The Danishefsky pyranone puzzle: an explanation based on the exterior frontier orbital extension model

The unusual facial stereoselection in the hydride reduction of the Danishefsky pyranones (2,3,5,6-tetrahydro-4-pyranones) with L-Selectride (Li-sec-Bu₃BH) has been explained based on the exterior frontier orbital extension model (the EFOE model).     

Sixfold bond orientational properties of a model liquid crystal in the dimensional crossover of B phases: a computer simulation study

A wide range of NPT simulations of a bead necklace liquid crystal model in the crystal B, smectic B, smectic A, and nematic phases have been performed. Systems with up to 21 600 molecules have been studied to observe the behavior of slowly decaying spatial correlation functions. The pair correlation function and its in-plane restriction are consistent with a crystalline phase made of independent two-dimensional crystalline layers. Smectic B phase is studied by the bond orientational pair correlation functions g(6) and its extension g(6ext). The first reaches a constant value, which seems to rule out a classical hexatic phase. The latter shows a power-law decay within the layers: its typical decay exponent (eta(6ext)) is evaluated. Relationships between multiple harmonics of the C(6n) order parameter have been evaluated through the whole range of existence of B phases (crystalline and smectic): the extension to the crystalline phase holds and provides an excellent fit of the simulation data.     

Vapor Pressures,Osmotic and Activity Coefficients of Electrolytes in Protic Solvents at Different Temperatures. 3. Lithium Bromide in 2-Propanol

Precise vapor pressure data for LiBr solutions in 2-propanol are given for temperatures ranging from (298.15 to 348.15) K. The molality range was from (0.0743 to 1.4825) mol-kg^–1. Osmotic coefficients were calculated by taking into account the second virial coefficient of 2-propanol. The parameters of the Archer extension of the Pitzer model, MSA-NRTL model, and chemical model of Barthel are evaluated. The parameters of the Archer extension of the Pitzer model were used to calculate the activity coefficients of LiBr in 2-propanol solutions.     

MDI与TDI封端聚酯预聚物和扩链剂间反应速率的研究

用ＦＴＩＲ法系统地研究了不同硬段结构对聚酯型聚氨酯扩链反应动力学的影响，结果显示：不同类二异氰酸酯ＭＤＩ和ＴＤＩ形成的ＮＣＯ封端的聚酯预聚体与二元胺（ＭＯＣＡ）和三元醇（ＴＭＰ）的扩链反应均为二级反应，由ＭＤＩ形成ＮＣＯ封端的聚酯预聚体与胺或醇的扩链反应速度比ＴＤＩ类的要快，ＭＯＣＡ扩链比ＴＭＰ扩链反应活化能低，从基团反应活性不同体系扩链反速度的差别进行了讨论。     

Thermodynamic Properties of Multicomponent Liquid Mixtures

Considering a ternary liquid mixture to be made up of three binary mixtures, by means of cell model using Sutherland type potential function for pair interaction between molecules, a statistical theory for binary liquid mixtures has been extended for ternary systems. In the light of above extension, excess volume ( V E ), excess energy ( E E ) and excess entropy ( TS E ) have been computed for three binary (benzene + cyclohexane, benzene + chlorobenzene and cyclohexane + chlorobenzene) and the resultant ternary system (benzene + cyclohexane + chlorobenzene) at 298.15 K. All the above mentioned excess properties have been computed from the data of ultrasonic velocity and density only.     

尼龙1010扩链反应的动力学模型和分析

双唑啉类化合物通过唑啉环与尼龙端羧基的开环加成反应使尼龙 10 10的分子量大幅提高 .以线性扩链反应的理想动力学模型为基础 ,通过对关键因素的修正使其更好地符合聚合物高粘度熔体扩链反应的实际过程 .通过数值积分方法 ,可以方便地模拟关键因素对扩链反应的影响 .理想线性模型与熔融扩链反应的前期规律相符合 ,但是其未考虑后期扩链产物的降解对偶联效果造成的影响 .通过基于降解的修正 ,该模型更好地符合了实际情况 .在扩链过程中 ,反应速率k和唑啉环与端羧基的比r是控制偶联效果的关键因素 ,温度和扩链剂加入量可以通过改变k和r进行方便的分析 .由于PA熔体反应体系的高粘性 ,2 ,2′ 双 (2 唑啉 ) (BOZ)和 1,4 双 (2 唑啉基 )苯 (PBO)的扩链效果的差别得以解释     

The generalized separated electron pair model. III. An application to three localization schemes for CO

The separated electron pair (SEP ) model (strongly orthogonal geminals) and methods for its systematic extension have been applied to three different localization schemes for C?O. The optimum SEP wave function is obtained for the particular localization scheme that involves three equivalent bent bonds. The major corrections to the SEP model arise from one-electron transfer terms. Two-electron transfer terms were important only for those pairs that were not well localized. It was found that the separate definitions of the total intrapair and interpair correlation energies did not depend strongly on the choice of localization scheme.     

双噁唑啉扩链尼龙1010

对尼龙 10 10的加成型化学扩链做了系统的研究 ,并对 2 ,2′ 双 (2 唑啉 ) (BOZ)和 1,4 双 (2 唑啉基 )苯 (PBO)的偶联效果和扩链产物的性能做了详细的探讨 ,前者的扩链效果较好 .研究结果表明 ,尼龙10 10熔体的转矩随着反应时间的延长显著增加 .但转矩达到极大值后开始缓慢下降 ,表明扩链产物在高温条件下发生热降解 .扩链剂的用量存在最佳值 ,用量不足时偶联反应不充分 ,用量过量时封端反应加剧 .扩链以后尼龙 10 10的端羧基含量大大降低 ,反应温度对反应速率的影响符合阿累尼乌斯关系式 .扩链产物极限粘度大大高于尼龙原料 ,表明分子量显著提高     

Continuous chromatographic separation process: simulated moving bed allowing simultaneous withdrawal of three fractions

Continuous counter-current chromatographic separation has been carried out in a simulated moving bed system (SMB). We have worked with a SMB pilot plant (8 columns, 4.4 litres of resin each) which allows the continuous withdrawal of two different fractions. A mixture of glucose-fructose has been separated. To calculate the concentration profile within the separator an axial dispersed plug flow model and an equilibrium stage model have been employed; software has been created to simulate the behaviour of the separator. The necessary parameters of the mode: the adsorption equilibrium constant, the height equivalent to a theoretical plate and the bed voidage, have been acquired experimentally from elution chromatography measurements. The results calculated by simulation give a good representation of the experimental concentration profiles; other separations like xylitol-arabitol have been simulated. The influence of some factors like desorbent flow-rate, feed flow-rate and the bed voidage have been studied using the software. Once the system has worked in a two withdrawal way, an extension of the pilot plant has been constructed so as to obtain a third one. The necessary parameters of the three withdrawal model will be studied.     

HOMA parameters for the boron–boron bond: How the introduction of a BB bond influences the aromaticity of selected hydrocarbons

An extension of the harmonic oscillator model of aromaticity (HOMA) model for systems with boron–boron bonds is presented. For the first time, the parameters of the HOMA model are estimated using only theoretically calculated bond lengths. The HOMA parameters obtained make geometric aromaticity studies possible for a large number of compounds containing the boron–boron bond. The derived HOMA parameters have been used to investigate how the introduction of the boron–boron moiety in the structure of selected hydrocarbons modifies their aromaticity. The conclusion is that the insertion of a boron–boron bond usually strongly decreases the aromaticity of the boron-containing compounds in comparison to their parent hydrocarbons.     

Wetting behavior of porous silicon surfaces functionalized with a fulleropyrrolidine

We report the immobilization of a fulleropyrrolidine, bearing a dec-9-ynyl functionality, on silicon surfaces through a thermal hydrosilylation protocol. Contact angle measurements on porous silicon (PS) surfaces reveal an unusual dependence of the angle with the PS roughness that apparently contradicts Wenzel's formula. This result has been explained by an extension of Wenzel's model in which the critical angle, which discriminates between the hydrophilic/hydrophobic character of a solid material, is substantially reduced below 90 degrees by surface roughness.     

Nanostructural Approach to Proton Ordering in Gas Hydrate Cages

A nanostructural approach to analysis of proton ordering in gas hydrate cages has been worked out within the framework of the topological model of strong and weak H-bonds. The approach involves rejection of the periodic boundary conditions, decomposition of the H-bond net into spherical layers, and two-dimensional drawing of the structure of spherical (spheroidal) fragments in the form of conjugate Schlegel diagrams. To analyze proton ordering in the spherical fragments composed of gas hydrate voids, we used the simulated annealing procedure and the correlation extension method proposed earlier.     

Dissociative electron transfer in donor-peptide-acceptor systems: results for kinetic parameters from a density functional/polarizable continuum model

The main structural and electronic factors playing a role in intramolecular dissociative electron transfer of a simple donor-peptide-acceptor (D-peptide-A) model have been investigated by an integrated computational protocol based on the density functional theory, its time-dependent extension, and the polarizable continuum model. Our results allow us to elucidate the electronic states involved in the process and how they are perturbed by the orientation of the donor and the acceptor with respect to the peptide chain and by the presence of the solvent. We also report a semiquantitative estimation of the rate constant governing electron transfer obtained by a direct quantum mechanical evaluation of all the terms entering the kinetic expressions based on the Marcus theory and its extensions.     

Constrained-pairing mean-field theory. V. Triplet pairing formalism

Describing strong (also known as static) correlation caused by degenerate or nearly degenerate orbitals near the Fermi level remains a theoretical challenge, particularly in molecular systems. Constrained-pairing mean-field theory has been quite successful, capturing the effects of static correlation in bond formation and breaking in closed-shell molecular systems by using singlet electron entanglement to model static correlation at mean-field computational cost. This work extends the previous formalism to include triplet pairing. Additionally, a spin orbital extension of the "odd-electron" formalism is presented as a method for understanding electron entanglement in molecules.     

Solubility prediction of anthracene in mixed solvents using a minimum number of experimental data

Numerical methods to predict the solubility of anthracene in mixed solvents have been proposed. A minimum number of 3 solubility data points in sub-binary solvents has been employed to calculate the solvent-solute interaction terms of a well established colsolvency model, i.e. the combined nearly ideal binary solvent/Redlich-Kister model. The calculated interaction terms were used to predict the solubility in binary and ternary solvent systems. The predicted solubilities have been compared with experimental solubility data and the absolute percentage mean deviation (APMD) has been computed as a criterion of prediction capability. The overall APMD for 25 anthracene data sets in binary solvents is 0.40%. In order to provide a predictive method, which is based fully on theoretical calculations, the quantitative relationships between sub-binary interaction terms and physicochemical properties of the solvents have been presented. The overall APMD value for 41 binary data sets is 9.19%. The estimated binary interaction terms using a minimum number of data points and the quantitative relationships have then been used to predict anthracene solubility data in 30 ternary solvent systems. The produced APMD values are 3.72 and 15.79%, respectively. To provide an accurate correlation for solubility in ternary solvent systems, an extension to the combined nearly ideal multicomponenet solvent/Redlich-Kister (CNIMS/R-K) model was proposed and the corresponding overall AMPD is 0.38%.     

Perturbed angular correlation studies of the relaxation process at 111Cd in the superviscous H3PO4 + H2O system

The time differential gamma—gamma perturbed angular correlation technique has been applied to the system phosphoric acid plus water in its phases of glass, superviscous liquid and normal liquid. For the superviscous case the extension of an existing model for the relaxation process has been used to obtain the temperature dependence of the rotational diffusion coefficient.     

Effects of thermal history on enthalpy relaxation

The effects of the thermal history on enthalpy relaxation in polymethylmethacrylate (PMMA) have been studied by differential scanning calorimetry (DSC). The temperature dependence of specific heat capacity in the liquid and glassy states, that of relaxation time and the exponent of the Kohlrausch–Williams–Watts function have been obtained by the measurement of the response of heat flux to the sinusoidal temperature variation. The phenomenological model equation as an extension of linear rheology has been applied to enthalpy relaxation. The evolution of entropy under a given thermal history same as the experiment has been calculated and compared with the DSC results. The calculated results reproduce two peaks of specific heat capacity at lower and higher temperatures in the glass transition region: the former is characteristic of PMMA and the latter is observed in typical glassy polymers.     

Photocatalytic degradation of pesticides in agricultural used waters

Various classes of organic contaminants can be easily destroyed and completely mineralized by titania-based photocatalysis. In the present work, we have studied the influence of the basic photocatalytic parameters on the catalyst activity as well as the identification of reaction intermediate products and degradation pathways in model and real solution, encountered in agricultural areas (rinsing waters). Various pesticides intensively used in agriculture such as herbicides (2,4-D(dichloro-phenoxy-acetic acid)) or insecticides (tetrachlorvinphos, fenitrothion, pirimiphos-methyl, fenamiphos) were successfully degraded, either as pure active agents or as commercial formulated reactants. A beneficial synergy effect has been obtained by adding activated carbon to titania. An extension to ‘helio-photocatalysis’ has been performed using the pilot solar photoreactor of Plataforma Solar de Almeria of the CIEMAT (Spain).     

The bending recovery of polymer films. I. A phenomenological model

When a flat film is bent to some fixed curvature, held at this state for some time, and then released, its curvature is usually observed to drop instantaneously to some finite value and then gradually decrease with time. This phenomenon, generally referred to as bending recovery, has been modeled by simple extension of the classical plate-bending theory to linear-viscoelastic materials. The model assumes that the relaxation spectra in tension and compression are dissimilar but are interrelated through a simple temporal shift. Experimental data, including instantaneous recovery at ambient and elevated temperatures as well as recovery over extended times, have been generated for four films with widely different relaxation characteristics. The data are in reasonable agreement with predictions of the model if a constant value of the compressive shift parameter (see text) is used for all materials. The model indicates that the bending recovery of a polymer film is intimately related to its relaxation spectrum and can be effectively manipulated by changes in temperature and winding time.     

The amylose triiodide-iodine complex: Its dependence with the degree of polymerization

An extension of the Statistical-Mechanics model for the inclusion complex of amylose with iodine is developed to take into account the influence of the degree of polymerization (x) of the polysaccharide. Good agreetment is found with the experimental data, in particular the iodine binding capacity (IBC) vs x is well reproduced by the model.