Электрондық көшу: механикалық сипаттама және теориялар

Relocation of an electron from an atom or molecule to another

Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons. Electrochemical processes are ET reactions. ET reactions are relevant to photosynthesis and respiration and commonly involve transition metal complexes. In organic chemistry ET is a step in some commercial polymerization reactions. It is foundational to photoredox catalysis.

In inner sphere ET, the two redox centers are covalently linked during the ET. This bridge can be permanent, in which case the electron transfer event is termed intramolecular electron transfer. More commonly, however, the covalent linkage is transitory, forming just prior to the ET and then disconnecting following the ET event. In such cases, the electron transfer is termed intermolecular electron transfer. A famous example of an inner sphere ET process that proceeds via a transitory bridged intermediate is the reduction of [CoCl(NH3)5]2+ by [Cr(H2O)6]2+. In this case, the chloride ligand is the bridging ligand that covalently connects the redox partners.

In heterogeneous electron transfer, an electron moves between a chemical species and a solid state electrode. Theories addressing heterogeneous electron transfer have applications in electrochemistry and the design of solar cells.

Especially in proteins, electron transfer often involves hopping of an electron from one redox active center to another. The hopping pathway, which is viewed as a vector, guides and facilitates ET within an insulating matrix. Typical redox centers are iron sulfur clusters, e. g. the 4Fe 4S ferredoxins. These site are often separated by 7 10 Å, a distance compatible with fast outer sphere ET.

The first generally accepted theory of ET was developed by Rudolph A. Marcus to address outer sphere electron transfer and was based on a transition state theory approach. The Marcus theory of electron transfer was then extended to include inner sphere electron transfer by Noel Hush and Marcus. The resultant theory called Marcus Hush theory, has guided most discussions of electron transfer ever since. Both theories are, however, semiclassical in nature, although they have been extended to fully quantum mechanical treatments by Joshua Jortner, Alexander M. Kuznetsov, and others proceeding from Fermi's golden rule and following earlier work in non radiative transitions. Furthermore, theories have been put forward to take into account the effects of vibronic coupling on electron transfer; in particular, the PKS theory of electron transfer. In proteins, ET rates are governed by the bond structures: the electrons, in effect, tunnel through the bonds comprising the chain structure of the proteins.