Conditional methods for continuum reacting flows in porous media

A special version of conditional moment closure—PCMC—is suggested for modeling reacting flows in porous media. The model involves conditioning on a special tracer scalar, which is introduced to characterize scalar transport in the gaseous phase. (i.e., for the flow in the interparticle space or in the pores). The model accounts for interparticle variations of species concentrations and emulates diffusion in the interparticle space. Special boundary conditions that are consistent with conventional conditions at the phase interface are obtained for the PCMC model. The model is tested against complete direct simulation of a reacting flow in porous media with favourable results.     

200GeV AuAu碰撞中前快度区的微分椭圆流

Identified particle elliptic flow results are presented for the Au+Au reaction at s_NN=200GeV as a function of transverse momentum and pseudorapidity. Data at pseudorapidities η≈0,1, and 3.4 were obtained using the two BRAHMS spectrometers. Differential v₂ (η,p_t) values for a given particle type are found to be essentially constant over the covered pseudorapidity range, in contrast to the integral v₂ values which have previously been observed to decrease at forward rapidities. A softening of the particle spectra at forward angles is found to account for at least part of the integral v₂ falloff. The data are found to be consistent with existing constituent quark scaling systematics.     

The surface science of enzymes

One of the largest challenges to science in the coming years is to find the relation between enzyme structure and function. Can we predict which reactions an enzyme catalyzes from knowledge of its structure—or from its amino acid sequence? Can we use that knowledge to modify enzyme function? To solve these problems we must understand in some detail how enzymes interact with reactants from its surroundings. These interactions take place at the surface of the enzyme and the question of enzyme function can be viewed as the surface science of enzymes. In this article we discuss how to describe catalysis by enzymes, and in particular the analogies between enzyme catalyzed reactions and surface catalyzed reactions. We do this by discussing two concrete examples of reactions catalyzed both in nature (by enzymes) and in industrial reactors (by inorganic materials), and show that although analogies exist and the two kinds of catalyst can be described by similar tools, nature and human effort have come up with different solutions. This on the other hand implies that new and improved catalysts may be made by learning from nature.     

Metal‐Free Nitrile Diboration through Activation by an Electron‐Rich Diborane

Diboration of unsaturated organic compounds is an extremely useful reaction in synthetic chemistry. Herein, we report the first diboration of a nitrile by an electron‐rich diborane, mediated by an electrophilic borane. The reaction is metal‐free, and all of the reagents are readily available.     

An STM study of the localized atomic reaction of 1,2- and 1,4-dibromobenzene at Si(1 1 1)-7 × 7

A comparative study is reported of the thermal reaction of 1,2- and 1,4-dibromobenzene (1,2- and 1,4-diBrPh) on Si(1 1 1)-7 × 7, investigated by STM. Some results are given for the intermediate case of 1,3-diBrPh. The STM images gave evidence of a different pattern of reaction to yield pairs of Br-Si for 1,2-, 1,3- and 1,4-diBrPh. The ratio of pairs of Br-Si to single bromination events was 1:2 for 1,2-diBrPh and 1:3 for 1,4-diBrPh. In many cases organic residue from the bromination reaction, R-Si, was evident in the STM image. The products R-Si and Br-Si were found to be bound to adjacent Si, for both 1,2- and 1,4-diBrPh. The mean Br?Br pair separation at the surface depended on the parent molecule, being 7.6 Å for 1,2-diBrPh, 10.3 Å for 1,3-diBrPh, and 11.3 Å for 1,4-diBrPh. These separations are, in each case, about 4 Å greater than the separation of the Br-atoms in the intact parent molecule, which increases systematically down the series. There was a marked decrease in the percentage of R-Si accompanying the Br-Si in going down the series, decreasing from 70% for 1,2- to 20% for 1,4-diBrPh; this was interpreted as being due to a decrease in the percentage of `benzene-mediated' reaction dynamics, in which the benzene ring was bound to the surface. At moderately increased surface temperature (45 °C) the reaction of 1,2- and also 1,4-diBrPh no longer resulted in R-Si formation, suggesting that the dynamics had altered from benzene-mediated to `bromine-mediated'.     

Nitroxyl free radical binding to Si(1 0 0): a combined scanning tunneling microscopy and computational modeling study

The ultra-high vacuum scanning tunneling microscope (UHV-STM) was used to investigate the addition of the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical to the Si(1 0 0) surface. Room temperature studies performed on clean Si(1 0 0)-2 × 1 confirm the proposed binding of the unpaired valence electron associated with the singly occupied molecular orbital (SOMO) of the molecule with a Si dangling bond. A strong bias dependence in the topography of isolated molecules was observed in the range of −2.0 to +2.5 V. Semiempirical and density functional calculations of TEMPO bound to a three-dimer silicon cluster model yield occupied state density isosurfaces below the highest occupied (HOMO) and unoccupied state densities isosurfaces above the lowest unoccupied molecular orbital (LUMO) which trend in qualitative agreement with the bias dependent STM topography. Furthermore, the placement of TEMPO molecules on dangling bonds was controlled with atomic precision on the monohydride Si(1 0 0) surface via electron stimulated desorption of H, demonstrating the compatibility of nitroxyl free radical binding chemistries with nanopatterning techniques such as feedback controlled lithography.     

Computational study of cycloaddition reactions on the SiC(1 0 0)-c(2 × 2) surface

Cycloaddition reactions between 1,3-butadiene and the C-terminated SiC(1 0 0)-c(2 × 2) surface have been addressed using quantum-chemical methods. The c(2 × 2) structure consists of ---CC--- bridges between underlayer Si atoms which themselves form Si---Si bonds. Of various possible reaction products, the one formed by a [2 + 4] reaction with the ---CC--- bridge (giving a species resembling 1,4-cyclohexadiene) is the lowest in energy. Density functional calculations for the bare c(2 × 2) surface, using a cluster model with mechanical embedding, gave good agreement with structural parameters obtained in previous fully ab initio studies. Similar calculations for the cycloaddition product and for the transition state gave a reaction energy of −50.3 kcal/mol and an activation energy of +6.1 kcal/mol to form a planar ring structure lying normal to the surface. Detailed results for the frequency and infrared polarization behavior of adsorbate vibrational modes have also been obtained.