Application of the Marcus theory to description of the kinetics of reduction processes of organic cations

The calculation of the rate constants for the processes in the reduction of diazonium, tropylium, verdazylium, and pyrylium cations by various organic electron donors, using the equations of the Marcus theory with allowance for the reorganization energy of only the outer coordination sphere, leads to values which are one to seven orders of magnitude higher than the experimental values. By quantum-chemical calculations it was shown that the reduction of diazonium and tropylium cations to the corresponding radicals is accompanied by a substantial change in the structure of the reagents. This leads to high values for the reorganization energy of the inner coordination sphere of the cations, which must be taken into account during calculation of the rate constants. The differences in the rate constants of the processes of direct electron transfer from the electron donors to the organic cations and the recombination of these reagents depend on the dissociation energy of the bond of the cation with the donor leading to the electron transfer products.     

Kinetics and mechanism of the reaction of organosiloxanes with the surface of pyrogenic silica

Infrared spectroscopy and gravimetry were used to study chemisorption kinetics and adsorption equilibrium in the reaction of hexamethyldisiloxane (HMDS), hexamethylcyclotrisiloxane (HMTS), octamethylcyclotetrasiloxane (OMTS), and 1,3,5 trivinyl-1,3,5-trimethylcyclotrisiloxane (MVCTS) with the surface of pyrogenic silica. It was determined that the electron-donor ability of the siloxanes in the formation of hydrogen bonds with surface hydroxyl groups decreases in the series HMDS > HMTS > OMTS > MVCTS. A reaction scheme is proposed, which includes electrophilic substitution of hydrogen of the surface silanol group in the limiting stage. It was found that the chemisorption kinetics is described by an equation of an inhomogeneous-surface model with allowance for interaction between species in a neutral-field approximation. The relative reactivity of the siloxanes in these transformations is governed mainly by the difference of the energies of the reactant siloxane bonds.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 167–172, March–April, 1992.     

Radical hydroxylation of aromatic hydrocarbons examined by MINDO/3 methods

MINDO/3 calculations have been made on the potential-energy surfaces for the attachment of OH^. radicals to benzene (1) and naphthalene (2) in the vapor state. The activation energies of these reactions are calculated as 88 and 58 kJ/mole. while the enthalpies at 298K are calculated as –211 and –199 kJ/mol. The transition states in (1) and (2) lie closer to the reagents than the products on the reaction coordinate, while (1) has an earlier transition state than does (2). The transition states in these reactions have high dipole moments: 3.1 and 3.6 D, respectively, which are due to charge transfer from the hydrocarbons to the OH^.. Quantum-chemical calculations and kinetic data on the reactions of aromatic hydrocarbons with OH^. in aqueous solution indicate that the mechanism is probably not one involving electron transfer and a rate-limiting stage in the attachment. These processes are of high performance because the radicals are of high stability, while polar effects determine the selectivity.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 20–26, January–February, 1986.     

Kinetics of the reduction of organic cations by cation radicals

It has been found that the reduction of substituted diazonium, iodonium, and tropylium salts by cation radicals of tetra-tert-butyl-bis-pyrylium, tetramethyl-p-phenylene diamine (I), tetraphenylbenzidine, and N-methylphenothiazine obey the kinetic equation of the second order and that it follows an electron transfer mechanism. The logarithms of the rate constants of these reactions decrease with increasing standard free energy of the electron transfer reaction. The rate of the reaction of C₆H₅N₂₊ BF₄ ^– with I and the selectivity of the reactions of the substituted diazonium salts with I increases with the electron donor capacity of the solvent. The addition of crown-ethers accelerates the reaction. The reaction rate increases also with increasing ionic strength of the solution, whereby the Cl^– anions have a specifically accelerating effect. The observed rules governing the influence of the medium contradict those found earlier for the interaction of organic cations with neutral radicals and anion radicals. This is attributed to the fact that the electron transfer takes place in ternary complexes cation-solvent (crown ether, Cl^– ion)-cation radical.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 45–51, January–February, 1985.The authors wish to express their gratitude to V. V. Lobonov for tje performance of the quantum chemical calculations.     

Adsorption of volatile atmospheric pollutants on an heterogeneous surface of surrogate solid aerosols

Adsorption of 23 nonpolar and polar volatile atmospheric organic pollutants on the surface of silica gel, carbon black (Carbopack), and carbon-coated silica gel (Carbosil), materials imitating mineral and carbonaceous aerosols, was studied by inverse gas chromatography at finite concentrations. The results obtained suggest that the surface is nonuniform. The experimental adsorption isotherms were described within the framework of an approach based on the local Langmuir equation and a rectangular surface area distribution over the adsorption energy. It was established how the mean and variance moments of these distributions depend on the polarizability of the sorbates and their acid-base characteristics and how these parameters influence the adsorption characteristics of the system under study.