electronentransfers

Electron transfers refer to processes in which an electron moves from a donor species to an acceptor species. These processes are central to chemistry, biology, electrochemistry, and materials science. Electron transfer can occur as a direct bimolecular collision in which no covalent bonds are formed or broken (outer-sphere mechanism), or through a shared ligand that bridges donor and acceptor (inner-sphere mechanism). In biological systems, electron transfer chains consist of a sequence of redox-active cofactors that shuttle electrons efficiently over molecular distances.

Two broad classes are recognized: outer-sphere transfers, where the redox centers interact without strong chemical bonding changes; inner-sphere transfers, where a ligand binds to both partners and provides a conduit for electron flow. The rate of transfer depends on the driving force, reorganization energy, and distance between centers, as described by theories such as Marcus theory for outer-sphere reactions. Marcus theory relates the rate constant to the free energy change and the reorganizational energy, predicting an optimal driving force and, for large driving forces, the inverted region.

Marcus theory is central to quantitative descriptions; reorganizational energy includes changes in solvent and internal coordinates; distance and medium influence electronic coupling; proton-coupled electron transfer (PCET) is important when protons participate concertedly with electrons, especially in biological redox reactions.

Applications include cellular respiration and photosynthesis, where electron transport chains move electrons through a series of carriers; electrochemical devices such as fuel cells, batteries, and dye-sensitized solar cells; corrosion and metal redox chemistry; catalysis and synthetic chemistry using mediators.

Understanding electron transfer informs design of materials and drugs; experimental techniques include spectroscopic redox potential measurements, cyclic voltammetry, and kinetic studies.