The structure of binary liquids. The Kirkwood-Buff theory of liquid mixtures,illustrated on the basis of the systems water/methanol,water/ethanol,and cyclohexane/2,3-dimethylbutane,as a link between thermodynamic data and x-ray and neutron scattering results

With the help of the Kirkwood-Buff theory of liquid mixtures it is demonstrated that there is a close relationship between, on the one hand, x-ray and neutron scattering intensities from liquid mixtures and, on the other hand, certain thermodynamic properties of the systems considered. This relationship can be utilized without having to specify the intermolecular interaction potential and without having to perform computer simulation calculations. In the present paper the Kirkwood-Buff theory is applied to liquid water and to the binary systems H₂O/MeOH, H₂O/EtOH, and cyclohexane/2,3-dimethyl-butane. On the basis of these examples it is explained that, from the thermodynamic quantities used, the detailed shape of the various molecular radial distribution functions cannot be established. However, in principle this information could well be obtained from the x-ray and neutron scattering intensities of the liquids that are being studied, provided an acceptable procedure could be found to isolate the different radial distribution functions. In this respect the information extracted from thermodynamic data would be essential since, in the first place, it provides a valuable way of normalization of the scattered intensity, and, in the second place, for multicomponent liquids it puts forward specific conditions to be satisfied by the various radial distribution functions.     

Transfer free energies of some ions from water to dimethylsulfoxide-water and urea-water mixtures

The standard free energies of transfer (G _t ^o ) of some electrolytes from water to aqueous mixtures of dimethylsulfoxide (DMSO) and of urea have been split into the contribution from individual ions by use of the reference electrolyte Ph ₄ AsBPh ₄ (RE), where Ph=phenyl. For each of the solvents, G _t ^o (Ph ₄ AsBPh ₄ ) was determined from the solubility products of the salts KBPh ₄ , Ph ₄ AsPi, and KPi, where Pi=picrate ion. The observed G _t ^o (i) values for the individual ions are strikingly different from the corresponding values obtained by the simultaneous extrapolation (SE) procedure reported earlier.     

Ionic interactions in solution: VI. The activity expansion and the association concept

The activity expansion equation which expresses the solute concentration as a series expansion of the activities of the solute species is used to reach a reformulation of the mean activity coefficient of a binary symmetrical electrolyte in solution. The result is displayed as the product of a shortrange term and a long-range term f_±=f _± ^S ·f^L _±, from which a straightforward derivation of the Bjerrum treatment is obtained. The analogy with an equivalent formulation for the molar conductance coefficient which results from the application of the echo-effect is emphasized. A simple formulation of the ion pair distribution function is proposed which is particularly powerful for ionic systems involving strong interactions. It is shown that the variable which controls the excess transport and thermodynamic quantities is not so much the stoichiometric concentration but rather the product of the concentration with the short-range factor f _± ^S of the activity coefficient product derived in the present work. In dilute solutions this factor becomes the so-called fraction of free ions originally introduced somewhat empirically by Bjerrum by means of a chemical model.Dedicated to Johannes Coetzee on the occasion of his 65th Birthday.     

The stokes and robinson hydration theory: A modification with application to concentrated electrolyte solutions

The proposed extension to the Stokes and Robinson hydration theory is based on the premise that H₂O molecules of electrolyte solutions reside in two unique environments, the solvent environment and the hydration envelope (or sheath) environment. Changes to the chemical potential of H₂O in either environment result in redistribution of H₂O molecules between the environments; consequently hydration numbers of electrolytes decrease as the solvent activity decreases. An equation derived from the theory and containing two adjustable parameters reproduces electrolyte activity coefficients more accurately than does the original Stokes and Robinson equation.     

SSOZ-HNC and SSOZ-PY integral equation studies of the structure of three-site polar fluids

Structure factors and site-site distribution functions for models of liquid carbon disulphide (CS₂) and acetonitrile (CH₃CN) are obtained by using the site-site Ornstein-Zernike (SSOZ) integral equation with the Percus-Yevick (PY) and the hypernetted chain (HNC) closures. The calculated structure factors are found to be in good agreement with the neutron and X-ray diffraction data as well as with the simulation data. The site charges have a significant effect on the distribution functions but not on the structure factors of both the systems. There is very good qualitative agreement between the calculated distribution functions and the results from computer simulations. Distinctive shoulders found in the simulation results for the first peaks of the C-N and CH₃-CH₃ distribution functions are enhanced in the calculations using the integral equations.     

Properties and structure of aqueous urea up to the singular temperature of overcooled water: Isotopy effects

In equations for the partial volume, dilatability, heat capacity, and adiabatic compressibility of aqueous urea, the properties are “fitted” to the singular temperature T _s of overcooled water T _s = 227.15 K (R. J. Speedy, J. Phys. Chem., 91, 3354 (1987)). The equations for the characteristics of D₂O-(ND₂)₂CO and T₂O-(NT₂)₂CO systems were obtained by scaling the temperature to 6 and 9.4 K, respectively. The properties of infinitely dilute solutions did not reveal the sign reversal of the isotope effect. The isoconcentrates of the partial volumes and dilatability form wagging lines in the region of low temperatures. The temperature dependences of the properties have extrema and inflection points, while the isotope effects in solutions of finite concentrations experience sign reversal. The maximum density temperature (MDT) of the solution decreases as the urea concentration increases, and also on passing from D₂O-(ND₂)₂CO to H₂O-(NH₂)₂CO. Thus, for the 8m solution, the MDT is 259.6 K (protium system) and 266.7 K (deuterium system). The difference is almost equal to the shift of the MDT after the H₂O → D₂O transition (7.2 K).     

Utilization of deformations in molecular quantum chemistry and application to density functional theory

The aim of this article is to present in a way accessible to most quantum chemists a general mathematical method which consists in deforming wave functions and density functions (in the spirit of the local scaling transformation). This deformation method allows us to obtain several new results, including a characterization of the set of wave functions that have the same given density function (which gives a new insight on a result of G. Zumbach and K. Maschke, Phys. Rev. A 28 , 544 (1983)) and an N-representability result where symmetry is taken into account. We also propose new theoretical ways to generate approximations of the exact density functional and give a numerical example. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 221–231, 1998     

On the theory of thermal density fluctuations in the glassy state: Application to poly(vinyl acetate)

A theory of the liquid state, suitably modified for the glass, contains a characteristic structure functionh, which represents a free volume fraction. As shown previously by means of experimental pressure-volume-temperature studies,h retains finite, nonvanishing temperature and pressure coefficients upon passing through the glass transition. These results are now employed to compute the mean-square thermal density fluctuations in poly(vinyl acetate). AboveT _g , the result attests again to the satisfactory quantitative performance of the equilibrium theory. BelowT _g , two glasses formed at low and elevated pressures, respectively, are considered under quasi-equilibrium conditions. The results show the anticipated initial accord with the approximation proposed by Fischer and Wendorff, involving the isothermal compressibility of the liquid atT _g . The theory delineates the increasing departures with decreasing temperature observed in the literature. We comment finally on the trend of the fluctuations on approaching absolute zero. Explicit low temperature calculations remain to be undertaken.Dedicated to Professor Dr. F. H. Müller.     

Post-adiabatic approach to atomic and molecular processes: The van der Waals interactions of some open shell systems

Summary Various properties of post-adiabatic representations of multichannel Schrödinger equations are described in the general context of adiabatic and classical path approximations as used in atomic and molecular physics. The van der Waals interactions of fluorine, chlorine, and oxygen atoms with rare gases, hydrogen, methane, and hydrogen halides are considered: it is found that in some of these systems, the first-order post-adiabatic scheme exhibits a smaller coupling than the adiabatic representation, thus providing an appropriate choice of the basis functions for a decoupling approximation.     

Solution viscosity behavior of polystyrene-based cationic ionomers: Effects of quaternary-group structure and counter ion

The dilute solution viscosity was investigated for several polystyrene-based cationic io-nomers. It was found that intramolecular aggregation among the ionic groups was strongly dependent on the sizes of quaternary onium groups and counter anions. The extent of the aggregation was controled by the solvent polarity and the solvation to the ionic groups. When there was a strong selective soivation to small counter ions, the structure of onium groups shows a minor effect on the viscosity behavior, indicating little aggregation among the ionic groups. A strong solvation to small quaternary cations also eliminates the intra-molecular aggregation and the influence of counter ions was barely observable. When the selective solvation to counter ions was disabled by the enlarged size of the counter ions, however, the viscosity depended on onium group structure or spacer chain length. If the solvent solvates neither counter ions nor quaternary groups, the smaller the sizes of onium cation and counter anion, the lower the reduced viscosity due to an enhancement of the ionic aggregate formation. © 1995 John Wiley & Sons, Inc.     

Conformational sampling with Poisson–Boltzmann forces and a stochastic dynamics/Monte Carlo method: Application to alanine dipeptide

We apply a combination of stochastic dynamics and Monte Carlo methods (MC/SD) to alanine dipeptide, with solvation forces derived from a Poisson–Boltzmann model supplemented with apolar terms. Our purpose is to study the effects of the model parameters, such as the friction constant and the size of the electrostatic finite difference grid, on the rate of conformational sampling and on the accuracy of the resulting free energy map. For dialanine, a converged Ramachandran map is produced in significantly less time than what is required by stochastic dynamics or Monte Carlo alone. MC/SD is also shown to be faster, per timestep, than explicit methods. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1750–1759, 1997     

Exceptional thermodynamic stability of DNA duplexes modified by nonpolar base analogues is due to increased stacking interactions and favorable solvation: Correlated ab initio calculations and molecular dynamics simulations

The geometries of DNA hexamer (5'-GGAACC-3') and DNA 13-mer (5'-GCGTACACATGCG-3') have been determined by molecular dynamics (MD) simulations using an empirical force field. The central canonical base pair was replaced by a pair of nonpolar base analogues, 2,2'-bipyridyl and 3-methylisocarbostyril. The stabilization energy of the model system (model A) consisting of a central base pair (base-analogue pair) and two neighboring base pairs was determined by the RI-MP2 method using an extended aug-cc-pVDZ basis set. The geometry of the model was averaged from structures determined by MD simulations. The role of the solvent was covered by the COSMO continuum solvent model and calculations were performed for a larger model system (model B) which also contained a sugar-phosphate backbone. The total stabilization energies of the unperturbed system and the system perturbed by a base-analogue pair (model A) were comparable to the stability of both duplexes experimentally determined. This is due to large stacking interaction energy of the base-analogue self-pair which compensates for the missing hydrogen-bonding energy of the replaced adenine...thymine base pair. The selectivity of the base-analogue pair was reproduced (model B) when their desolvation energy was included with the interaction energy of both strands determined by the approximate SCC-DFTB-D method.     

Continuous Miscibility from Dilute Solution to Fused Salt: Volumetric Properties of Binary Solutions of Nitrobenzene, 1-Butanol, and Anisole with Tetra-n-Butylammonium Picrate from 298.15 to 371.1 K

The densities and speeds of sound of binary solutions of nitrobenzene, 1-butanol, and anisole with tetra-n-butylammonium picrate have been measured over the full composition range from 298.15 to 371.1 K, in order to study the volumetric behavior of continuously miscible systems, made from an organic fused salt and a molecular organic liquid, at just above the melting point of the salt. The calculated apparent molar volumes and compressibilities are analyzed by using the equations of Petrenko and Pitzer. The thermal expansion coefficients of the above systems are also reported.     

Characterization of specific interactions capacity of solid surfaces by adsorption of alkanes and alkenes. Part II: Adsorption on crystalline silica layer surfaces

Summary Adsorption of branched octanes and linear hydrocarbons on crystalline lamellar silica surfaces has been studied by inverse gas chromatography at infinite dilution. Taking the adsorption of the n-alkanes as a reference, the influence of the double bond on the hydrocarbon adsorption phenomena has been demonstrated. Thermodynamical parameters have been calculated which permit conclusions to be made on the adsorption mechanisms of lamellar materials.     

Statistical theory for hydrogen bonding fluid system of AaDd type (Ⅲ): Equation of state and fluctuations

The equation of the state of the hydrogen bonding fluid system of A_aD_d type is studied by the principle of statistical mechanics. The influences of hydrogen bonds on the equation of state of the system are obtained based on the change in volume due to hydrogen bonds. Moreover, the number density fluctuations of both molecules and hydrogen bonds as well as their spatial correlation property are investigated. Furthermore, an equation describing relation between the number density correlation function of “molecules-hydrogen bonds” and that of molecules and hydrogen bonds is derived. As application, taking the van der Waals hydrogen bonding fluid as an example, we considered the effect of hydrogen bonds on its relevant statistical properties. Supported by the National Natural Science Foundation of China (Grant Nos. 20303006 and 20574016), the Natural Science Foundation of Hebei Province (Grant Nos. B2006000959 and B2004000093) and the Natural Science Foundation of Education Committee of Hebei Province (Grant No. 2003101)     

ChemInform Abstract: Structure and Bonding of IrB12‐: Converting a Rigid Boron B12 Platelet to a Wankel Motor.

DFT‐based calculations reveal that the global minimum of IrB₁₂^‐ is a C_3v symmetric bowl‐like structure in which the Ir atom is located on the concave side of the bowl similar to its lighter congeners, CoB₁₂^‐ and RhB₁₂^‐.     

Raman spectroscopic investigation of aqueous FeSO4 in neutral and acidic solutions from 25‡C to 303‡C: inner- and outer-sphere complexes

Raman spectra of aqueous FeSO₄ and (NH₄)₂SO₄ solutions have been recorded over broad concentration and temperature ranges. Whereas the v₁-SO ₄ ^2- band profile is symmetrical in noncomplexing (NH₄)₂SO₄ solutions, in FeSO₄ solutions a shoulder appears on the high-frequency side, which increases in intensity with increasing concentration and temperature. The molar scattering coefficient of the v_1-SO ₄ ^2- band is the same for all forms of sulfate in (NH₄)₂SO₄ and FeSO₄ solutions and is independent of temperature up to 150‡C, the highest temperature studied. The high-frequency shoulder is attributed to the formation of a contact ion pair, Fe²⁺OSO^3/2-, as is the splitting of the v_3-SO ₄ ^2- antisymmetric stretching mode which is observed in the FeSO₄ solution. The bending modes v_2-SO ₄ ^2- and v_4-SO ₄ ^2- , normally forbidden in the isotropic spectrum, show a gain in intensity with increasing ion-pair formation. A polarized band has been assigned to the Fe²⁺-O ligand vibration. No higher associates or anionic complexes are required to interpret the spectroscopic data. No evidence of contact ion pairing between Fe²⁺ and HSO4 ₄ ^- could be detected at temperatures up to 303‡C in 1 molal solutions of FeSO₄ with an excess of 2 molal H₂SO₄.     

Generalized model of the linear theory of electromigration and its application to electrokinetic chromatography: Theory and software PeakMaster 6—Next Generation

The linear theory of electromigration, including the first‐order nonlinear approximation, is generalized to systems with any equilibria fast enough to be considered instantaneous in comparison with the timescale of peak movement. For example, this theory is practically applied in the electrokinetic chromatography (EKC) mode of the CZE. The model enables the calculation of positions and shapes of analyte and system peaks without restricting the number of selectors, the complexation stoichiometry, or simultaneous acid–base equilibria. The latest version of our PeakMaster software, PeakMaster 6—Next Generation, implements the theory in a user‐friendly way. It is a free and open‐source software that performs all calculations and shows the properties of the background electrolyte and the expected electropherogram within a few seconds. In this paper, we mathematically derive the model, discuss its applicability to EKC systems, and introduce the PeakMaster 6 software.