Structural parameters of alanine zwitterion hydration from the data of the integral equation method in the RISM approximation

The structural parameters of alanine zwitterion hydration in water were studied by means of the integral equation method in the framework of the 1D-RISM approximation. According to the calculations, ～5.9 water molecules are located in the nearest environment of the COO-group, and the nearest environments of the NH₃ ⁺ and CH₃ groups contain ～5.5 and ～11.6 water molecules, respectively. The number of hydrogen bonds formed by the COO- and NH₃ ⁺ groups is 4.3 and 2.4, respectively. The data obtained did not reveal hydrogen bonding of the nitrogen atom of the NH₃ ⁺ group, whereas there is a possibility for hydrogen bonding of the methyl group with water molecules. No structuredness of water molecules near the CH fragment of the hydrophobic moiety was found.     

Structural parameters of glycine zwitterion hydration from the data of the integral equation method in the RISM approach

The structural parameters of glycine zwitterion in water were studied by means of the integral equation method in the framework of the RISM approximation. According to calculations, five water molecules are located in the nearest environment of the -NH ₃ ⁺ group, and two of them are the H-bonded with this group. At the same time, six water molecules are located in the nearest environment of the ?COO^? group, and three of them are the H-bonded with this group. The average number of water molecules in the first hydration shell of ?CH₂ group is four. It has been shown that the probability of hydrogen bond formation between water molecules and the hydrogen atom H1 of the ?NH ₃ ⁺ group is low, and there is no H-bonding between water molecules and the nitrogen atom the ?NH ₃ ⁺ group.     

Accurate and local formulation for thermodynamic properties directly from integral equation method

A local formulation for determination of excess chemical potential is derived out by applying an assumption of linear dependence of correlation function and bridge function on the charging parameter to the Kirkwood charging formula and scaling the bridge function, the scaling parameter is specified by a Gibbs–Duhem relation. The local formulation for the excess chemical potential only requires the correlation function and bridge function of the investigated state as input and is therefore free of an unwieldy thermodynamic integration. A comprehensive comparison between the presently calculated thermodynamic quantities for a Lennard–Jones (LJ) fluid including two key quantities, i.e. the excess chemical potential and excess entropy, corresponding simulation data available in literature, and corresponding calculated results by several other global and local formulations, indicates that the present formulation is the only one capable of predicting locally and excellently all of the thermodynamic properties of the LJ fluid. The GCMC simulation is carried out for a core-softened potential fluid and the LJ fluid near critical state and at subcritical state near the gas–liquid coexistence line to obtain the excess chemical potential which is also in excellent agreement with the theoretical prediction from the present formalism; this indicates that the present formalism is of general interest in fluid statistical mechanics and applicable to parameter space covering over the entire phase diagram.     

Estimating the Gibbs energy of hydration from molecular dynamics trajectories obtained by integral equations of the theory of liquids in the RISM approximation

A method of integral equations of the theory of liquids in the reference interaction site model (RISM) approximation is used to estimate the Gibbs energy averaged over equilibrium trajectories computed by molecular mechanics. Peptide oxytocin is selected as the object of interest. The Gibbs energy is calculated using all chemical potential formulas introduced in the RISM approach for the excess chemical potential of solvation and is compared with estimates by the generalized Born model. Some formulas are shown to give the wrong sign of Gibbs energy changes when peptide passes from the gas phase into water environment; the other formulas give overestimated Gibbs energy changes with the right sign. Note that allowance for the repulsive correction in the approximate analytical expressions for the Gibbs energy derived by thermodynamic perturbation theory is not a remedy.     

Study of thermal properties of the metastable supersaturated vapor with the integral equation method

Pressure, excess chemical potential, and excess free energy data for different densities of the supersaturated argon vapor at reduced temperatures from 0.7 to 1.2 are obtained by solving the integral equation with perturbation correction to the radial distribution function [F. Lado, Phys. Rev. 135, A1013 (1964)]. For those state points where there is no solution, the integral equation is solved with the interaction between argon atoms modeled by Lennard-Jones potential plus a repulsive potential with one controlling parameter, alpha exp(-rsigma) and in the end, all the thermal properties are mapped back to the alpha=0 case. Our pressure data and the spinodal obtained from the current method are compared with a molecular dynamics simulation study [A. Linhart et al., J. Chem. Phys. 122, 144506 (2005)] of the same system.     

Adsorption integral equation via complex approximation with constraints: kernel of general form

For the monolayer adsorption on a homogeneous surface, including arbitrary range lateral interactions, the isotherm can be written as a power series of the Langmuir isotherm. If this isotherm is used as the kernel in the adsorption integral equation, this integral equation can be solved in an analytical form. Because the global isotherm is usually known as a set of experimental values, the use of a numerical method is inevitable. A new numerical method for solving the adsorption integral equation with a kernel of general form is developed. It is based on recent results concerning the structure of the local isotherm and on the ideas of complex approximation with constraints, and allows reduction of the problem under consideration to a linear‐quadratic programming problem. Results of numerical experiments are presented. The method can be useful for the evaluation of the adsorption energy distribution from experimental data. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1058–1066, 2001     

The infrared spectrum of the methylammonium ion in bis(methylammonium)tetrachlorocadmate(II)

The IR spectrum of polycrystalline (CH₃NH₃)₂CdCl₄ has been recorded at temperatures between 300 and 90 K. The phase transition reported to take place at 173 K was observed and appears to involve a significant change in hydrogen-bond strength. Factor-goup splitting, affecting both parallel and perpendicular vibrational modes, was found to occur in the low-temperature spectra; its presence was demonstrated by the use of isotopically dilute CD₃NH₃⁺ ions. The formally IR-active torsional mode was not observed.     

Structure formation features of water and concentrated aqueous lithium halide solutions at low temperatures from the data of integral equation method

The structure of water and the influence of halide ions on the structure formation of concentrated LiX : H₂O (1 : 5; X = Cl, Br, I) solutions at low temperatures were studied by the method of integral equations. Based on the results obtained, supercooling of pure water is expected to significantly enhance the tetrahedral ordering of its molecules, strengthen hydrogen bonding in the system, and decrease the number of the nearest-neighbor water molecules. The effects for the solutions on lowering the temperature include a partial restoration of the tetrahedral network of H-bonds of the solvent molecules, insignificant increase in the number of the nearest-neighbor water molecules, enhancement of the coordination ability of Li⁺ cation, strengthening of hydrogen bonding between anions and water molecules in the first hydration shell, increase in the number of solvent-separated ion pairs, and weakening of the temperature effect on these structural parameters in the following order of solutions: LiCl > LiBr > LiI. The probability of contact ion pair formation in the systems studied should appreciably decrease. The temperature should to a greater extent influence the associative ability of larger anions.     

‘Exact’ integral equation theory and local formulation for excess thermodynamic properties of hard spheres

A hybrid hard sphere bridge function is proposed, which, in combination with the standard Ornstein–Zernike integral equation, can predict extremely accurately hard sphere compressibility, virial pressure, and correlation function. Second, a local formulation for determination of excess chemical potential is derived out, which, in combination with the present hybrid hard sphere bridge function and OZ integral equation, can predict the excess chemical potential also extremely accurately. The resultant excess entropy is in excellent agreement with that from the Carnahan–Starling equation of state. The present formalism performs excellently over the whole density range, i.e. from zero to freezing density, and is largely superior to a formalism available in the literature.     

Thermodynamic and structural properties of repulsive hard-core Yukawa fluid: integral equation theory, perturbation theory and Monte Carlo simulations

The thermodynamic and structural properties of purely repulsive hard-core Yukawa particles in the fluid state are determined through Monte Carlo simulation and modeled using perturbation theory and integral equation theory in the mean spherical approximation (MSA). Systems of particles with Yukawa screening lengths of 1.8, 3.0, and 5.0 are examined with results compared to variations of MSA and perturbation theory. Thermodynamic properties were predicted well by both theories in the fluid region up to the fluid-solid phase boundary. Further, we found that a simplified exponential version of the MSA is the most accurate at predicting radial distribution function at contact. Radial distribution function of repulsive hard-core Yukawa particles are also reported. The results show that methods based on MSA and perturbation theory that are typically applied to the attractive hard-core Yukawa potential can also be extended to the purely repulsive hard-core Yukawa potential.     

Calculating the thermodynamic and structural properties of liquid noble gases

The structural and energy parameters in the liquid branch of the saturation line of noble gases, including ⁴He, are determined.     

Electronic factor in electron self-exchange reaction of hydrated redox ion pairs from thermodynamic data

An accurate scheme for determining the electronic factor of the electron self-exchange reaction in solution is presented in this paper. The used various activation parameters and slopes of potential energy surfaces are obtained in terms of an improved activation model and the accurate potential function determined from the vibrational spectroscopic and thermodynamic data. The coupling matrix elements are determined using numerical integral method over the perturbed double-zeta Slater-type state functions. Theoretical results of electronic factor in this work are found in close agreement with those extracted from experimental rate constant data and to be less than unity. Results indicate that outer-sphere electron transfer reactions in solution involving hydrated transition metal ions are nonadiabatic in nature.     

Structure and thermodynamics of polymeric liquids: Polymer kirkwood integral equation method

A statistical mechanical theory is employed to predict the structural and thermodynamic properties of polymeric liquids. The theory consists of a coupled set of self-consistent integral equations for the pair correlation functions which relates the polymer structure to molecular interaction potentials. In this article, the essentials of the method are reviewed and the results are shown in comparison with other theories and simulation data. Quantitative agreements are found between the results of the theory and those of the simulations for a wide range of density and temperature.     

Insight into molecular interactions and solvation properties of methylammonium perchlorate in acetonitrile-dimethylsulfoxide binary mixtures: Compressibility,viscometric and dissolution thermodynamic studies

Densities (ρ), ultrasonic velocities (u) and viscosities (ƞ) of Methylammonium perchlorate (CH₃NH₃ClO₄) in binary solvent mixtures of Acetonitrile (AN) and Dimethylsulfoxide (DMSO) containing 0.0, 0.20, 0.40, 0.60, 0.80 and 1.0 ​mol fraction of DMSO at various temperatures ranging from 298K–328K at atmospheric pressure, have been measured. From the experimental data important parameters, namely, apparent molal volume (V_Φ), isentropic compressibility (K_s), limiting apparent molal isentropic compressibility (K^o_s,ɸ), A and B-viscosity coefficients from Jones-Dole equation, B/V_ɸ^o values and viscous flow related thermodynamic properties have been derived. Interpretation of the obtained thermodynamic parameters was done with regards to solute-solvent and solute-solute interactions and structure breaking and making potential of the solutes in solution. To evaluate the extent of preferential solvation, K^o_s,ɸ values and B-coefficients for the electrolytes have been further split up into the contribution of individual ion (K^o_s,ɸ± and B_± values) by reference electrolyte method.     

Applications of the RISM equation to diatomic fluids: the liquids nitrogen,oxygen and bromine

The reference interaction site model (RISM) integral equation is used to study the equilibrium pair correlation for one-component liquids composed of homonuclear diatomic molecules. The integral equation is first tested by comparing the results obtained from it with those of computer simulation calculations. An internal consistency test is developed which seems to provide an a priori measure of the accuracy of the RISM equation. Then the theory is used to interpret the neutron scattering structure factor data taken on the liquids nitrogen and oxygen. For both of these liquids, a satisfactory explanation of the data is obtained by assuming that the short ranged repulsive forces between molecules are mimicked by a two-site hard core model. However, this simple model does not provide a satisfactory explanation of the data taken from liquid bromine. But, it is shown that a slightly more sophisticated model for the short-ranged repulsion between Br₂ molecules does provide an adequate explanation. With the molecular models determined by fitting the neutron scattering data, the RISM equation provides a method for determining the atom, atom to center-of-mass, and center-of-mass to center-of-mass intermolecular distribution functions in the diatomic liquids. From these functions, the local structures in the three liquids are analyzed. While orientational pair correlations are nearly negligible in both the liquids nitrogen and oxygen, these correlations are fairly substantial in liquid bromine. Furthermore, even when the orientation of a molecule does not greatly influence the orientation of its neighbors, it is found that the orientation of a molecule does have an important effect on the location of its neighbors. Thus, the coupling of translational and orientational coordinates is significant, even in the liquids nitrogen and oxygen.     

Hydration of the bisulfate ion: atmospheric implications

Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO(4)(-)(H(2)O)(n=1-6) and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO(4)(-)/H(2)O system to the neutral H(2)SO(4)/H(2)O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO(4)(-)/H(2)O and H(2)SO(4)/H(2)O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO(4)(-)(H(2)O)(n=1-5) is favorable at 298.15 K, and that of HSO(4)(-)(H(2)O)(n=1-6) is favorable for T < 273.15 K. The HSO(4)(-) ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.     

Exploring the structural,mechanical and thermodynamic properties of Ti-V solid solutions

Although Ti-V based high-temperature alloys are used in aerospace engine, rocket engine and hot sections, the structure and mechanical properties of Ti-V alloys remains controversy. To explore the correlation between structural and mechanical properties, we apply employed the DFT method to study the phases stability, mechanical and thermodynamic properties of Ti-V solid solution. Two Ti-V solid solutions: Ti(V)_ss solid solution and V(Ti)_ss solid solution are discussed. Two Ti-V solid solutions are thermodynamic stability. In particular, the Ti-V solid solution prefers to form V(Ti)_ss solid solution, in while the V(Ti)_ss solid solution remains cubic structure. Furthermore, the Ti(V)_ss solid solution is a mechanical instability. However, the V(Ti)_ss solid solution is a mechanical stability. Here, the bulk modulus, shear modulus and Young's modulus of V(Ti)_ss solid solution are 136.9, 23.5 and 66.7 GPa. In particular, the bulk modulus of V(Ti)_ss solid solution is higher than the bulk modulus of the pure Ti. In addition, the V(Ti)_ss solid solution shows better ductility compared to the pure Ti and V. Naturally, the stability and mechanical properties of V(Ti) solid solution is related to the Ti-V metallic bond because of the localized hybridization between the Ti(3d) and V(3d).