Ernst D. German

Theory of homogeneous reactions involving proton transfer

Abstract A quantum-mechanical theory for proton transfer processes in solution is given. The Bronsted rule and isotope effect for these processes are also discussed.

Theory of the kinetic isotope effect in proton transfer reactions in a polar medium

A theory of the hydrogen kinetic isotope effect (KIE) is presented. It is based on the quantum-mechanical theory of the elementary act of chemical reactions in polar media. The theory takes into account both the non-adiabatic and adiabatic ways of proton (deuteron) transitions between various vibrational energy levels and possible unharmonicity of the proton vibrations. The theory describes the dependence of the KIE on various characteristics of the reactants and the medium. Simplified equations for the KIE are presented.

Dependence of the hydrogen kinetic isotope effect on the reaction free energy

The kinetic isotope effect for proton transfer reactions between ethyl nitroacetate and a series of O-bases has been calculated for the linear complex model. The influence of both the adiabaticity of proton transitions from some vibrational energy levels and of the anharmonicity of proton vibrations on the dependence of the kinetic isotope effect on reaction free energy has been investigated.

A theoretical analysis of the kinetics of reductive cleavage of the carbon—halogen bond in tert-butyl halides in polar solvent

Rate constants and activation energies of dissociative electron transfer reactions of t-butyl halides proceeding in polar solvent are calculated. A model is used in which potential energy of the bond C-X to be broken is described by a Morse function in reactant and an exponential curve in product. The parameters of the exponential function are taken from quantum chemical calculations of the radical anions of t-butyl halides. The reorganization energy of reaction is computed in the framework of the cavity model. Results of calculations are compared with experimental data.

A study of the carbon-halogen bond breaking in tert-butyl halides by the PM3 quantum chemical method

Abstract Quantum chemical calculations by the semiempirical PM3 method are performed for halogen tert-butanes t-BuX (X = Cl, Br) and their radical anions in the gas phase and model polar solvent. It is shown that these radical anions are unstable in polar solvents. Potential energy surfaces for t-BuBr−.and t-BuI−. have shallow minima in the gas phase. In the range of the values of the distances R(C-halogen) from the equilibrium one R 0 to R ≈ 2.2–2.3 A the potential curves for t-BuCl−.and t-BuBr−. may be well approximated by the exponential functions Uf = B exp[−2β(R−R0)]. Activation energies and the values R ∗ for the transitional configuration for the reactions t-BuX + e → t-Bu. + X− in the gas phase are calculated.

Hydrogen production by solar energy: Optimization of the plant “solar array + electrolyzer”

Abstract The optimization of the performance of a plant consisting of a solar photovoltaic converter and a water electrolyzer has been theoretically analyzed. It is shown that the criterion of the maximum productivity of the plant coincides with that of the operation of a solar array at the point of maximum power. The algorithm of design optimization of the system has been developed allowing the ratio of the number of the series-connected solar cells to that of the electrolysis cells at given solar radiation power density to be found. The regimes of switching the number of series-connected solar cells and electrolysis cells to maintain the maximum running capacity of the plant have been computed for the operation of the system at varying solar radiation power density. A program has also been compiled for the operation of the system without automatic regime switching, and the parameters effecting the maximum integral daily capacity have been computed. The optimum relation between the solar array area and that of the electrolyzer electrodes, providing the maximum capacity of the system per unit of the investment, has been calculated with reference to the relation of the cost of unit solar cell area to that of the electrolyzer electrode.

Elementary act of dissociative adsorption of two-atom molecules on a solid surface in a polar solvent

A model is suggested for the description of an elementary act of dissociative adsorption of molecules of the X2 type at a solid surface in a polar solvent. Within this model, the potential energy of interaction between the molecule and the surface and the intramolecular energy of X2 are approximated by Morse functions. The model accounts for the reorganization of the substrate and polar solvent structures. The model is compared with other approximations.

Calculation of the polar media reorganization energy for the model of two dielectric spheres

Abstract The effect of medium dielectric inhomogeneity on the solvent reorganization energy in charge transfer processes is investigated for the model describing reactants as two dielectric spheres. It is shown that this effect is of the order of 16% for the contact position of the reactants and decreases when the inter-reactant distance increases.

The electronic factor for outer-sphere electron-transfer reactions

Electronic matrix elements L have been estimated for symmetrical electon-transfer reactions between transition-metal complexes, based on an analysis of the experimental data in terms of the theory of non-adiabatic transitions. The rate constants calculated using the values of L thus obtained and the cross-relationship for the matrix elements agree well with the experimental data for a wide range of cross electron-transfer reactions.

A quantum-mechanical model for nucleophilic substitution reactions in methyl halides

A quantum-mechanical model for nucleophilic substitution reactions is suggested in which the reactions are assumed to be partially adiabatic. The model takes into account the motion of both incoming and outgoing groups in the course of the reaction and the reorganization of the medium. A considerable contribution to the activation energy comes from the outersphere reorganization of the medium. The theoretical results for the activation energy and pre-exponential factor are in good agreement with the experimental data.