Outer sphere electron transfer
Encyclopedia
Outer sphere refers to an electron transfer
(ET) event that occurs between chemical species that remain separate intact before, during, and after the ET event. In contrast, for inner sphere electron transfer
the participating redox sites undergoing ET become connected by a chemical bridge. Because the ET in outer sphere electron transfer occurs between two non-connected species, the electron is forced to move through space from one redox
center to the other.
in the 1950s. A major aspect of Marcus theory
is the dependence of the electron transfer rate on the thermodynamic driving force (difference in the redox potentials of the electron-exchanging sites). For most reactions, the rates increase with increased driving force. A second aspect is that the rate of outer-sphere electron-transfer depends inversely on the "reorganizational energy." Reorganization energy describes the changes in bond lengths and angles that are required for the oxidant and reductant to switch their oxidation states. This energy is assessed by measurements of the self-exchange rates (see below). Outer sphere electron transfer is the most common type of electron transfer, especially in biochemistry
, where redox centers are separated by several (up to about 11) angstroms by intervening protein. In biochemistry, there are two main types of outer sphere ET: ET between two biological molecules or fixed distance electron transfer, in which the electron transfers within a single biomolecule (e.g., intraprotein).
reaction between the tetrahedral ions permanganate
and manganate
:
For octahedral metal complexes, the rate constant for self-exchange reactions correlates with changes the population of the eg orbitals, the population of which most strongly affects the length of metal-ligand bonds:
s. The Fe centers are typically further coordinated by cysteinyl ligands. The [Fe4S4] electron-transfer proteins ([Fe4S4] ferredoxin
s) may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins. Low- and high-potential ferredoxins are related by the following redox scheme:
Because of the small structural differences between the individual redox states, ET is rapid between these clusters.
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
(ET) event that occurs between chemical species that remain separate intact before, during, and after the ET event. In contrast, for inner sphere electron transfer
Inner sphere electron transfer
Inner sphere or bonded electron transfer proceeds via a covalent linkage between the two redox partners, the oxidant and the reductant. In Inner Sphere electron transfer , a ligand bridges the two metal redox centers during the electron transfer event. Inner sphere reactions are inhibited by...
the participating redox sites undergoing ET become connected by a chemical bridge. Because the ET in outer sphere electron transfer occurs between two non-connected species, the electron is forced to move through space from one redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
center to the other.
Marcus Theory
The main theory that describes the rates of outer-sphere electron transfer 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...
in the 1950s. A major aspect 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 the dependence of the electron transfer rate on the thermodynamic driving force (difference in the redox potentials of the electron-exchanging sites). For most reactions, the rates increase with increased driving force. A second aspect is that the rate of outer-sphere electron-transfer depends inversely on the "reorganizational energy." Reorganization energy describes the changes in bond lengths and angles that are required for the oxidant and reductant to switch their oxidation states. This energy is assessed by measurements of the self-exchange rates (see below). Outer sphere electron transfer is the most common type of electron transfer, especially in biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
, where redox centers are separated by several (up to about 11) angstroms by intervening protein. In biochemistry, there are two main types of outer sphere ET: ET between two biological molecules or fixed distance electron transfer, in which the electron transfers within a single biomolecule (e.g., intraprotein).
Self-exchange
Outer sphere electron transfer can occur between chemical species that are identical save for their oxidation state. This process is termed self-exchange. An example is 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 the tetrahedral ions 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 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]-
For octahedral metal complexes, the rate constant for self-exchange reactions correlates with changes the population of the eg orbitals, the population of which most strongly affects the length of metal-ligand bonds:
- For the [Co(bipy)3]+/[Co(bipy)3]2+ pair, self exchange proceeds at 109 M-1s-1. In this case, the electron configuration changes from (t2g)6(eg)2 (for Co(I)) to (t2g)5(eg)2 (for Co(II)).
- For the [Co[bipy)3]2+/[Co(bipy)3]3+ pair, self exchange proceeds at 18 M-1s-1. In this case, the electron configuration changes from (t2g)5(eg)2 (for Co(II)) to (t2g)6(eg)0 (for Co(III)).
Iron-sulfur proteins
Outer sphere ET is the basis of the biological function of the iron-sulfur proteinIron-sulfur protein
Iron-sulfur proteins are proteins characterized by the presence of iron-sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states...
s. The Fe centers are typically further coordinated by cysteinyl ligands. The [Fe4S4] electron-transfer proteins ([Fe4S4] ferredoxin
Ferredoxin
Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
s) may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins. Low- and high-potential ferredoxins are related by the following redox scheme:
Because of the small structural differences between the individual redox states, ET is rapid between these clusters.