Dispersion-corrected energy barriers for silylene addition to white phosphorus, a density functional investigation into substituent effects

A density functional investigation into differently substituted silylenes with respect to the first step in the addition to white phosphorus is presented. The investigations include dispersion corrections in the density functional treatment. They become sizable for the transition state geometries for the silylenes as they become increasingly substituted by bulky groups. Hence, dispersion corrections are essential for a quantum chemical treatment of real molecules using density functional theory. The different singlet–triplet energy separations of differently substituted silylenes were also investigated and compared with calculated activation barriers for the first step in the addition reaction.     

Sample dispersion with chemical reaction in a flow-injection system

Computer simulations of signals based on a dispersion equation involving diffusion, convection and chemical reaction terms are reported for systems with and without chemica reaction. 2-(2-Thiazolylazo)-4-methyl-5-(sulfomethylamino)benzoic acid (TAMSMB) is used as the reagent because it reacts very quickly with copper(II) and much more slowly with nickel(II). Comparison of experimental signal profiles with the simulated ones enables sample dispersion from the reaction rates to be elucidated. Concetration profiles of the reaction product in a straight, narrow tube were also stimulated; they explain satisfactorily the signal profiles obtained experimentally.     

TATB二聚体分子间作用力及其气相几何构型研究

采用对称性匹配微扰理论(SAPT)定量地求得TATB分子间的静电、交换排斥、诱导和色散等分子间作用能项, 从理论上揭示了TATB分子间作用本质; 在此基础上, 阐明了密度泛函在研究TATB二聚体时的适合性问题. 结果表明: (1)在有分子间氢键的TATB二聚体中, 库仑力足以与交换排斥力相抗衡, 起主导作用. (2)含分子间氢键的气相TATB二聚体的合理几何构型为平面型结构, 此结构的产生与色散力无关, 因此不管泛函是否含有近程色散作用, 均应预测到这种强极性的平面型结构. (3)在无分子间氢键的TATB二聚体中, 库仑力难以与交换排斥力相抗衡, 色散作用起到了关键作用; (4)在这种情况下, 未含有近程色散作用的密度泛函不可能给出合理构型. 恰好相反, 含有近程色散作用的密度泛函PBE0却能正确地预测到具有“平行重叠”结构且呈微弱极性的TATB二聚体, 色散力是导致这种构型产生的根本原因. “平行重叠”TATB二聚体是典型的色散体系, 其色散力占绝对主导地位并极有可能起源于两个TATB分子上π电子的相互作用. (5)对于所有TATB二聚体, 色散力或很显著或起主导作用. 由于密度泛函或未含有近程色散, 或只能部分地把近程色散表达出来, 这样使得当前所有密度泛函不可能精确求得这些二聚体的作用能.     

Migration study of iodine in Beishan granite by a column method

The fate and migration behavior of radionuclides in environment are influenced by a series of physical and chemical processes such as advection, hydrodynamic dispersion (including mechanical dispersion and molecular diffusion), retention, chemical reaction and so on. In this study, the migration of ¹²⁵I^? in Beishan granite and the potential retention of iodine by silver halide additives were investigated by a pulse injection column method. All breakthrough curves were analyzed according to the analytical solution of transport equation and the dispersion coefficient (D), and first-order sorption constant (k) were obtained. For conservative nuclide, the dispersion behavior is only related to the dispersion medium. Silver halides were proved having sorption ability for ¹²⁵I^? in the order of AgCl > AgBr > AgI. The transport of iodine in the crushed granite column can be adequately described by the advection–dispersion equation with a first-order, irreversible sorption term. The pulse injection column method can be used as a fast method to evaluate the sorption or retention ability of solid phase.     

Kinetics in continuous flow sample processing Chemical Contributions to Dispersion in Flow-Injection Techniques

A numerical model based on a basic dispersion equation modified by simultaneous consideration of the chemical effect introduced by a chemical reaction taking place as the sample plug disperses has been tested. Computer analysis and simulation (by use of an alternating direction implicit finite difference approximation) by solving the laminar dispersion equation and the rate expression simultaneously, have been evaluated with experimental studies of a reaction of moderate speed (oxidation of ascorbic acid by dichromate ion). Theoretically generated signal profiles indicated the need for three correction factors representative of the deviation of the experimental system from ideality. The rate coefficient in the chemical kinetic term (if pseudo-first-order is assumed) appears to change in an oscillating manner reflecting the complexity of the overall kinetic process. A straight-line relationship was found between the time at peak maximum and the rate coefficient in computer-simulated curves. Such a correlation provided a way to estimate a value for the rate coefficient of the L-ascorbic acid/dichromate reaction which agreed with the one obtained from batch kinetic determinations. In some of the cases studied, the unbalanced evolution of dispersion forces resulted in chemically induced doubled-humped peaks.     

Rheological Modeling of Dispersion System of Nano/Microsized Polymer Particles Considering Swelling Behavior

Rheological behavior of dispersion system containing nano/microsized cross-linked polymer particle was studied considering particle hydration and swelling. Viscosity of the dispersion system depends on swelling kinetics of polymer particles. Under shear flow, dispersion of swollen polymer particles is shear thinning. According to experimental results, kinetics of particle swelling and hydration was described well by second-order kinetic equation. Relational expression between equilibrium particle size and influencing factors of swelling such as salt concentration and temperature was presented. Assume that swollen polymer particles are uniform and have a simple core-shell structure, interacting through a repulsive steric potential. The rheological modeling of such dispersion system at low shear rate was presented using the concept of effective volume fraction, which depends on swelling kinetics and interparticle potential. Cross model was introduced to describe shear-thinning behavior. The viscosity equation allows correlation of experimental data of relative viscosity versus shear rate or hydration time; accounting for effect of temperature and salt concentration on viscosity. Predictions of the model have a good agreement with experimental results.     

An Equation for Viscosity and Isothermal Compressibility of Simple Liquids from a Closed-Form Expression for the Effective Viscosity of a Dispersed System

Abstract

Viscosity and isothermal compressibility of a simple liquid have been related by means of a closed-form equation for the low-shear viscosity of a liquid dispersion of solid particles. It involves macroscopic density, total correlation function and free volume. The equation achieved, which does not have an explicit solution, was approximated and applied to two sets of experimental data, where the free volume contribution was evaluated on the basis of experimental values of either volumes at zero fluidity or force constants for second virial coefficients and pair interaction potentials.     

Peak tailing in electrophoresis due to alteration of the wall charge by adsorbed analytes a: Numerical simulations and asymptotic theory

When analytes containing cationic components, such as proteins, are separated in fused silica capillaries or micro-chips, they adsorb strongly to the negatively charged channel walls. Broadened and highly asymmetric peaks in the detector signal is symptomatic of the presence of such wall interactions. Band broadening is caused by the introduction of shear into the electroosmotic flow which leads to Taylor dispersion. The shearing flow in turn is caused by axial variations in zeta-potential due to adsorbed analytes. In this paper, numerical solutions of the coupled electro-hydrodynamic equations for fluid flow and the advection-diffusion equation for analyte concentration are presented in the limit of thin Debye layers. The simulations reproduce many of the qualitative effects of wall adsorption familiar from observation. Further, the simulation results are compared, and found to agree very well (to within a percent for characteristic values of the parameters) with a recently developed asymptotic theory.     

Mechanism of initiation in the γ-radiation-induced polymerization of ethylene

The relation between the gaseous products and the reaction conditions such as pressure, temperature, and dose rate in the γ-radiation-induced polymerization of ethylene was studied. The main gaseous products were hydrogen and acetylene, and the amounts of these products increased linearly with reaction time, monomer density, and dose rate, while they were independent of reaction temperature. The ratio of rate of formation of hydrogen to acetylene was about one-half. Further, it was found that the number of moles of polymer chain formed was almost equal to that of acetylene at room temperature. An initiation mechanism in which both hydrogen and acetylene are formed is proposed. The equation which is derived on the basis of the initiation mechanism is shown to be in good accordance with the experimental results.     

Assessment of TD-DFT- and TD-HF-based approaches for the prediction of exciton coupling parameters, potential energy curves, and electronic characters of electronically excited aggregates

The reliability of linear response approaches such as time-dependent Hartree-Fock (TD-HF) and time-dependent density functional theory (TD-DFT) for the prediction of the excited state properties of 3,4;9,10-tetracarboxylic-perylene-bisimide (PBI) aggregates is investigated. A dimer model of PBI is investigated as a function of a torsional motion of the monomers, which was shown before to be an important intermolecular coordinate in these aggregates. The potential energy curves of the ground state and the two energetically lowest neutral excited and charge-transfer (CT) states were obtained with the spin-component scaling modification of the approximate coupled-cluster singles-and-doubles (SCS-CC2) method as a benchmark for dispersion corrected TD-HF and a range of TD-DFT approaches. The highly accurate SCS-CC2 results are used to assess the other, computationally less demanding methods. TD-HF predicts similar potential energy curves and transition dipole moments as SCS-CC2, as well as the correct order of neutral and CT states. This supports an exciton trapping mechanism, which was found on the basis of TD-HF data. However, the investigated TD-DFT methods provide generally the opposite character for the excited states. As a consequence, these TD-DFT results have unacceptably large errors for optical properties of these dye aggregates.     

Role of Joule heating in dispersive mixing effects in electrophoretic cells: convective-diffusive transport aspects

This contribution addresses the problem of solute dispersion in a free convection electrophoretic cell for the batch mode of operation, caused by the Joule heating generation. The problem is analyzed by using the two-problem approach originally proposed by Bosse and Arce (Electrophoresis 2000, 21, 1018-1025). The approach identifies the carrier fluid problem and the solute problem. This contribution is focused on the latter. The strategy uses a sequential coupling between the energy, momentum and mass conservation equations and, based on geometrical and physical assumptions for the system, leads to the derivation of analytical temperature and velocity profiles inside the cell. These results are subsequently used in the derivation of the effective dispersion coefficient for the cell by using the method of area averaging. The result shows the first design equation that relates the Joule heating effect directly to the solute dispersion in the cell. Some illustrative results are presented and discussed and their implication to the operation and design of the device is addressed. Due to the assumptions made, the equation may be viewed as an upper boundary for applications such as free flow electrophoresis.     

Consistent theoretical description of 1,3-dipolar cycloaddition reactions

The cycloaddition reactions of 18 1,3-dipolar molecules to ethylene and acetylene have been reinvestigated by quantum chemical methods that are based on a second-order perturbation treatment of electron correlation. It is found that SCS-MP2 and the new perturbative B2-PLYP density functional provide accurate reaction barriers and outperform MP2 as well as standard density functionals such as B3-LYP. The new second-order based methods have the additional advantage that they perform better with increasing quality of the one-particle space, as is desired for a good quantum chemical method. The errors for the reaction enthalpies are in general larger than for the barriers when compared to CBS-QB3 literature values, which is related to strong changes in the electronic structures, but the deviations are again smaller than with MP2 or B3-LYP and are also more systematic. The results of a detailed basis set study suggest that properly polarized triple-zeta AO basis sets represent a good compromise between accuracy and computational speed. The combination of very inaccurate density functionals with small (double-zeta) basis sets, which yields good results for the initial part of the reactions due to error compensation, is not recommended.     

Analysis of binary adsorption of polar and nonpolar molecules in narrow slit-pores by mean-field perturbation theory

In the present research study, we present the development of a model for predicting the adsorption of binary mixtures of nonpolar molecules, as well as polar molecules, based on density functional theory with mean-field approximation in narrow slit-pores. The first system under consideration is comprised of a binary mixture of nonpolar molecules, modeled by considering intermolecular dispersion forces, whereas the second system comprised of a binary mixture of polar molecules is modeled by considering orientation averaged electrostatic interactions, namely dipole-dipole and dipole-induced dipole interactions as well as dispersion interactions. An explicit equation for the Helmholtz free energy of the pore phase binary fluid mixture is derived. The proposed model is used to simulate the selective sorption of ethane from an ethane-methane mixture and water from a methanol-water mixture in the slit-pore. The simulated results are interpreted by studying the relative contributions of fluid-wall and fluid-fluid interactions. Finally, simulation results obtained are compared with the results of existing models and molecular simulations in the literature.     

Viscosity-change-induced density fingering in polyelectrolytes

We have studied the density fingering of an acid-catalyzed autocatalytic reaction in the presence of carboxylate containing polyelectrolyte. The decrease in viscosity as a result of the change in the ionic character of the polymer due to the pH-change during the reaction is the major driving force for the spatiotemporal pattern formation. The front evolution is quantitatively characterized by dispersion curves.