Boris Ya. Simkin

Theoretical methods of calculating solvation effects

Abstract In this review modern theoretical methods of considering solvent effects are discussed and systematized, together with their comparative characteristics. The model of macroscopic representation of the medium and its effects being incorporated into quantum chemical calculations is discussed. The methods of constructing model Hamiltonians in microscopic and macroscopic approximations and the method of electrostatic potential and supermolecule approach have been analyzed in detail. A comparative characteristic of the applicability of various quantum chemical methods for calculating intermolecular interactions is given. Particular attention is drawn to the rapidly developing methods of calculating the structure and energetics of a solution: the Monte-Carlo method, the molecular dynamics method and others. The possibilities of methods are illustrated by typical examples.

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.

Monte Carlo investigation of structure and dielectric properties of the electrode-water interface depending on electrode charge density

Dependence of the dielectric and structural characteristics on the surface charge density of electrode was investigated using a Monte Carlo method (MCM) version developed for studying liquids near electrode. A similar problem was solved also for the case associated with the presence of the H3O+ cation in the electrode-water interface.

Effect of boundary conditions in the Monte Carlo method on energy and structural characteristics of liquids. Calculation of water and the aqueous methane solution

Abstract New boundary conditions for the calculation of pure liquids and solutions by the Monte Carlo method have been introduced. The main spherical cell is surrounded by the boundary layer of the solvent molecules whose positions are fixed. Calculations of structural and energy characteristics of water and the aqueous methane solution indicate the significance of the approach proposed. To take only one example, the calculated methane hydratation energy of − 7.63 ± 8.40 kJ/mol agrees with the experimental value of − 10.9 kJ/mol better than in all other similar Monte Carlo calculations.

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.

Theoretical investigation of changes in the thermodynamical properties of water in electrode-liquid interface during hydroxonium discharge

Abstract Using the Monte Carlo method, the water reorganization energies during electron transfer from the unpolarizable surface of the absolutely hard charged electrode to the hydroxonium and during the reverse process were calculated for various surface charge densities of the electrode. The dependence is studied of the solvation energy of ions (taking by way of example H3O+) and non-electrolytes (for H3O·) in electrode-solution interface on the surface charge density.

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.

A microscopic simulation of solvent reorganization in heterogeneous hydroxonium discharge dependening on the interface polarizability and the distance to interface

Abstract The paper investigates the dependences of the energetics of the polar solvent reorganization during the electrochemical electron transfer reactions near electrode on polarizability of the electrode surface and the separation between reduced/oxidized particle and electrode. The Monte Carlo and Langevin dipoles methods have been used. A significant influence has been found of taking into account the electrode surface and the solvent molecule polarizabilities on the solvent reorganization energetics studied.

Some aspects of the homogeneous (chemical) electron exchange near an interface

Abstract The influence of electrode on the homogeneous (chemical) electron transfer (ET) reaction occurring near it has been considered. Using the Langevin dipole (LD) method a model electron exchange reaction between a donor (aluminium atom) and an acceptor (hydroxonium cation) is investigated. The solvent (water) reorganization energies. ET activation energies and their different contributions have been calculated depending on the localization of this reaction pair regarding electrode. A catalytic effect of electrode on the ET homogeneous reaction occurring near it has been studied.