Electron transfer
Encyclopedia
Electron transfer is the process by which an electron
moves from an atom
or a chemical species
(e.g. a molecule
) to another atom or chemical species. ET is a mechanistic description of the thermodynamic concept of redox
, wherein the oxidation states of both reaction partners change.
Numerous biological processes involve ET reactions, including oxygen binding, photosynthesis, respiration, and detoxification routes. Additionally, the process of energy transfer
can be formalized as a two-electron exchange (two concurrent ET events in opposite directions) in case of small distances between the transferring molecules. ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic chemistry
.
is the bridging ligand that covalently connects the redox partners.
reaction between permanganate
and its one-electron reduced relative manganate
:
In general, if electron transfer is faster than ligand substitution, the reaction will follow the outer-sphere electron transfer.
Often occurs when one/both reactants are inert or if there is no suitable bridging ligand.
A key concept of Marcus theory
is that the rates of such self-exchange reactions are mathematically related to the rates of "cross reactions". Cross reactions entail partners that differ by more than their oxidation states. One example (of many thousands) is the reduction of permanganate by iodide
to form iodine
and, again, manganate.
. Theories addressing heterogeneous electron transfer have applications in electrochemistry
and the design of solar cell
s.
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
, Alexender M. Kuznetsov, and others proceeding from the Fermi's Golden Rule
and following earlier work in non-radiative transitions. Furthermore, theories have been forwarded to take into account the effects of vibronic coupling
on electron transfer. In particular the PKS theory of electron transfer.
Before 1991, ET in metalloproteins was thought to affected primarily be the average, diffuse properties of the non-metal atoms forming an insulated barrier between the metals, but Beratan, Betts and Onuchic subsequently showed that the ET rates are governed by the bond structures of the proteins, that the electrons, in effect, tunnel through the bonds comprising the chain structure of the proteins..
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
moves from an atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or a chemical species
Chemical species
Chemical species are atoms, molecules, molecular fragments, ions, etc., being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a...
(e.g. a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
) to another atom or chemical species. ET is a mechanistic description of the thermodynamic concept of redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, wherein the oxidation states of both reaction partners change.
Numerous biological processes involve ET reactions, including oxygen binding, photosynthesis, respiration, and detoxification routes. Additionally, the process of energy transfer
Energy transfer
Energy transfer is the transfer of energy from one body to another.There are a few main ways that energy transfer occurs:*Radiant energy *Heat conduction*Convection*Electrical power transmission*Mechanical work...
can be formalized as a two-electron exchange (two concurrent ET events in opposite directions) in case of small distances between the transferring molecules. ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
.
Classes of electron transfer
There are several classes of electron transfer, defined by the state of the two redox centers and their connectivityInner-sphere electron transfer
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 ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
is the bridging ligand that covalently connects the redox partners.
Outer-sphere electron transfer
In outer-sphere ET reactions, the participating redox centers are not linked via any bridge during the ET event. Instead, the electron "hops" through space from the reducing center to the acceptor. Outer sphere electron transfer can occur between different chemical species or between identical chemical species that differ only in their oxidation state. The later process is termed self-exchange. As an example, self-exchange describes the degenerateDegenerate energy level
In physics, two or more different quantum states are said to be degenerate if they are all at the same energy level. Statistically this means that they are all equally probable of being filled, and in Quantum Mechanics it is represented mathematically by the Hamiltonian for the system having more...
reaction between permanganate
Permanganate
A permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent. The ion has tetrahedral geometry...
and its one-electron reduced relative manganate
Manganate
In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom. However, the name is usually used to refer to the tetraoxidomanganate anion, MnO, also known as manganate because it contains manganese in the +6 oxidation state...
:
- [MnO4]- + [Mn*O4]2- → [MnO4]2- + [Mn*O4]-
In general, if electron transfer is faster than ligand substitution, the reaction will follow the outer-sphere electron transfer.
Often occurs when one/both reactants are inert or if there is no suitable bridging ligand.
A key concept of Marcus theory
Marcus Theory
Marcus Theory is a theory originally developed by Rudolph A. Marcus, starting in 1956, to explain the rates of electron transfer reactions – the rate at which an electron can move or jump from one chemical species to another...
is that the rates of such self-exchange reactions are mathematically related to the rates of "cross reactions". Cross reactions entail partners that differ by more than their oxidation states. One example (of many thousands) is the reduction of permanganate by iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
to form iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
and, again, manganate.
Five steps of an outer sphere reaction
- 1. reactants diffuse together out of their solvent shells => precursor complex (requires work =wr)
- 2. changing bond lengths, reorganize solvent => activated complex
- 3. Electron transfer
- 4. Relaxation of bond lengths, solvent molecules => successor complex
- 5. Diffusion of products (requires work=wp)
Heterogeneous electron transfer
In heterogeneous electron transfer, an electron moves between a chemical species and a solid-state electrodeElectrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
. Theories addressing heterogeneous electron transfer have applications in electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
and the design of solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s.
Theory
The first generally accepted theory of ET was developed by Rudolph A. MarcusRudolph A. Marcus
Rudolph "Rudy" Arthur Marcus is a Canadian-born chemist who received the 1992 Nobel Prize in Chemistry for his theory of electron transfer. Marcus theory, named after him, provides a thermodynamic and kinetic framework for describing one electron outer-sphere electron transfer.He was born in...
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
Marcus Theory
Marcus Theory is a theory originally developed by Rudolph A. Marcus, starting in 1956, to explain the rates of electron transfer reactions – the rate at which an electron can move or jump from one chemical species to another...
, 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
Joshua Jortner
Joshua Jortner is an Israeli physical chemist. He is a Professor Emeritus at School of Chemistry, The Sackler Faculty of Exact Sciences, Tel Aviv University in Tel Aviv, Israel.- Birth and education :...
, Alexender M. Kuznetsov, and others proceeding from the Fermi's Golden Rule
Fermi's golden rule
In quantum physics, Fermi's golden rule is a way to calculate the transition rate from one energy eigenstate of a quantum system into a continuum of energy eigenstates, due to a perturbation....
and following earlier work in non-radiative transitions. Furthermore, theories have been forwarded to take into account the effects of vibronic coupling
Vibronic coupling
In theoretical chemistry, the vibronic coupling terms, , are proportional to the interaction between electronic and nuclear motions of molecules. The term "vibronic" originates from the concatenation of the terms "vibrational" and "electronic"...
on electron transfer. In particular the PKS theory of electron transfer.
Before 1991, ET in metalloproteins was thought to affected primarily be the average, diffuse properties of the non-metal atoms forming an insulated barrier between the metals, but Beratan, Betts and Onuchic subsequently showed that the ET rates are governed by the bond structures of the proteins, that the electrons, in effect, tunnel through the bonds comprising the chain structure of the proteins..