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Major Channel (major + channel)
Selected AbstractsTheoretical study on the Br + CH3SCH3 reactionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2007Hui Zhang Abstract The multiple-channel reactions Br + CH3SCH3 , products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained at the MP2/6-31+G(d,p) level, and energetic information is further refined by the G3(MP2) (single-point) theory. The rate constants for every reaction channels, Br + CH3SCH3 , CH3SCH2 + HBr (R1), Br + CH3SCH3 , CH3SBr + CH3 (R2), and Br + CH3SCH3 ,CH3S + CH3Br (R3), are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200,3000 K. The total rate constants are in good agreement with the available experimental data, and the two-parameter expression k(T) = 2.68 × 10,12 exp(,1235.24/T) cm3/(molecule s) over the temperature range 200,3000 K is given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smallest barrier height among three channels considered, and the other two channels to yield CH3SBr + CH3 and CH3S + CH3Br are minor channels over the whole temperature range. © 2007 Wiley Periodicals, Inc. J Comput Chem 2007 [source] On the properties of young multiple starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004E. J. Delgado-Donate ABSTRACT We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) +N -body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ,1,103 au. At the end of the hydrodynamic stage (0.5 Myr), we find that ,60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low-mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (,80 per cent) and that this fraction is an increasing function of primary mass. After N -body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low-mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour,magnitude diagram of the Praesepe cluster and from spectroscopic and high-resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4,1 M, range appears to require that stars are indeed commonly assembled into high-order multiple systems. Similarly, observational results from Taurus and , Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio (q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour,magnitude diagram and independent evidence from field binary surveys. Secondly, very low-mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations. [source] Ligand diffusion in the catalase from Proteus mirabilis: A molecular dynamics studyPROTEIN SCIENCE, Issue 10 2001Patricia Amara Abstract The role of the channels and cavities present in the catalase from Proteus mirabilis (PMC) was investigated using molecular dynamics (MD) simulations. The reactant and products of the reaction, H2O2 ,1/2 O2 + H2O, catalyzed by the enzyme were allowed to diffuse to and from the active site. Dynamic fluctuations in the structure are found necessary for the opening of the major channel, ideied in the X-ray model, which allows access to the active site. This channel is the only pathway to the active site observed during the dynamics, and both the products and reactant use it. H2O and O2 are also detected in a cavity defined by the heme and Ser196, which could play an important role during the reaction. Free energy profiles of the ligands diffusing through the major channel indicate that the barriers to ligand diffusion are less than 20 kJ mol,1 for each of the species. It is not clear from our study that minor channels play a role for access to the protein active site or to the protein surface. [source] The first structure of a cold-active catalase from Vibrio salmonicida at 1.96,Å reveals structural aspects of cold adaptationACTA CRYSTALLOGRAPHICA SECTION D, Issue 2 2007Ellen Kristin Riise The cold-adapted catalase from the fish-pathogenic bacterium Vibrio salmonicida (VSC) has recently been characterized and shown to be two times more catalytically efficient compared with catalase from the mesophilic human pathogen Proteus mirabilis [PMC; Lorentzen et al. (2006), Extremophiles, 10, 427,440]. VSC is also less temperature-stable, with a half-life of 5,min at 333,K compared with 50,min for PMC. This was the background for solving the crystal structure of the cold-adapted VSC to 1.96,Å and performing an extensive structural comparison of VSC and PMC. The comparison revealed that the entrance (the major channel) leading to the catalytically essential haem group, is locally more flexible and slightly wider in VSC. This might explain the enhanced catalytic efficiency of the nearly diffusion-controlled degradation of hydrogen peroxide into water and molecular oxygen in VSC. The reduced thermal stability of the cold-adapted VSC may be explained by a reduced number of ion-pair networks. The four C-terminal ,-helices are displaced in the structures, probably owing to missing ionic interactions in VSC compared with PMC, and this is postulated as an initiation site for unfolding the cold-adapted enzyme. VSC is the first crystal structure reported of a cold-adapted monofunctional haem-containing catalase. [source] Ab initio chemical kinetics for the NH2 + HNOx reactions, part II: Kinetics and mechanism for NH2 + HONOINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2009Shucheng Xu The kinetics and mechanism for the reaction of NH2 with HONO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The reaction producing the primary products, NH3 + NO2, takes place via precomplexes, H2N,,,c -HONO or H2N,,,t -HONO with binding energies, 5.0 or 5.9 kcal/mol, respectively. The rate constants for the major reaction channels in the temperature range of 300,3000 K are predicted by variational transition state theory or Rice,Ramsperger,Kassel,Marcus theory depending on the mechanism involved. The total rate constant can be represented by ktotal = 1.69 × 10,20 × T2.34 exp(1612/T) cm3 molecule,1 s,1 at T = 300,650 K and 8.04 × 10,22 × T3.36 exp(2303/T) cm3 molecule,1 s,1 at T = 650,3000 K. The branching ratios of the major channels are predicted: k1 + k3 producing NH3 + NO2 accounts for 1.00,0.98 in the temperature range 300,3000 K and k2 producing OH + H2NNO accounts for 0.02 at T > 2500 K. The predicted rate constant for the reverse reaction, NH3 + NO2 , NH2 + HONO represented by 8.00 × 10,26 × T4.25 exp(,11,560/T) cm3 molecule,1 s,1, is in good agreement with the experimental data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 678,688, 2009 [source] Similarities and differences in the historical development of flood management in the alluvial stretches of the Lower Mississippi Basin and the Rhine Basin,§IRRIGATION AND DRAINAGE, Issue S1 2006Dick de Bruin ingénierie hydraulique fluviale; développement historique des bassins du Rhin et du Mississippi inférieur; plaines alluviales Abstract Although the rivers Rhine and Mississippi cannot be compared as features of nature,the Mississippi River as a feature of nature is much bigger and more impressive than the Rhine,one can still observe striking similarities on flood management in both river basins, in particular in the alluvial flat reaches. But there are also some fundamental differences, not only technically but also institutionally. Since industrialization (around 1800), inland navigation became a major user on both river systems and later flood control started developing more fundamentally. Large intervention works were needed, mainly developed and based on trial and error. In both cases it has led to irreversible effects, which demand continuous attention. For the alluvial stretches in both river basins, a review is given on the most important developments in river engineering over the last two centuries. For both rivers, nautical management and flood control were held in one institutional hand at national/federal level, because both uses/sectors need the creation and regular maintenance of one similar issue: a stable and deep main channel. But the way in which in particular flood management gradually developed institutionally, as an essential part of integrated water management in the alluvial flat lower reaches of both river systems, has diverged. Discussions on financing, priorities, public disclosure, multifunctional aspects, etc. have led in both basins to lengthy procedures and complicated policy making. This paper elaborates on the historic development of fundamentals in river engineering and river management in the alluvial plains of the Rhine Basin and the Lower Mississippi Basin, more in particular focusing on the development of flood protection dikes, and on the stabilization of major channels. Copyright © 2006 John Wiley & Sons, Ltd. Bien que le Rhin et le Mississippi ne puissent pas être comparés en termes physiques,le Mississippi est beaucoup plus grand et impressionnant que le Rhin,on peut pourtant observer des similitudes saisissantes dans la protection contre les inondations des deux bassins, en particulier dans les plaines alluviales. Mais il existe également quelques différences fondamentales, non seulement techniques mais institutionnelles. Depuis les débuts de l'industrialisation (vers 1800), la navigation est devenue un usage très important sur les deux fleuves et la protection contre les inondations à commencer à se mettre en place de façon plus systématique. De grands travaux d'intervention ont été nécessaires, principalement basés sur la règle empirique de l'essai/erreur. Dans les deux cas, ceci a entraîné des effets irréversibles, qui nécessitent une surveillance permanente. Pour les plaines alluviales des deux bassins, l'article passe en revue la plupart des développements de l'ingénierie hydraulique fluviale des deux cent dernières années. Pour les deux fleuves, la gestion de la navigation et la protection contre les inondations ont été regroupées dans une même institution au niveau national/fédéral, parce que les deux usages/secteurs demandaient la création et l'entretien régulier d'une même infrastructure: un canal principal stable et profond. Mais la manière dont la protection contre les inondations s'est progressivement développée sur le plan institutionnel, comme élément essentiel de la gestion intégrée de l'eau dans les plaines alluviales des deux fleuves, s'est différenciée. Des débats sur le financement, les priorités, l'information du public, les aspects multi fonctionnels, etc., ont conduit les deux bassins à mettre en place des procédures lourdes et des prises de décision complexes. Cet article présente le développement historique des principes fondamentaux de l'hydraulique fluviale et de la gestion de fleuve dans les plaines alluviales du bassin du Rhin et du bassin inférieur du Mississippi, en mettant l'accent sur le développement des digues de protection et la stabilisation des canaux principaux. Copyright © 2006 John Wiley & Sons, Ltd. [source] THE LACUSTRINE LIANGJIALOU FAN IN THE DONGYING DEPRESSION, EASTERN CHINA: DEEP-WATER RESERVOIR SANDSTONES IN A NON-MARINE RIFT BASINJOURNAL OF PETROLEUM GEOLOGY, Issue 4 2005Jin Qiang A lacustrine fan covering an area of about 175sq. km has been identified in the Liangjialou area in the SW of the Dongying Depression, a Tertiary non-marine rift basin in eastern China. Fluvial and deltaic sandstones are established reservoir rocks in the basin, and the deep-water sandstones of the fan succession, which are assigned to Member 3 of the lower Tertiary Shahejie Formation, are also thought to have important reservoir potential. Available data for this study included some 800m of core from 16 wells, well-log data from 426 wells, and 220 sq.km of 3D surveys together with well-test and other production data. From geomorphological reconstructions of the fan, we distinguish first-order (major) fan channels from second-order branched and more distal tip channels. Crevasse splays and overbank shales occur between channels, and sandstone lobes were deposited at channel mouths. Conglomeratic sandstones deposited in major channels are probably the most promising reservoir facies (average porosity c. 20%; average permeability > 1D). Fan construction took place during a single complete cycle of lake level variation which was composed of several sub-cycles. During initial highstand conditions, the fan was dominated by small-scale branched and tip channels and intervening sandy lobes. Fan size increased rapidly during the following lowstand, and then decreased during the ensuing highstand. [source] Supersoft X-ray sources and the progenitors of Type Ia supernovaeASTRONOMISCHE NACHRICHTEN, Issue 2 2010Ph. Podsiadlowski Abstract Supersoft X-ray sources have been proposed as one of the major channels to produce Type Ia supernovae (SNe Ia). However, the true nature of the progenitors has remained an unsolved problem. In this review I summarize the present status of our understanding of SN Ia progenitors, the main classes of progenitor models and recent observational constraints. At present, neither the single-degenerate nor the double-degenerate model can be ruled out, and indeed more than one channel may be required to explain the observed SN Ia diversity. Finally, I discuss the origin of the lightcurve peak , lightcurve width relation (the ,Phillips relation') and show that it is expected to depend on metallicity; this needs to be taken into account in high-precision cosmological applications (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||