Atomic charges based on spherical harmonics expansion at the atomic centers

Molecular orbitals are expanded in spherical harmonics functions around atomic centers. The expansion coefficient is a function of the distance from the nucleus and the quotient between this function and a corresponding atomic orbital is almost constant in the core region. The square of the quotient is used as a definition of an atomic charge component. The erratic dependence on the type of basis functions in the Mulliken method is thereby avoided. The relationship between the new charge and the Mulliken population is investigated and a new invariant Mulliken population is suggested.     

A lubricated stagnation point flow of nanofluid with heat and mass transfer phenomenon: Significance to hydraulic systems

The improved thermal association of heat transfer is considerably observed due to interaction of nanoparticles in recent days. The lubrication phenomenon with heat and mass transfer effects plays a key role in the hydraulic systems. In current research, the thermal impact of nanofluid over a lubricated stretching surfaces near a stagnation point analytical has been studied. A thin layer of lubricating fluid with a variable thickness provides lubrication. The inspection of thermophoresis and Brownian motion phenomenon is illustrated via Boungrino model. The analytical finding of refurbished boundary layer ordinary differential equations is obtained by a reliable and proficient technique namely variational iteration method (VIM). The Lagrange Multiplier is a potent tool in proposed technique to reduce the computational work. In addition, a numerical comparison is presented to show the effectiveness of this study. The range of flow parameters is based on theoretical flow assumptions. Physical inspection of involved parameters on velocities, temperatures, concentrations, and other quantities of interest when lubrication is presented. The current results present applications in polymer process, manufacturing systems, heat transfer and hydraulic systems.     

Fabrication of a Simple and Cheap Screen-printed Silver/Silver Chloride (Ag/AgCl) Quasi-reference Electrode

In this work, a silver/silver chloride ink is fabricated using two steps. First the silver ink is prepare using silver, nail polish and acetone. Then the silver ink is painted in a paper substrate and a silver chloride layer is deposited using a bleach solution. The result is the silver/silver chloride conductive ink. The silver ink is cheap ($2.49/g), well-dispersive and very easy to fabricate. The materials were characterized by SEM and XRD. The Ag ink showed the formation of a continuous network throughout the silver ink film with fewer agglomeration. The effective chlorination process was also observed in the Ag/AgCl characterization. Since the Ag/AgCl substrate will be used as a quasi-reference electrode, it is important to investigate the electrical properties. The Ag ink showed an average ohmic resistance of 2.27 Ω. The addition of the AgCl layer decreases the conductivity, as expected. In summary, the Ag/Ag/Cl ink developed is simple, well-dispersed, cheap and with good conductivity. Therefore, it can be used as a conductive ink in the fabrication of quasi-reference electrodes.     

Determination of formaldehyde in frozen fish with formaldehyde dehydrogenase using a flow injection system with an incorporated gel-filtration chromatography column

A flow injection analysis (FIA) system for determination of formaldehyde in frozen fish products is described. The system provides a rapid and selective determination of formaldehyde in aqueous fish extracts by the combination of a deproteinization procedure and a stopped-flow enzymatic approach in a FIA system. The FIA system is furnished with a gel-filtration chromatography column for on-line removal of the proteins from the extract before the enzymatic analysis is performed. Compared with the standard methods for determination of formaldehyde in fish products the present method is much faster and less affected by interferences. The limit of detection for the proposed method is 2.5 mg/l of formaldehyde. The sampling frequency is about 10 determinations per hour.     

Dominant representations and a markaracter table for a group of finite order

Summary The concept of markaracter is proposed to discuss marks and characters for a group of finite order on a common basis. Thus, we consider a non-redundant set of dominant subgroups and a non-redundant set of dominant representations (SDR), where coset representations concerning cyclic subgroups are named dominant representations (DRs). The numbers of fixed points corresponding to each DR are collected to form a row vecter called a dominant markaracter (mark-character). Such dominant markaracters for the SDR are collected as a markaracter table. The markaracter table is related to a subdominant markaracter table of its subgroup so that the corresponding row of the former table is constructed from the latter. The data of the markaracter table are in turn used to construct a character table of the group, after each character is regarded as a markaracter and transformed into a multiplicity vector. The concept of orbit index is proposed to classify multiplicity vectors; thus, the orbit index of each DR is proved to be equal to one, while that corresonding to an irreducible representation is equal to zero.     

Monte Carlo simulations of a coarse grained model for an athermal all-polystyrene nanocomposite system

The structure of a polystyrene matrix filled with tightly cross-linked polystyrene nanoparticles, forming an athermal nanocomposite system, is investigated by means of a Monte Carlo sampling formalism. The polymer chains are represented as random walks and the system is described through a coarse grained Hamiltonian. This approach is related to self-consistent-field theory but does not invoke a saddle point approximation and is suitable for treating large three-dimensional systems. The local structure of the polymer matrix in the vicinity of the nanoparticles is found to be different in many ways from that of the corresponding bulk, both at the segment and the chain level. The local polymer density profile near to the particle displays a maximum and the bonds develop considerable orientation parallel to the nanoparticle surface. The depletion layer thickness is also analyzed. The chains orient with their longest dimension parallel to the surface of the particles. Their intrinsic shape, as characterized by spans and principal moments of inertia, is found to be a strong function of position relative to the interface. The dispersion of many nanoparticles in the polymeric matrix leads to extension of the chains when their size is similar to the radius of the dispersed particles.     

Measuring the electrophoretic mobility of concentrated suspensions in nonaqueous media

We present a new method of measuring the electrophoretic mobility of a particle in a concentrated suspension. The method is used to measure the electrophoretic mobility of PMMA particles (diameter 10 microm) suspended in a mixture of liquid hydrocarbons. The particle volume fraction of the suspension is varied from 0 up to 0.30 and the resulting variation of the electrophoretic mobility is discussed. The suspending liquid is such that its refractive index is very close to that of the particles. Thus the suspension is almost transparent and it is possible to follow through a microscope the motion of one particle. The suspension is subjected to a low-frequency electric field (0.5 Hz). The cell containing the suspension is mounted on a piezoelectric crystal. The displacement that compensates for the particle motion (when the particle image is steady) is determined.     

A novel study on CHEMCAD simulation of isopropyl alcohol dehydrogenation process development

Acetone is a product which is obtained via several processes. It is produced mainly by cumene hydroperoxide process. As an alternative process, acetone is obtained by isopropyl alcohol (IPA) dehydrogenation. The conversion of IPA to acetone is an endothermic gas-phase reaction which produces hydrogen as a byproduct. The process consists of an equilibrium reactor or kinetic reactor (EREA), a distillation column (SCDS), and a flash tank. As the fresh feed, liquid-phase IPA, combined with a recycle stream which contains slight amount of unseparated acetone, is vaporized and given into the equilibrium reactor. Reactor effluent is then cooled by two heat exchangers and pressurized by a compressor until unreacted IPA and formed acetone are liquidised. This stream is fed to a flash tank in order to separate the gas-phase hydrogen, which contains slight amount of acetone. The distillation column distinguishes the acetone from unreacted IPA. The distillate stream has an acetone purity over 99%. The bottoms stream, which consists of residual IPA and slight amount of acetone, is recycled back into the mixer. The purpose of this study is to improve the simulation of isopropyl alcohol (IPA) dehydrogenation process by using the ChemCad program in terms of industrial applications.     

Determination of dissolved inorganic phosphorus in natural waters at nanomolar concentrations using a long capillary cell detector

A high sensitivity spectrophotometric system using a long capillary cell (LCC) detector was developed to determine dissolved inorganic phosphorus (DIP) at nanomolar concentrations in natural waters. The colorimetric chemistry used is based on the classical molybdophosphate blue technique. The radiation source is a near-infrared light-emitting diode and a silicon phototransistor is used as a detector. Method parameters were optimized for the final LCC design. With a 600-mm LCC, a detection limit (twice the standard deviation of the blank) of 1 nmol l^?1 was obtained with a relative standard deviation of 6%. The working range of the instrument is 1–500 nmol l^?1 DIP, and a sample volume of 10 ml is required for each analysis. The technique was applied to both fresh and marine water. The instrument is compact, relatively simple and easy to use in the laboratory and the field.     

Fluorimetric kinetic method for the determination of total ascorbic acid with o-phenylenediamine

A fluorimetric reaction-rate method for the determination of L-ascorbic acid (AA) in aqueous solution is presented. The technique is based on the rapid oxidation of AA by mercury(II) chloride to dehydro-L-ascorbic acid, which then reacts with o-phenylenediamine to form a fluorescent quinoxaline. The formation of the product is monitored fluorimetrically with a data acquisition system based on a microcomputer, a voltage-to-frequency converter and a timer-counter board. The initial rate is estimated with a fixed-time computational method. With a 20-s measurement time (after a 5-s delay from initiation of the reaction), the detection limit for AA is 0.02 μg ml^?1 with a linear dynamic range extending to 10 μg ml^?. The procedure is applied to the determination of the AA in vitamin pills and juice. The relative standard deviation is 1.9% or better.     

The molecular anvil model of an enzyme taking into consideration the flexibility of enzyme molecules

The concept of a molecular anvil model of an enzyme, assuming a rigid enzyme molecule, is introduced. Two distinct features of enzymes, high catalytic power and high specificity, are reasonably and consistently explained. The dynamic nature of molecular anvil action is stressed. The origin of the high catalytic power is the spontaneous creation of a high energy state at the anvil site. The origin of the high specificity is a high sensitivity of the maximum accessible potential energy to the relatively extruded distance of the molecular anvil. The flexible model is developed by assuming a flexible enzyme molecule. It is deduced from this flexible model that enzyme activity shows a maximum with a wide range of monotonous change of the configuration of the enzyme molecule. This is the origin of the general property of enzymes that enzyme activity shows a maximum with monotonous variation of environmental parameters such as pH, temperature, pressure or some times concentration of chemical substances. The induced fit theory of Koshland is reasonably explained. The relation and differences between individual theories of enzymes are discussed. The enzymological basis of the complex regulation of biological organisms is discussed. The inversion of the sign of control of effectors is predicted when environmental parameters are varied. This concept may be useful in designing artificial enzymes or high specificity catalysts.Presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on Cyclodextrins, Lancaster, U.K., 20–25 July 1986.     

Locating conical intersections relevant to photochemical reactions

A new computerized method for locating conical intersections of interest in photochemistry is presented. The search is based on the Longuet-Higgins phase change theorem (Berry phase) which provides the subspace required for the initial search. The subspace is approximated as a plane containing three stable structures lying on a Longuet-Higgins loop. The search is conducted for a minimum of ΔE, the energy difference between two electronic states. It is started using up to three points within the circle defined by the three structures; symmetry, if relevant, is helpful but not essential. Since a two-dimensional subspace of the large 3N − 6 space is used, the search that uses either Cartesian or internal coordinates is efficient and yields a degeneracy after a few iterations. Given that not all degrees of freedom are included in the search, usually a high lying part of the conical intersection is initially located. The system is subsequently optimized along all coordinates keeping ΔE as close to zero as desired. The method is demonstrated for the symmetric H₃ system and also for the butadiene–cyclobutene–bicyclobutane system in which the three stable structures are not equivalent. The method is general and can be extended to any photochemical system.     

Synthesis of 7-deoxypancratistatin from carbohydrates by the use of olefin metathesis

The stereocontrolled synthesis of (+)-7-deoxypancratistatin is described. The convergent synthesis has been achieved by two different strategies, both of which commence from a pentose and piperonal. The latter is converted into allylic bromide 7, which is then coupled with a protected methyl 5-deoxy-5-iodo-D-ribofuranoside in the presence of zinc metal. The first strategy involves a total of only 13 steps from D-ribose and piperonal, but suffers from a low yield in the zinc-mediated reaction between ribofuranoside 9, benzylamine, and bromide 7. The second strategy involves a total of 18 steps from D-xylose and piperonal. The former is converted into ribofuranoside 28, which is coupled with bromide 7 in the presence of zinc, and this is followed by ring-closing olefin metathesis. Subsequent Overman rearrangement, dihydroxylation, and deprotection then affords the natural product.     

A NOVEL INTERPRETATION OF CONCENTRATION DEPENDENCE OF VISCOSITY OF DILUTE POLYMER SOLUTION*

The concentration dependence of the reduced viscosity of dilute polymer solution is interpreted in the light of anew concept of the self-association of polymer chains in dilute solution. The apparent self-association constant is defined asthe molar association constant divided by the molar mass of individual polymer chain and is numerically interconvertiblewith the Huggins coefficient. The molar association constant is directly proportional to the effective hydrodynamic volume ofthe polymer chain in solution and is irrespective of the chain architecture. The effective hydrodynamic volume accounts forthe non-spherical conformation of a short polymer chain in solution and is a product of a shape factor and hydrodynamicvolume. The observed enhancement of Huggins coefficient for short chain and branched polymer is satisfactorily interpretedby the concept of self-association. The concept of self-association allows us to predict the existence of a boundaryconcentration C_s (dynamic contact concentration) which divides the dilute polymer solution into two regions.     

Modeling the Electrochemical Response of Mesoporous Materials Toward Its Application to Biomolecular Detection

The performance of an electrochemical sensor based on the ability of a probe to cross a mesoporous membrane partially blocked by an analyte is predicted using a numerical model. The system comprehends a membrane placed close to the working electrode and the signal is generated by applying square wave voltammetry. The digital simulation allows comparing the responses for different situations regarding the way in which the membrane is blocked by the sample. The developed model is compared with experimental results. The effect of the sizes of the pore, analyte and probe on the system response is evaluated.     

新型二氧化硫气体电化学传感器的研究

SO2 是大气的主要污染物 ,并能与环境中的 NOx,O3等协同作用 ,产生更严重的污染 .因此 ,SO2的测定在环境保护、职业健康、工业排放控制等方面起着非常重要的作用 .SO2 的测定方法包括气相色谱法 [1]、离子色谱法 [2 ]、电导测定 [3]和电解电导率 [4]、光纤传感器 [5 ,6 ]等 ,但这些方法所需仪器昂贵、操作繁琐、灵敏度较低、难以实时连续测定 .一些研究者致力于 SO2 电化学传感器的开发 [7～ 8] ,但存在着灵敏度低、需在高温下使用、稳定性差等缺点 .我们成功地研制了一种微型 SO2 气体电化学传感器 ,该传感器稳定性好、灵敏度高、价…     

导数-固体基质室温燐光法同时测定痕量对位三联苯和间位三联苯

本文提出间三联苯和对三联苯的二阶导数-固体基质室温燐光法(d²SS-RTP)。本法在λ_ax=288nm,用448nm处正峰和460nm处负峰的峰峰高度值定量测定间三联苯,对三联苯不干扰测定.线性范围0.46～46ng.检出限为0.1ng/斑点.对三联苯用526nm处正峰和548nm处负峰的峰峰高度值测定.间三联苯不干扰.线性范围0.46～46ng.检出限0.07ng/斑点.     

Startup of electrophoresis in a suspension of colloidal spheres

The transient electrophoretic response of a homogeneous suspension of spherical particles to the step application of an electric field is analyzed. The electric double layer encompassing each particle is assumed to be thin but finite, and the effect of dynamic electroosmosis within it is incorporated. The momentum equation for the fluid outside the double layers is solved through the use of a unit cell model. Closed‐form formulas for the time‐evolving electrophoretic and settling velocities of the particles in the Laplace transform are obtained in terms of the electrokinetic radius, relative mass density, and volume fraction of the particles. The time scale for the development of electrophoresis and sedimentation is significantly smaller for a suspension with a higher particle volume fraction or a smaller particle‐to‐fluid density ratio, and the electrophoretic mobility at any instant increases with an increase in the electrokinetic particle radius. The transient electrophoretic mobility is a decreasing function of the particle volume fraction if the particle‐to‐fluid density ratio is relatively small, but it may increase with an increase in the particle volume fraction if this density ratio is relatively large. The particle interaction effect in a suspension on the transient electrophoresis is much weaker than that on the transient sedimentation of the particles.     

淀粉系高吸水性树脂反应中间体稳定性及反应活性的量子化学研究

运用abinitioUHF方法和DFTUB3LYP方法在6-31G基组水平下对淀粉系高吸水性树脂反应中间体的模型体系进行几何构型优化.针对每类自由基对应得到的两种典型的构象异构体(1_a,1_b和2_a,2_b),进行稳定性和反应活性分析.构型1_a和2_b的总能量相对较低.比较4个模型体系的前线分子轨道,构型1_a的能隙较小,同时HOMO能量较高,易给出电子,LUMO能量较低,易得电子,有利于电子转移,因而有较好的反应活性.自由基自旋密度主要集聚在C2或C3上,未有较大弥散,同时也是前线MO主要布居位置和重要的反应活性部位.对构型1_a与丙烯酸模型分子形成的复合物分析表明,两者的反应未经过过渡态,在瞬间即可完成.     

Principles of rapid polymerase chain reactions: mathematical modeling and experimental verification

Polymerase chain reaction (PCR) is an important diagnostic tool for the amplification of DNA. The PCR process can be treated as a problem in biochemical engineering. This study focuses on the development of a mathematical model of the polymerase chain reaction. The PCR process consists of three steps: denaturation of target DNA, annealing of sequence-specific oligonucleotide primers and the enzyme-catalyzed elongation of the annealed complex (primer:DNA:polymerase). The denaturation step separates the double strands of DNA; this model assumes denaturation is complete. The annealing step describes the formation of a primer-fragment complex followed by the attachment of the polymerase to form a ternary complex. This step is complicated by competitive annealing between primers and incomplete fragments including primer-primer reactions. The elongation step is modeled by a stochastic method. Species that compete during the elongation step are deoxynucleotide triphosphates dCTP, dATP, dTTP, dGTP, dUTP, and pyrophosphate. Thermal deamination of dCTP to form dUTP is included in the model. The probability for a species to arrive at the active site is based on its molar fraction. The number of random insertion events depends on the average processing speed of the polymerase and the elongation time of the simulation. The numerical stochastic experiment is repeated a sufficient number of times to construct a probability density distribution (PDF). The moment of the PDF and the annealing step products provide the product distribution at the end of the elongation step. The overall yield is compared to six experimental values of the yield. In all cases the comparisons are very good.