Intrinsic reaction parameters for electron transfer from aromatic radical anions to vicinal dibromoalkanes in alkane...

In diffusion-assisted bimolecular reactions, the elementary reaction mechanism is typically difficult to study, because the kinetics of the intrinsic reaction is masked by the diffusive motion of the reactants. One of the possible experimental approaches to solve the problem is the precise determination of the effective reaction radii, R = k/(4πD(S)), where k is the stationary value of the reaction rate constant and D(S) is the sum of the diffusion coefficients of the reactants involved. In this work, this approach has been applied to study the reaction between radical anions of aromatic compounds (diphenylacetylene and diphenylsilane) and vicinal dibromoalkanes (1,2-dibromoethane and trans-1,2-dibromocyclohexane) in liquid alkanes. The reaction rates were determined using the effect that the bromoalkanes exert on the decay kinetics of the fluorescence from the irradiated solutions of the aromatics. The effect of the external electric field on the fluorescence decays was exploited to measure aromatic radical ion mobilities. Diffusion coefficients of the bromoalkane molecules were determined by means of the thermal diffusion forced Rayleigh scattering technique. The experimental radii obtained in the temperature range of 273-333 K were compared with those calculated from the diffusion equation with a distance-dependent relative diffusion coefficient and electron-transfer rate. The most important factors that have been taken into account are (i) the ion-dipole interaction between the reagents, (ii) the shift of the equilibrium between rotational isomers of dibromoalkanes in the electric field of its charged partner, and (iii) the hydrodynamic interaction. The study shows that the apparent activation energy of the intrinsic electron-transfer rate from the anions to the dibromoalkanes does not exceed a few kilocalories per mole, in agreement with the results of quantum chemical calculations within the DFT approach. The evaluated electron-transfer rates at the reagents' contact (intrinsic rate) were estimated to be on the order 5 × 10(11) s(-1), corresponding to transfer matrix elements of about 30-50 cm(-1).