chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, a diradical is a molecular species with two

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s occupying

molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...

s (MOs) which are degenerate. The term "diradical" is mainly used to describe

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s, where most diradicals are extremely reactive and

non-Kekulé molecule A non-Kekulé molecule is a conjugated system, conjugated hydrocarbon that cannot be assigned a classical Kekulé structure. Since non-Kekulé molecules have two or more formal charges or radical (chemistry), radical centers, their Spin (physics ...

s that are rarely isolated. Diradicals are even-electron molecules but have one fewer bond than the number permitted by the

octet rule The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The ru ...

. Examples of diradical species can also be found in

coordination chemistry A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

, for example among bis(1,2-dithiolene) metal complexes.

Spin states

Diradicals are usually

triplets A multiple birth is the culmination of a multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such births ...

. The phrases ''singlet'' and ''triplet'' are derived from the multiplicity of states of diradicals in

electron spin resonance Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spin ...

: a singlet diradical has one state (S=0, M_s=2*0+1=1, m_s=0) and exhibits no signal in EPR and a triplet diradical has 3 states (S=1, M_s=2*1+1=3, m_s=-1; 0; 1) and shows in EPR 2 peaks (if no hyperfine splitting). The triplet state has total

spin quantum number In physics and chemistry, the spin quantum number is a quantum number (designated ) that describes the intrinsic angular momentum (or spin angular momentum, or simply ''spin'') of an electron or other particle. It has the same value for all ...

S=1 and is

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

. Therefore, diradical species display a triplet state when the two electrons are unpaired and display the same spin. When the unpaired electrons with opposite spin are antiferromagnetically coupled, diradical species can display a singlet state (S=0) and be

diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...

Examples

Stable, isolable, diradicals include

singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemistry, inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are Radical (chemistry), spin p ...

and

triplet oxygen Triplet oxygen, 3O2, refers to the ''S'' = 1 electronic ground state of molecular oxygen (dioxygen). Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradi ...

. Other important diradicals are certain

carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a Valence (chemistry), valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. Th ...

nitrene In chemistry, a nitrene or imene () is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and valence (chemistry)#monovalent, monovalent, so it has only 6 electrons in its valence level—two covalent bonded and four non-bonded e ...

s, and their main-group elemental analogues. Lesser-known diradicals are nitrenium ions, carbon chains, and organic so-called

s in which the electrons reside on different carbon atoms. Main-group cyclic structures can also exhibit diradicals, such as disulfur dinitride, or diradical character, such as

diphosphadiboretanes 1,3-Diphospha-2,4-diboretanes, or B2P2, is a class of 4-member cyclic compound, cyclic compounds of alternating boron and phosphorus atoms. They are often found as dimers during the synthesis of Group 13/15 multiple bonds#Boraphosphenes(P=B), borap ...

. In inorganic chemistry, both homoleptic and

heteroleptic In inorganic chemistry, a homoleptic chemical compound is a Coordination complex, metal compound with all ligands identical. The term uses the "homo-" prefix to indicate that something is the same for all. Any metal species which has more than one ...

1,2-dithiolene complexes of d⁸ transition metal ions show a large degree of diradical character in the ground state.

Spin states

Examples

References

Further reading