no organophosphorus compound was known to induce the enzymes required for its own degradation. Whole-genome and metagenome sequence analysis indicates that the genes encoding these newly described C-P hydrolases are distributed widely among prokaryotes. As they are able to function under conditions in which C-P lyases are inactive, the three enzymes may play a hitherto-unrecognized role in phosphonate breakdown in the environment and hence make a significant contribution to global biogeochemical P-cycling.

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The C-H activation on metal oxides is a fundamental process in chemistry. In this paper, we report a density functional theory study on the process of the C-H activation of CH4 on Pd(111), Pt(111), Ru(0001), Tc(0001), Cu(111), PdO(001), PdO(110), and PdO(100). A linear relationship between the C-H activation barrier and the chemisorption in the dissociation final state on the metal surfaces is obtained, which is consistent with the work in the literature. However, the relationship is poor on the metal oxide surfaces. Instead, a strong linear correlation between the barrier and the lattice O-H bond strength is found on the oxides. The new linear relationship is analyzed and the physical origin is identified. (c) 2008 American Institute of Physics.

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Transition metal catalyzed bond formation is a fundamental process in catalysis and is of general interest throughout chemistry. To date, however, the knowledge of association reactions is rather limited, relative to what is known about dissociative processes. For example, surprisingly little is known about how the bond-forming ability of a metal, in general, varies across the Periodic Table. In particular, the effect of reactant valency on such trends is poorly understood. Herein, the authors examine these key issues by using density functional theory calculations to study CO and CN formations over the 4d metals. The calculations reveal that the chemistries differ in a fundamental way. In the case of CO formation, the reaction enthalpies span a much greater range than those of CN formation. Moreover, CO formation is found to be kinetically sensitive to the metal; here the reaction barriers (E-a) are found to be influenced by the reaction enthalpy. CN formation, conversely, is found to be relatively kinetically insensitive to the metal, and there is no correlation found between the reaction barriers and the reaction enthalpy. Analysis has shown that at the final adsorbed state, the interaction between N and the surface is relatively greater than that of O. Furthermore, in comparison with O, relatively less bonding between the surface and N is observed to be lost during transition state formation. These greater interactions between N and the surface, which can be related to the larger valency of N, are found to be responsible for the relatively smaller enthalpy range and limited variation in E-a for CN formation. (C) 2007 American Institute of Physics.