V. N. Levchuk

Calculation of the hydration energy of ethanol by the Monte Carlo method

Abstract The hydration energy of ethanol has been calculated by the Monte Carlo method. The boundary conditions have been employed, different from the standard ones, represented by a spherical cell surrounded by a layer of the boundary solvent molecules whose positions are fixed in the process of simulation of the solution. The calculated value of −57.1 ± 6.5 kJ/mol agrees much better with the xperimental estimate of −54 kJ/mol of the hydration enthalpy of ethanol than in the previous calculations where the periodic boundary conditions were used. The scheme proposed has permitted reliable data to be obtained also when simulating pure water.

Investigation of infinitely diluted solutions of organic compounds in tetrachloromethane using the Monte Carlo method

Abstract Solvation energies of ten organic compounds have been calculated by the Monte Carlo method. Deviations from the experimentally determined enthalpies of solutions are less than 11 kJ/mol. Excellent linear dependence has been found of solvation enthalpies on the energies of interaction between solute and solvent molecules. The tetrachloromethane molecule has been simulated as a one-center particle. Parameters of its interaction potential functions have been fitted to reproduce the properties of pure tetrachloromethane.

Effect of the boundary conditions in the Monte Carlo method on energy and structure characteristics of liquids. A calculation on aqueous solutions of methanol and benzene.

Abstract A new model of boundary conditions, suggested earlier by the present authors [1], for calculating infinitely dilute solutions by the Monte Carlo method has been subjected to one more test. This model, represented by a main spherical cell surrounded by a layer of the solvent molecules with fixed positions, has permitted us to calculate fairly accurately the hydration energies of benzene and methanol, which all previous MC calculations have been incapable of achieving. The structural characteristics of solutions, too, have been shown to depend on the manner in which the boundary effects are simulated.

Theoretical investigation of the factors which determine the high rate of metal dissolution in the mixture of Tetrahydofuran with HCl

Using the quantum chemical methods and the continuum representation of the medium, the causes of the metal dissolution high rates in the mixture “HCl-THF” were examined. A search was made for possible molecular and ionic associates. The associates (C4H8O)2 and C4H8O HCl were found to be thermodynamically the most advantageous. Then, their oxidizing activities were compared with one another and with that of the H3O+(H2O)3 cation which exists in the mixture “HClH2O”. For this purpose, the rates of electron transfer from the same donor (Al atom) to the associates in solutions studied were compared. The highest rate was obtained for the (C4H8O)2H+ accociate. This allows one to explain the causes of rapid metal dissolution in “HClTHF” and the chemical mechanism of dissolution, which predominates over the electrochemical in this case.

Calculation of the solvation contributions to free reorganization energy in electron transfer reactions and to vertical electron transition energy. continuum representation of medium

Abstract A method for taking into account the influence of medium on processes in solutions, which are associated with electron transfer, is presented. The calculation scheme may be realized in terms of continuum representation of the medium using, for example, the method of virtual charges developed by Tomasis grous. This scheme allowes description of non-equilibrium medium for rapid processes in solutions. The description is in a most general form and, at the same time, more rigorous than a number of the analogous schemes available at present.

Theoretical study of the aqueous medium effect on the reaction CO2+ OH− ⇄ HCO−3

Abstract A method for the theoretical study of chemical reaction mechanisms in aqueous solutions has been developed. Approximate reaction pathways are determined with the aid of a model Hamiltonian in which medium is described by a set of Langevin point dipoles. Solvation of stationary points on the potential energy surface so obtained is performed by a more exact calculation of the medium contribution using the method of minimization of the solvation shell potential energy. Usefulness of our approach is exemplified by the calculation of the reaction CO 2 + OH − ⇄ HCO − 3 . Method MIND0/13 has been employed in constructing the model Hamiltonian. A good agreement with experiment has been achieved.

Theoretical aspects of the homogeneous electron transfer reaction kinetics in electrolytes containing chlorohydrogen

Abstract A theoretical analysis of the factors conditioning high and independent of electrode potential rate of metal dissolution in dry (without water) chlorohydrogen electrolytes, in tetrahydrofuran and the lowest alcohols has been performed. A relationship between the dissolution rates and the electron affinity values of the oxonium associates stable in the considered electrolytes has been discussed.

Calculation of the properties of liquid chloroform by the Monte Carlo method

A version of the Monte Carlo method for the study of the properties of pure liquids and solutions that was previously developed by Levchuk et al. was modified for the case of liquid chloroform. A single-center potential of the interaction of chloroform molecules was plotted by using trial Monte Carlo calculations. The potential makes it possible to calculate satisfactorily the energy and structural characteristics of chloroform.

A theoretical method of researching ion-molecule reactions in solution

A method has been devised for researching the mechanisms of ion-molecule reactions in aqueous solutions, which involves deriving an approximate reaction path by means of a model Hamiltonian, in which the medium is described by a set of point Langevin dipoles. At the stationary points on the PES derived in that way, the solvation may be simulated by means of a more accurate scheme for the medium, namely by direct optimization of the potential energy for the solvate shell. The method has been tested on the reaction CO2+OH−⇄HCO3−. The model Hamiltonian has been constructed by MINDO/3. Good agreement with experiment is obtained.

Structure of methylformamide and its enolic form in aqueous solutions

The electronic and structural characteristics of methylformamide and its enolic form were calculated by the MINDO/3 method with due regard to the effects of the solvent in the model of point charges. The coordinates of the point charges were determined both from calculation of the water cluster by the Monte Carlo method (112 water molecules) and from model theories about the structure of the intermolecular hydrogen bonds. An analysis is given of the dependence of the calculated characteristics on the structure of the solvated shell.