Drag on two nonuniformly structured flocs moving along the axis of a cylindrical tube

The boundary effect on the drag on two identical, nonuniformly structured flocs moving along the axis of a cylindrical tube filled with a Newtonian fluid is investigated at a small to medium larger Reynolds number. A two-layer model is adopted to simulate various possible structures of a floc, and the flow field inside is described by Darcy–Brinkman model. The results of numerical simulation reveal that a convective flow is present in the rear region of a floc when Reynolds number is on the order of 40. The presence of the tube wall and/or the porous structure of a floc has the effect of reducing that convective flow. For a fixed level of the volume-average permeability of a floc, the influence of the tube wall on the drag depends upon floc structure; the influence on a nonuniformly structured floc is more significant than that on a uniformly structured floc. The more nonuniform the floc structure, the more appreciable the deviation of the drag coefficient–Reynolds number curve from a Stokes’-law-like relation becomes. The smaller the volume-average permeability of a floc and/or the smaller the separation distance between the two flocs, the greater is the deviation, but the presence of the tube wall has the effect of reducing that deviation.     

The Many Measures of Molecular Dimensions

In contrast to the high precision with which masses of atoms and molecules can be specified, their external dimensions remain intrinsically fuzzy. Attention is directed to the difference between measurements of interatomic separations within molecules, that in many instances are cited with error bars of +/– 0.002 Å, and the available approximate magnitudes of over-all sizes, needed for estimating packing densities (in solids or liquids). The following account, is a critical examination of the underlying concepts and of the diversity of data in the literature, regarding internal dimensions (bond lengths), overall extensions, cross-sectional areas, and volumes of molecules that may be derived from a variety of experiments. Historical markers of the evolution of the underlying theories and experiments are included.     

MM3 parametrization of four‐ and five‐coordinated rhenium complexes by a genetic algorithm—Which factors influence the optimization performance?

Genetic algorithms (GA) were used to solve one of the multidimensional problems in computational chemistry, the optimization of force field parameters. The correlation between the composition of the GA, its parameters (p(c), p(m)) and the quality of the results were investigated. The composition was studied for all combinations of a Simple GA/Steady State GA with a Roulette Wheel/Tournament Selector using different values each for crossover (0.5, 0.7, 0.9) and mutation rates (0.01, 0.02, 0.05, 0.10, 0.20). The results show that the performance is strongly dependent on the GA scheme, where the Simple GA/Tournament Selector yields the best results. Two new MM3 parameters were introduced for rhenium compounds with coordination number four (204) and coordination number five (205), the formal oxidation states of rhenium ranging from +V to +VII. A manifold of parameters (Re-C, N, O, S) was obtained by using a diverse set of CSD structures. The advantage of the GA vs. UFF calculations is shown by comparison of several examples. The GA optimized parameters were able to reproduce the geometrical data of the X-ray structures.     

用原子捕获技术提高火焰原子吸收光谱法的灵敏度

采用简易的石英原子捕获器，使Ｃｕ，Ａｇ，Ａｕ，Ｚｎ，Ｃｄ，Ｐｂ，Ｓｂ，Ｂｉ等易挥发元素在空气－乙炔火焰原子吸收光谱法中的灵敏度提高了３～５倍，并有较好的精密度，相对标准偏差在１％左右，方法中用此技术测定了标准物质中Ｐｂ、Ｃｄ的含量，获得了满意的结果，它较之石墨炉原子吸收光地具有快速，简便的特点，具有较大的实用价值。     

硼烷缺电子多中心桥键的H-F力研究

Hellmann-Feynman静电力表现了分子中电荷分布及对各原子核的作用。H-F力的方向性可描述电子电荷的数量和位置,具有定量、形象和直观的特点,已在弯键的研究中显示出来。这一方法尚可用于多中心缺电子桥键的研究。 1 理论与方法 按LCAO-MO理论对H-F力进行分解,除有重叠力、极化力和屏蔽力外,还有一     

A method to construct polyelectrolyte multilayers film containing gold nanoparticles

Gold nanoparticles in polyelectrolyte multilayers film can be easily prepared by repeating immersion of a substrate in poly(diallyl dimethylammonium) chloride (PDDA)-AuCl₄⁻ complexes solution followed by reduction Au³⁺ through heating. UV-vis spectroscopy, cyclic voltammetry (CV) and tapping-mode atomic force microscopy (AFM) are used to confirm the successful construction of the polyelectrolyte multilayers film and the formation of gold nanoparticles. The multilayers film shows electrocatalytic activity to dioxygen reduction.     

Comparison of linear-scaling semiempirical methods and combined quantum mechanical/molecular mechanical methods for enzymic reactions. II. An energy decomposition analysis

QM/MM methods have been developed as a computationally feasible solution to QM simulation of chemical processes, such as enzyme-catalyzed reactions, within a more approximate MM representation of the condensed-phase environment. However, there has been no independent method for checking the quality of this representation, especially for highly nonisotropic protein environments such as those surrounding enzyme active sites. Hence, the validity of QM/MM methods is largely untested. Here we use the possibility of performing all-QM calculations at the semiempirical PM3 level with a linear-scaling method (MOZYME) to assess the performance of a QM/MM method (PM3/AMBER94 force field). Using two model pathways for the hydride-ion transfer reaction of the enzyme dihydrofolate reductase studied previously (Titmuss et al., Chem Phys Lett 2000, 320, 169-176), we have analyzed the reaction energy contributions (QM, QM/MM, and MM) from the QM/MM results and compared them with analogous-region components calculated via an energy partitioning scheme implemented into MOZYME. This analysis further divided the MOZYME components into Coulomb, resonance and exchange energy terms. For the model in which the MM coordinates are kept fixed during the reaction, we find that the MOZYME and QM/MM total energy profiles agree very well, but that there are significant differences in the energy components. Most significantly there is a large change (approximately 16 kcal/mol) in the MOZYME MM component due to polarization of the MM region surrounding the active site, and which arises mostly from MM atoms close to (<10 A) the active-site QM region, which is not modelled explicitly by our QM/MM method. However, for the model where the MM coordinates are allowed to vary during the reaction, we find large differences in the MOZYME and QM/MM total energy profiles, with a discrepancy of 52 kcal/mol between the relative reaction (product-reactant) energies. This is largely due to a difference in the MM energies of 58 kcal/mol, of which we can attribute approximately 40 kcal/mol to geometry effects in the MM region and the remainder, as before, to MM region polarization. Contrary to the fixed-geometry model, there is no correlation of the MM energy changes with distance from the QM region, nor are they contributed by only a few residues. Overall, the results suggest that merely extending the size of the QM region in the QM/MM calculation is not a universal solution to the MOZYME- and QM/MM-method differences. They also suggest that attaching physical significance to MOZYME Coulomb, resonance and exchange components is problematic. Although we conclude that it would be possible to reparameterize the QM/MM force field to reproduce MOZYME energies, a better way to account for both the effects of the protein environment and known deficiencies in semiempirical methods would be to parameterize the force field based on data from DFT or ab initio QM linear-scaling calculations. Such a force field could be used efficiently in MD simulations to calculate free energies.     

Single polymer molecules adsorbed to mica and the oppositely charged polymer/surfactant complexes formed at the air–water interface visualized by atomic force microscopy

In the present study, the structure and morphology of single sodium poly(styrenesulfonate) (PSS) molecules adsorbed to mica surface from the natural aqueous solution is investigated using atomic force microscopy technique. Results show that single PSS molecules are observed which show a morphology of wormlike coils. Meanwhile, single sodium poly(styrenesulfonate) (PSS)/Hexadecyltrimethylammonium bromide (CTAB) complexes deposited on mica from the air–water interface are also observed. However, the PSS/CTA⁺ complexes show different conformations by appearing in the morphology of circular patches. Experimental data are in fair agreement with the theoretical analysis.     

用选择离子流动管质谱测定汽油和柴油蒸汽成分

采用选择离子流动管质谱(SIFT／MS)装置，以H30^ 、N0^ 为初始离子对汽油和柴油蒸汽进行了研究，质谱分析表明，汽油和柴油主要由C—H化合物组成，包括烷烃、环烷烃、烯烃、炔烃、二烯烃以及芳香烃。在这几种成分中，烷烃都占有最大的比例；汽油蒸汽和柴油蒸汽最大的区别是柴油蒸汽中长链大分子的挥发性C—H化合物所占的比例远远高于汽油蒸汽。文中还给出了以H30^ 、N0^ 为初始离子所得到的汽油蒸汽的质谱图，以及汽油、柴油蒸汽中各种成分的定量分析结果。     

A quantitative method for dual‐pass electrostatic force microscopy phase measurements

Electrostatic force microscopy (EFM) has become a powerful tool for investigating charges on surfaces. The use of phase measurement in EFM is a direct and fast way to detect electrostatic force gradients, but only qualitatively. With the dual‐pass scheme, the phase signal at lifted height is often assumed to exclude any influences from the topography, but it does not. We report the collection of both topography and phase data by EFM on charged, micron‐sized metal wires. In order to quantify the electrostatic force, a cone model and finite element analysis are provided to integrate the force gradient from the phase signal. Copyright © 2007 John Wiley & Sons, Ltd.