多重平衡标准平衡常数的计算方法及应用

介绍了多重平衡标准平衡常数的技巧性计算公式及适用范围,利用该公式解题简捷、快速、准确,还可以直观判断反应的方向性及反应程度。     

A novel method for estimation of transition temperature for polymorphic pairs in pharmaceuticals using heat of solution and solubility data

A novel method for thermodynamic stability studies of polymorphic drug substances has been developed. In order to estimate the transition temperature for an enantiotropic polymorphic pair, a formula for calculating the temperature at which the solubilities of each polymorph become equal has been derived with heat of solution and solubility as the variables. This formula is based on the assumption that van't Hoff plots (logarithmic solubility versus reciprocal of absolute temperature plots) of each polymorph show a straight line (heat of solution is independent of temperature) whose slope can be expressed as a function of heat of solution. The transition temperatures for seratrodast, acetazolamide and carbamazepine polymorphic pairs calculated by the formula were in good agreement with the results of previous studies. Furthermore, the calculated transition temperature for the indomethacin polymorphic pair was above the melting point, an unrealistic temperature range, suggesting that these polymorphs are monotropically related. Since this formula requires solubility data at only one arbitrary temperature other than heat of solution data for both polymorphs in a polymorphic pair, the proposed method is much faster than the conventional method requiring solubility data at five or more different temperatures for the preparation of van't Hoff plots.     

A constant entropy increase model for the selection of parallel tempering ensembles

The present paper explores a simple approach to the question of parallel tempering temperature selection. We argue that to optimize the performance of parallel tempering it is reasonable to require that the increase in entropy between successive temperatures be uniform over the entire ensemble. An estimate of the system's heat capacity, obtained either from experiment, a preliminary simulation, or a suitable physical model, thus provides a means for generating the desired tempering ensemble. Applications to the two-dimensional Ising problem indicate that the resulting method is effective, simple to implement, and robust with respect to its sensitivity to the quality of the underlying heat capacity model.     

Zone broadening in electrophoresis with special reference to high-performance electrophoresis in capillaries: an interplay between theory and practice

Approximate equations have been derived for the total (final) zone width (plate height, plate number and resolution) as a function of the width of the starting zone and of the zone broadening caused by diffusion. Joule heat, adsorption and the difference in conductivity (delta kappa) between a solute zone and the surrounding buffer. Two cases are treated: (A) the conductivity differences eliminate entirely or (B) partially, the diffusional broadening at one boundary of a zone. When adsorption is negligible one can derive from these equations the field strength - and for case A also the electrical conductivity - that gives the minimum zone broadening (plate height). Interestingly, at this minimum, contributions to the zone broadening from diffusion. Joule heat and conductivity differences have the ratio 4:1:1 in case A. In case B the ratio between the broadening caused by diffusion (including that caused by conductivity and pH differences) and broadening due to Joule heat is 4:1. The total zone width, plate height and optimal field strength calculated from the derived equations agree satisfactorily with experimental values. A simple method to estimate the variance of the zone broadening caused by the Joule heat led to a formula similar to that calculated mathematically. An appropriate width of the starting zone can be calculated rapidly by means of a simple formula. Following a run the true width can be estimated graphically from measurements of plate heights or zone widths at low field strengths. For high resolution the width of the starting zone usually should not exceed 0.5 mm. A new principle for the design of multi-buffer systems which generate sufficiently narrow starting zones has been developed for carrier-free zone electrophoresis. This zone sharpening method permits application of wide zones of concentrations below the detection limit of the monitor. The diffusion coefficient (D) and the universal parameter D/mu (mu = mobility) appear in many of the equations derived and are often the only variables which are not easily accessible. Simple methods have therefore been developed by which they can be determined with sufficient accuracy. Fortunately, they are raised to the power of 1/5 in many formulas and therefore only a rough estimation is required. True plate numbers (calculated in the absence of electroendosmosis) often differ considerably from apparent plate numbers (calculated in the presence of electroendosmosis). A mathematical relationship between the true and apparent plate numbers has been derived.(ABSTRACT TRUNCATED AT 400 WORDS)     

Temperature modulated differential scanning calorimetry (TMDSC) in the region of phase transitions. Part 1: theoretical considerations

For temperature modulated differential scanning calorimetry (TMDSC) a simple model, the low pass filter, is presented which allows to see and calculate the influence of heat transfer into the sample on magnitude and phase shift of the modulated part of the measured heat flow rate and the heat capacity determined from it. A formula is given which enables to correct the measured magnitude of the periodic heat flow rate function and the calculated heat capacity in dependence on the thermal resistance and heat capacity of the sample. The correction becomes very important in regions where the heat capacity changes considerably as in the melting region. The approach is successfully tested with model substances with well-known excess heat capacity in the transition region.     

Trace-level determination of polar flavour compounds in butter by solid-phase extraction and gas chromatography-mass spectrometry.

Volatile compounds are responsible for the aromas of butter. A simple technique for the determination of these components is described which is based on solid-phase extraction (SPE) after melting of the butter and separation of the aqueous phase from the fat. Volatile flavours present in the water fraction are collected by off-line SPE on cartidges packed with a copolymer sorbent. After desorption with 500 microliters of methyl acetate, 1-microliter aliquots are quantified and/or identified by gas chromatography-mass spectrometry. The procedure was tested with respect to recovery, linearity and limit of detection in real-life samples using five polar model analytes. It allows the characterisation of polar flavour compounds in butter prior to and after heat treatment at 170 degrees C. From the five model compounds, vanillin, traces of diacetyl and maltol were found to be present in the butter samples. After heat treatment 500-1000-fold increased concentration of maltol, and substantial amounts of furaneol were detected.     

Scaling law for the radius of gyration of proteins and its dependence on hydrophobicity

The scaling law between the radius of gyration and the length of a polymer chain has long been an interesting topic since the Flory theory. In this article, we seek to derive a unified formula for the scaling exponent of proteins under different solvent conditions. The formula is obtained by considering the balance between the excluded volume effect and elastic interactions among monomers. Our results show that the scaling exponent is closely related to the fractal dimension of a protein's structure at the equilibrium state. Applying this formula to natural proteins yields a 2/5 law with fractal dimension 2 at the native state, which is in good agreement with other studies based on Protein Data Bank analysis. We also study the dependence of the scaling exponent on the hydrophobicity of a protein chain through a simple two‐letters HP model. The results provides a way to estimate the globular structure of a protein, and could be helpful for the investigation of the mechanisms of protein folding. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 207–214, 2009     

临界点二级相变的有限时间热力学逼近

应用内可逆卡诺循环的方法,导出了各种物质在临界点附近可逆与不可逆二级相变普遍适用的比热跃变公式以及广义的爱伦菲斯特方程。对简单(P,V,T)系统、超导、电介质顺电一铁电二级相变进行了应用讨论。     

Prediction of the heat capacity of ionic liquids using the mass connectivity index and a group contribution method

A simple and accurate group contribution method to estimate the heat capacity of ionic liquids is presented. The method considers groups previously defined for a successful method used to estimate critical properties of ionic liquids. Additionally a structural parameter known as mass connectivity index recently defined by the authors has been incorporated to define the model equation. To better define the values of the groups, heat capacity data at 298 K for 126 organic substances were used with the 469 heat capacity data for 32 ionic liquids. The results were compared with experimental data and with values reported by other available estimation methods. Results show that the new group contribution method gives low deviations and can be used with confidence in thermodynamic and engineering calculations.     

Estimation of the hydration of polar groups of α-amino acids by differential scanning calorimetry

The heat capacity of hydration of zwitterions derived from aliphatic amino acids depends linearly on the surface area of the amino acid side radicals accessible to water molecules with the slopeb = 2.35±0.11 J mol^–1 K^–1 Å^–2 at 298 K. The linear correlation between hydration heat capacities of zwitterions of aliphatic amino acids and the corresponding aliphatic alcohols with a coefficient of approximately unity confirms the assumption that hydrophobic hydration does not depend on the nature of the surrounding groups. Using the assumption that the hydration of hydrocarbon radicals is independent of the neighboring groups, theb value has been used to calculate the contributions of polar groups. The contributions of OH, COON, and CONH groups of the side radicals in polar amino acids in the zwitterion form are close to zero; in the case of organic nonionic molecules, these contributions are negative. The increments for polar groups obtained for the zwitterions can be used for the calculation of the heat capacities of proteins and polypeptides incorporating charged amino acid residues. The difference between hydrophilic and hydrophobic hydration mechanisms is manifested not only as different magnitudes and signs of heat capacities and temperature coefficients but also in the fact that the neighboring polar (charged) groups have an effect on hydrophilic hydration but have no effect on hydrophobic hydration.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2237–2242, September, 1996.     

An Efficient Algorithm for Calculating the Heat Capacity of a Large‐Scale Molecular System

We present an efficient algorithm for computing the heat capacity of a large‐scale molecular system. The new algorithm is based on a special Gaussian quadrature whose abscissas and weights are obtained by a simple Lanczos iteration. Our numerical results have indicated that this new computational scheme is quite accurate. We have also shown that this method is at least a hundred times faster than the earlier approach that is based on estimating the density of states and integrating with a simple quadrature formula.     

Simple Estimates for Eutectic Behavior

A simple formula is derived for the eutectic point of an A–B system in terms of the monomer melting points and melting enthalpies. This estimate is tested on several non-ionic or ionic systems, with or without common ions, including choline chloride/urea mixtures. The results are compared with the Schröder-van Laar equation.     

Dipole interactions and electrical polarity in nanosystems: the Clausius-Mossotti and related models

Point polarizable molecules at fixed spatial positions have solvable electrostatic properties in classical approximation, the most familiar being the Clausius-Mossotti (CM) formula. This paper generalizes the model and imagines various applications to nanosystems. The behavior is worked out for a sequence of octahedral fragments of simple cubic crystals, and the crossover to the bulk CM law is found. Some relations to fixed moment systems are discussed and exploited. The one-dimensional dipole stack is introduced as an important model system. The energy of interaction of parallel stacks is worked out, and clarifies the diverse behavior found in different crystal structures. It also suggests patterns of self-organization which polar molecules in solution might adopt. A sum rule on the stack interaction is found and tested. Stability of polarized states under thermal fluctuations is discussed, using the one-dimensional domain wall as an example. Possible structures for polar hard ellipsoids are considered. An idea is formulated for enhancing polarity of nanosystems by intentionally adding metallic coatings.     

A new angle on heat capacity changes in hydrophobic solvation

The differential solubility of polar and apolar groups in water is important for the self-assembly of globular proteins, lipid membranes, nucleic acids, and other specific biological structures through hydrophobic and hydrophilic effects. The increase in water's heat capacity upon hydration of apolar compounds is one signature of the hydrophobic effect and differentiates it from the hydration of polar compounds, which cause a decrease in heat capacity. Water structuring around apolar and polar groups is an important factor in their differential solubility and heat capacity effects. Here, it is shown that joint radial/angular distribution functions of water obtained from simulations reveal quite different hydration structures around polar and apolar groups: polar and apolar groups have a deficit or excess, respectively, of "low angle hydrogen bonds". Low angle hydrogen bonds have a larger energy fluctuation than high angle bonds, and analysis of these differences provides a physical reason for the opposite changes in heat capacity and new insight into water structure around solutes and the hydrophobic effect.     

On the energy loss of slow electrons due to dielectric relaxation

We derive an approximate formula for a novel form of energy loss experienced by 1–10 eV electrons passing through polar liquids. This loss mechanism, which depends on the translational motions of polar molecules in the Coulomb field of an electron is shown to yield a dissipation rate comparable in magnitude with the prediction of the Fröhlich-Platzman theory, which allows only for the rotational motion of polar molecules (Debye relaxation mechanism).     

定标粒子液体理论应用于极性溶质体系及其盐效应系数的计算

1970年W. L. Masterton和T. P. Lee将定标粒子理论应用于非极性溶质在电解质水溶液中的盐效应系数的计算, 推导出便于应用的盐效应表达式。本文将定标粒子理论应用于极性溶质, 推导出一个求极性溶质的盐效应常数表达式。作者用此表达式分别计算了乙酸、正丙酸、正丁酸、正己酸及正庚酸各在KCl、KBr、NaCl和NaBr四种小离子盐水溶液中的理论盐析常数。修正后的盐析常数比用Masterton和Lee公式计算的盐析常数更接近实验值。     

四苯硼钠水溶液的系列热力学性质

在288.15～318.15K范围内,用等压法测定了四苯硼钠在不同浓度水溶液中的平均活度系数和渗透系数,计算了不同浓度时的超额自由能、相对偏摩尔焓、偏摩尔热容。经计算机曲线拟合给出热力学性质的经验计算公式。     

Calorimetry and potentiometry of chemical oscillations in Briggs-Rauscher reactions with simultaneous measurements of the produced oxygen volume

The Briggs-Rauscher reaction is known as a nonlinear and non-equilibrium chemical oscillation reaction. The reaction solution is composed of malonic acid as organic substrate, hydrogen peroxide and iodate in sulphuric acid as oxidizing agent, and manganese (II) as metal catalyst. The calorimetric behaviour, in terms of the total heat Q and the heat evolving rate q for each chemical oscillation, was followed with the use of a heat exchange calorimeter of the batch type assembled by the authors. Simultaneously, the concentration of iodide ions produced as the intermediate species was measured as the potential difference E by the common potentiometric cell incorporated in the calorimeter and the released oxygen volume G was also observed by a simple flowmeter. The starting point of the peak on the curve of q against time t coincided with that in the curve of E against t. The switching concentration of iodide between radical and nonradical paths was calculated from literature values of rate constants. The heat evolving period of Q coincided with the period in the radical path. The curve of Q against t also coincided with that of G against t. The total volume of released oxygen was larger than that calculated from the stoichiometric reaction formula.     

W4 theory for computational thermochemistry: In pursuit of confident sub-kJ/mol predictions

In an attempt to improve on our earlier W3 theory [A. D. Boese et al., J. Chem. Phys. 120, 4129 (2004)] we consider such refinements as more accurate estimates for the contribution of connected quadruple excitations (T4), inclusion of connected quintuple excitations (T5), diagonal Born-Oppenheimer corrections (DBOC), and improved basis set extrapolation procedures. Revised experimental data for validation purposes were obtained from the latest version of the Active Thermochemical Tables thermochemical network. The recent CCSDT(Q) method offers a cost-effective way of estimating T4, but is insufficient by itself if the molecule exhibits some nondynamical correlation. The latter considerably slows down basis set convergence for T4, and anomalous basis set convergence in highly polar systems makes two-point extrapolation procedures unusable. However, we found that the CCSDTQ-CCSDT(Q) difference converges quite rapidly with the basis set, and that the formula 1.10[CCSDT(Q)cc-pVTZ+CCSDTQcc-pVDZ-CCSDT(Q)cc-pVDZ] offers a very reliable as well as fairly cost-effective estimate of the basis set limit T4 contribution. The T5 contribution converges very rapidly with the basis set, and even a simple double-zeta basis set appears to be adequate. The largest T5 contribution found in the present work is on the order of 0.5 kcal/mol (for ozone). DBOCs are significant at the 0.1 kcal/mol level in hydride systems. Post-CCSD(T) contributions to the core-valence correlation energy are only significant at that level in systems with severe nondynamical correlation effects. Based on the accumulated experience, a new computational thermochemistry protocol for first- and second-row main-group systems, to be known as W4 theory, is proposed. Its computational cost is not insurmountably higher than that of the earlier W3 theory, while performance is markedly superior. Our W4 atomization energies for a number of key species are in excellent agreement (better than 0.1 kcal/mol on average, 95% confidence intervals narrower than 1 kJ/mol) with the latest experimental data obtained from Active Thermochemical Tables. Lower-cost variants are proposed: the sequence W1-->W2.2-->W3.2-->W4lite-->W4 is proposed as a converging hierarchy of computational thermochemistry methods. A simple a priori estimate for the importance of post-CCSD(T) correlation contributions (and hence a pessimistic estimate for the error in a W2-type calculation) is proposed.     

Electrostatic properties of cyano-containing mesogens

We analyse the electrostatic properties of a set of cyano-containing mesogen molecules with different rigid cores and variable alkyl chain lengths, computing the molecular charge distributions. Using the simple prototype benzonitrile, we analyse the reliability of the quantum chemical methods used to estimate the electrostatic dipole moments of polar conjugated molecules. We show that the electronic properties of the long mesogenic molecules can be treated by combining HF geometry optimization procedures with single point MP2 calculations. We compare the results of these computations with the available experimental phase transition data of mesogens and discuss some examples of how the non-trivial mesomorphic behaviour, which is usually observed in these cyano compounds, can be qualitatively explained by the molecular electrostatic interaction potential.