Free electron transfer—relations between molecule dynamics and reaction kinetics

Abstract

Electron transfer in non-polar media (alkanes, alkyl chlorides) exhibits some essential peculiarities. For instance the reaction of hetero-substituted aromatics with parent solvent radical cations results in the parallel formation of metastable donor radical cations and fragmentation products, in comparable amounts. The fragmentation products originate from a dissociative donor radical cation which decays extremely rapidly, i.e., in a few femtoseconds. This phenomenon is explained in terms of intramolecular dynamic motions which cause changes of the electron density (π- and n-orbitals) in dependence on the deformation angle between the substituents and the aromatic ring. Hence femtosecond dynamics is reflected in the nanosecond time range and can be observed with real-time spectroscopy. Therefore, the process is named free electron transfer (FET) which corresponds to an unhindered electron jump occurring in the first approach of the reactants. This dynamic controlled process is compared with the classical electron transfer theories which are based on equilibrium kinetics. From the FET mechanism some new aspects for chemical reaction kinetics can be derived (critical review, 69 references).