Proton coupled electron transfer
Encyclopedia
Proton-coupled electron transfer (PCET) is a reaction mechanism
that is thought to be common in redox
reactions. It involves the concerted transfer of an electron
and proton
to or from a substrate.
In PCET, the proton and the electron (i) start from different orbitals and (ii) are transferred to different orbitals. They transfer in a concerted elementary step. PCET contrast to step-wise mechanisms in which the electron and proton are transferred sequentially.
, nitrogen fixation
and oxygen reduction in many pathways for respiration
. Inorganic chemists often study simple reactions to test this mechanism, one example being the comproportionation of a Ru(II) aquo and a Ru(IV) oxo reactants:
cis-[(bipy
)2(py)RuIV(O)]2+ + cis-[(bipy)2(py)RuII(OH2)]2+ → 2cis-[(bipy)2(py)RuIII(OH)]2+
PCET is also often invoked in electrochemical reactions where reduction is coupled to protonation or where oxidation is coupled to deprotonation.
Although it is relatively simple to demonstrate that the electron and proton begin and end in different orbitals, it is more difficult to prove that they do not move sequentially. General sequential pathways are lower in energy than concerted pathways. The main evidence that PCET exists is that a number of reactions occur faster than expected for the sequential pathways. In the initial electron transfer (ET) mechanism the initial redox event has a minimum thermodynamics barrier associate with the first step. Similarly the initial proton transfer (PT) mechanism has a minium barrier associated with the protons initial pKa. Variations on these minimum barriers are also considered. The important finding is that there are number of reactions with rates greater than these minimum barriers would permit. This suggests a third mechanism lower in energy, the concerted PCET has been offered as this third mechanism. This assertion has also been supported by the observation of unusually large kinetic isotope effects (KIE).
A typical method for establishing PCET pathway is to show that the individual ET and PT pathways operate at higher activation energy
than the concerted pathway.
In some literature the definition of PCET has been extended to include the sequential mechanisms listed above. This confusion in the definition of PCET has led to the proposal of alternate names including electron transfer-proton transfer (ETPT), electron-proton transfer (EPT), and concerted proton-electron transfer (CPET).
Also distinct is hydrogen atom transfer (HAT) in which the proton and electron start in the same orbital
s and move together to the final orbital. HAT is recognized as a radical
pathway, although the stoichiometry is similar to that for PCET.
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
that is thought to be common in redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reactions. It involves the concerted transfer of an electron
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...
and proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
to or from a substrate.
In PCET, the proton and the electron (i) start from different orbitals and (ii) are transferred to different orbitals. They transfer in a concerted elementary step. PCET contrast to step-wise mechanisms in which the electron and proton are transferred sequentially.
- ET
- [HX] + [M] → [HX]+ + [HM]-
- PT
- [HX] + [M] → [X]- + [HM]+
- PCET
- [HX] + [M] → [X] + [HM]
Examples
PCET is thought to be pervasive in redox reactions appear to be net hydrogenations and dehydrogenations. Relevant examples include water oxidation in photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
and oxygen reduction in many pathways for respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
. Inorganic chemists often study simple reactions to test this mechanism, one example being the comproportionation of a Ru(II) aquo and a Ru(IV) oxo reactants:
cis-[(bipy
2,2'-Bipyridine
2,2'-Bipyridine is a organic compound with the formula . This colorless solid, commonly abbreviated bipy or bpy , is an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals...
)2(py)RuIV(O)]2+ + cis-[(bipy)2(py)RuII(OH2)]2+ → 2cis-[(bipy)2(py)RuIII(OH)]2+
PCET is also often invoked in electrochemical reactions where reduction is coupled to protonation or where oxidation is coupled to deprotonation.
Although it is relatively simple to demonstrate that the electron and proton begin and end in different orbitals, it is more difficult to prove that they do not move sequentially. General sequential pathways are lower in energy than concerted pathways. The main evidence that PCET exists is that a number of reactions occur faster than expected for the sequential pathways. In the initial electron transfer (ET) mechanism the initial redox event has a minimum thermodynamics barrier associate with the first step. Similarly the initial proton transfer (PT) mechanism has a minium barrier associated with the protons initial pKa. Variations on these minimum barriers are also considered. The important finding is that there are number of reactions with rates greater than these minimum barriers would permit. This suggests a third mechanism lower in energy, the concerted PCET has been offered as this third mechanism. This assertion has also been supported by the observation of unusually large kinetic isotope effects (KIE).
A typical method for establishing PCET pathway is to show that the individual ET and PT pathways operate at higher activation energy
Activation energy
In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
than the concerted pathway.
In some literature the definition of PCET has been extended to include the sequential mechanisms listed above. This confusion in the definition of PCET has led to the proposal of alternate names including electron transfer-proton transfer (ETPT), electron-proton transfer (EPT), and concerted proton-electron transfer (CPET).
Also distinct is hydrogen atom transfer (HAT) in which the proton and electron start in the same orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
s and move together to the final orbital. HAT is recognized as a radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
pathway, although the stoichiometry is similar to that for PCET.