Triplet oxygen
Encyclopedia
Triplet oxygen is the ground state
of the oxygen
molecule. The electron configuration
of the molecule has two unpaired electrons occupying two degenerate
molecular orbitals. These orbitals are classified as antibonding
, so the O-O bond is weaker than the N-N bond in molecular nitrogen
, where all bonding molecular orbitals are filled.
The s=1/2 spin
s of the two electrons in degenerate orbitals gives rise to 2x2=4 independent spin states in total. Exchange interaction
splits these into a singlet
state (total spin S=0) and a set of 3 degenerate triplet
states (S=1). In agreement with Hund's rules, the triplet states are energetically more favorable, and the ground state of the molecule carries an electron spin of S=1. Excitation to the S=0 state results in much more reactive, metastable singlet oxygen
.
Because the molecule in its ground state has a non-zero spin magnetic moment
, oxygen
is paramagnetic; i.e., it can be attracted to the poles of a magnet
. The Lewis structure
O=O does not accurately represent the diradical
nature of molecular oxygen; molecular orbital theory
must be used to adequately account for the unpaired electrons. Triplet oxygen is better described by a bond order of one and two halves instead of two to better reflect its unpaired bonding electrons. This allows for easier reasoning of the bond length.
The unusual electron configuration prevents molecular oxygen from reacting directly with many other molecules, which are often in the singlet
state. Triplet oxygen will, however, readily react with molecules in a doublet state, such as radicals, to form a new radical. Conservation of spin quantum number would require a triplet transition state
in a reaction of triplet oxygen with a closed shell (a molecule in a singlet state). The extra energy required is sufficient to prevent direct reaction at ambient temperatures with all but the most reactive substrates, e.g. white phosphorus. At higher temperatures or in the presence of suitable catalysts the reaction proceeds more readily. For instance, most flammable substances are characterised by an autoignition temperature
at which they will undergo combustion in air without an external flame or spark.
Singlet oxygen
, where the electron spins are opposed in a higher energy state, is many times more reactive than triplet oxygen, and extremely hazardous to organic matter. It is used for the extermination of heavy infestations of persistent pests in houses and buildings.
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
of the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
molecule. The electron configuration
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
of the molecule has two unpaired electrons occupying two degenerate
Degenerate 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...
molecular orbitals. These orbitals are classified as antibonding
Antibonding
Antibonding is a type of chemical bonding. An antibonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei...
, so the O-O bond is weaker than the N-N bond in molecular nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, where all bonding molecular orbitals are filled.
Overview
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
s of the two electrons in degenerate orbitals gives rise to 2x2=4 independent spin states in total. Exchange interaction
Exchange interaction
In physics, the exchange interaction is a quantum mechanical effect without classical analog which increases or decreases the expectation value of the energy or distance between two or more identical particles when their wave functions overlap...
splits these into a singlet
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
state (total spin S=0) and a set of 3 degenerate triplet
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
states (S=1). In agreement with Hund's rules, the triplet states are energetically more favorable, and the ground state of the molecule carries an electron spin of S=1. Excitation to the S=0 state results in much more reactive, metastable singlet oxygen
Singlet oxygen
Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment...
.
Because the molecule in its ground state has a non-zero spin magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is paramagnetic; i.e., it can be attracted to the poles of a magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
. The Lewis structure
Lewis structure
Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. A Lewis structure can be drawn for any covalently bonded molecule, as well as coordination compounds...
O=O does not accurately represent the diradical
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
nature of molecular oxygen; molecular orbital theory
Molecular orbital theory
In chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule...
must be used to adequately account for the unpaired electrons. Triplet oxygen is better described by a bond order of one and two halves instead of two to better reflect its unpaired bonding electrons. This allows for easier reasoning of the bond length.
The unusual electron configuration prevents molecular oxygen from reacting directly with many other molecules, which are often in the singlet
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
state. Triplet oxygen will, however, readily react with molecules in a doublet state, such as radicals, to form a new radical. Conservation of spin quantum number would require a triplet transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
in a reaction of triplet oxygen with a closed shell (a molecule in a singlet state). The extra energy required is sufficient to prevent direct reaction at ambient temperatures with all but the most reactive substrates, e.g. white phosphorus. At higher temperatures or in the presence of suitable catalysts the reaction proceeds more readily. For instance, most flammable substances are characterised by an autoignition temperature
Autoignition temperature
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion...
at which they will undergo combustion in air without an external flame or spark.
Singlet oxygen
Singlet oxygen
Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment...
, where the electron spins are opposed in a higher energy state, is many times more reactive than triplet oxygen, and extremely hazardous to organic matter. It is used for the extermination of heavy infestations of persistent pests in houses and buildings.
External Sources
- http://missouri.edu/~chemrg/wade4e/chapter15/dioxygen_t.html
- http://meta-synthesis.com/webbook/16_diradical/diradical.html