Statistical thermodynamics of liquids using the Monte Carlo method. II. Liquid chloroform.

Thermodynamic properties and radial distribution functions for liquid chloroform were calculated using the Monte Carlo method implemented with Metropolis algorithm in the NpT ensemble at 298 K and 1 atm, A five site model was developed to represent the chloroform molecules. A force field composed by Lennard-Jones and Coulomb potential functions was used to calculate the intermolecular energy. The partial charges needed to represent the Coulombic interactions were obtained from quantum chemical ab initio calculations, The Lennard-Jones parameters were adjusted to reproduce experimental values for density and enthalpy of vaporization for pure liquid, All thermodynamic results are in excellent agreement with experimental data, The correlation functions calculated are in good accordance with theoretical results available in the literature. The free energy for solvating one chloroform molecule into its own liquid st 298 IC and 1 atm was computed as an additional test of the potential model. The result obtained compares well with the experimental value. The medium effects an cis/trans convertion of a hypotetical solute in water TIP4P and chloroform solvents were also accomplished. The results obtained from this investigation are in agreement with estimates of the continuous theory of solvation.22457458

Statistical thermodynamics of liquids using the Monte Carlo method. I. Methodology.

Statistical mechanics Monte Carlo simulation is reviewed as a formalism to study thermodynamic properties of liquids. Considering the importance of free energy changes in chemical processes, the thermodynamic perturbation theory implemented in the Monte Carlo method is discussed. The representation of molecular interaction by the Lennard-Jones and Coulomb potential functions is also discussed. Charges derived from quantum molecular electrostatic potential are also discussed as an useful methodology to generate an adequate set of partial charges to be used in liquid simulation.22225426

Free energy calculation using Monte Carlo simulation

The combination of statistical perturbation theory and Monte Carlo method to calculate free energy of solvation is discussed. In the model presented the solvent-solvent and solute-solvent interaction energies are obtained from appropriated Lennard-Jones and coulomb potential functions. Molecular interactions beyond a suitable cut-off range are neglected and long-range correction for ion-solvent molecule interactions is included by using the Born solvation model. Free energies of hydration for methane and chloride ion were obtained in the NpT ensemble at T = 289K and p = 1.0 atm. The behavior of free energy of solvation as a function of the ion charge and ionic radius were investigated in methanol and acetonitrile. Tile results are in good agreement with qualitative predictions from the Born model. The influence of temperature in the energy of hydration was also investigated, The results are in very good agreement with theoretical and experimental data from literature.19216617

Monte Carlo simulation of water-pyridine mixtures

Thermodynamic properties and radial distribution functions for a water-pyridine mixture as a function of composition were calculated with the Monte Carlo method in the isothermal and isobaric ensemble at T = 298.15K and p = 1.0 atm. The TIP4P model was used for water and optimized potentials for liquid simulations (OPLS) force field parameters were used for pyridine. The effect of including the hydrogen atoms of pyridine was explicitly investigated by comparing the results from calculations with six and eleven site models. The results obtained for the average configurational interaction energy as a function of the mole fraction are in good agreement with experimental data. The partitioning of the total configurational energy of the water-pyridine mixture is presented and compared with similar results from other systems. Comparatively low energy values were obtained for the water-pyridine interaction. This interaction becomes slightly more negative when the hydrogen atoms of pyridine are explicitly considered. The radial distribution functions for the water-pyridine interaction show characteristic features indicating the formation of hydrogen-bonded dimers. The amplitude of the peaks observed for these distribution functions depends on the particular model used to represent the pyridine molecule. Radial distribution functions and molecular graphic representations for water-water interaction indicate the formation of small clusters of water molecules in the bulk of the water-pyridine system. This clustering enhances the water-water coordination numbers and interaction energy. The contribution from solute-solvent electrostatic interaction to the free energy of solvation of water in pure water, pure pyridine and in the equimolar water-pyridine and water-methanol mixtures were calculated using statistical perturbation theory (SPT). The results obtained for the electrostatic contribution to the free energy of solvation, in absolute values, are pyridine < water-pyridine < water-methanol < water. These free energy results indicate the migration of water molecules towards an environment rich in hydrogen bonds and explain the formation of water clusters in the bulk of water-pyridine mixtures.7213314

Theoretical studies of liquids by computer simulations: the water-acetone mixture.

Monte Carlo simulation results for pure liquid acetone and water-acetone mixtures calculated in the isothermal and isobaric (NPT) ensemble at T=298K and p=1.0atm are presented. The TIP4P model was used for water and optimized potential for liquid simulation (OPLS) force field parameters used for acetone. The results obtained for the average configurational energy as a function of the mole fraction are in good accord with experimental data. Energy partitioning and co-ordination numbers results calculated for equimolar water-acetone solution are compared to similar data obtained for other water-organic liquid mixtures. These results show an increase in water-water interaction energy and co-ordination numbers when the interaction between water and organic liquid molecules decrease. Distribution functions for pure liquid acetone and water-acetone mixtures are presented. Dipole-dipole angular correlation functions obtained for pure liquid acetone show a predominance of dimers with parallel alignment of dipole moments. Radial distribution functions from water-acetone interaction show characteristic features of hydrogen bonded liquids. Radial and angular distribution functions for water-water correlation calculated in pure water and in equimolar water-acetone mixture are compared, showing very similar features in both systems. (C) 1999 Elsevier B.V. B.V. All rights reserved

Theoretical studies of concentrated aqueous urea solutions using computacional Monte Carlo simulation

Monte Carlo simulations were carried out for concentrated aqueous urea solutions at 4, 5, 6, 7 and 8 mol L-1 in the NpT ensemble at 298 K and 1 atm. The potential energy surfaces were represented by the sum of Coulomb and Lennard-Jones potentials in which pair-wise additivity was considered. Water was described by the TIP4P model, while urea was described by the OPLS planar model and also by a proposed non-planar model. The simulation results for both urea models showed that urea does not induce any significant changes in the structure of water over all the concentrations studied. However, it was noted that the number of hydrogen bonds between the water molecules decrease as the concentration increases. The simulation results of the OPLS planar model suggest the formation of urea dimers in the more concentrated solutions. The simulations with the non-planar model showed several features similar to those of the OPLS planar model.13223824

Structure activity relationship between calculated molecular properties and biological activities against Leishmania donovani of the natural neolignan analogues studied with pattern recognition techniques: a possible explanation for the activity/inactivity of some neolignans

Structure activity relationship (SAR) between calculated molecular properties and biological activities against Leishimania donovani of 20 analogues of the natural neolignans was studied. Since the arginine residue in the adenosine-kinase from Leishmania donovani is a possible receptor of the neolignans, fitting of some neolignans with the arginine was performed in order to define the binding sites of neolignans. The ab initio Hatree-Fock method was employed to calculate the molecular properties of the 20 analogues. Four different pattern recognition techniques such as principal component analysis, SIMCA, etc. were used to establish SAR. The compounds were classified correctly in 95% of success by SIMCA, more than 80% of success by the other three methods. The neolignans that have the aromatic rings substituted in the ortho and/or para positions are generally active. It was demonstrated that SAR studies were possible using the calculated molecular properties. (C) 2001 Elsevier Science B.V. All rights reserved.54314715