Non-Kekulé Molecule
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A non-Kekulé molecule is a conjugated
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
that cannot be assigned a classical Kekulé structure. Since non-Kekulé molecules have two or more formal charges or
radical Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...
centers, their spin-spin interactions can cause
electrical conductivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
or
ferromagnetism Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...
(
molecule-based magnets Molecule-based magnets (MBMs) or molecular magnets are a class of materials capable of displaying ferromagnetism and other more complex magnetic phenomena. This class expands the materials properties typically associated with magnets to include lo ...
), and applications to functional materials are expected. However, as these molecules are quite reactive and most of them are easily decomposed or
polymerized In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many form ...
at room temperature, strategies for stabilization are needed for their practical use. Synthesis and observation of these reactive molecules are generally accomplished by
matrix-isolation Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous solid phase in which ''guest'' particles (atoms, molecules, ...
methods.


Biradicals

The simplest non-Kekulé molecules are biradicals. A biradical is an even-electron
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with two
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...
centres which act ''independently'' of each other. They should not be confused with the more general class of
diradical In chemistry, a diradical is a chemical species, molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate energy level, degenerate. The term "diradical" is mainly used to describe organic compounds, where most ...
s. One of the first biradicals was synthesized by Wilhelm Schlenk in 1915 following the same methodology as
Moses Gomberg Moses Gomberg (February 8, 1866 – February 12, 1947) was a chemistry professor at the University of Michigan. He was elected to the National Academy of Sciences and the American Philosophical Society, and served as president of the American Chem ...
's
triphenylmethyl radical The triphenylmethyl radical (often shortened to trityl radical after 1927 suggestion by Burckhardt Helferich, Helferich et al.) is an organic compound with the formula (C6H5)3C. It is a persistent radical. It was the first radical (chemistry), rad ...
. The so-called Schlenk-Brauns hydrocarbons are: Eugene Müller, with the aid of a
Gouy balance The Gouy balance, invented by the French physicist Louis Georges Gouy, is a device for measuring the magnetic susceptibility of a sample. The Gouy balance operates on magnetic torque, by placing the sample on a horizontal arm or beam suspended by ...
, established for the first time that these compounds are
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, ...
with a triplet ground state. Another classic biradical was synthesised by
Aleksei Chichibabin Alekséy Yevgényevich Chichibábin (; – 15 August 1945) was a Soviet Union, Soviet Russian organic chemist. His name is also written ''Alexei Yevgenievich Chichibabin'' and ''Alexei Euguenievich Tchitchibabine''. Life Chichibábin was born ...
in 1907. Other classical examples are the biradicals described by Yang in 1960 and by Coppinger in 1962.


Trimethylenemethane

A well studied biradical is
trimethylenemethane Trimethylenemethane (often abbreviated TMM) is a chemical compound with molecular formula, formula . It is a Charge (chemistry), neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, ...
(TMM), . In 1966 Paul Dowd determined with
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 ...
that this compound also has a
triplet state In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = â ...
. In a crystalline host the 6 hydrogen atoms in TMM are identical.


Quinodimethanes and PAHs

Other examples of non-Kekulé molecules are the
biradicaloid Biradicaloids or diradicaloids are molecules with two radical electrons that have significant interaction with each other. The two unpaired electrons are coupled and can either form a singlet ground state ( antiferromagnetic coupling) or a tripl ...
quinodimethane In organic chemistry, a xylylene (sometimes quinone-dimethide) is any of the constitutional isomers having the formula C6H4(CH2)2. These compounds are related to the corresponding quinones and quinone methides by replacement of the oxygen atoms ...
s, that have a six-membered ring with methylene substituents. Non-Kekulé
polynuclear aromatic hydrocarbons A Polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incine ...
are composed of several fused six-membered rings. The simplest member of this class is
triangulene Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid. It exists as a biradical with the chemical formula . It was first hypothesized by Czech chemist Erich Clar in 1953. Its first confirmed synthesi ...
. After unsuccessful attempts by
Erich Clar Erich Clar (23 August 1902 – 27 March 1987) was a German organic chemistry, organic chemist, born in Hřensko (a village directly at the border of Germany and the Czech Republic; Czech: Hřensko, German: Herrnskretschen), who studied polycyclic a ...
in 1953, trioxytriangulene was synthesized by Richard J. Bushby in 1995, and kinetically stabilized triangulene by Kazuhiro Nakasuji in 2001. However, in 2017 a project led by David Fox and Anish Mistry from the University of Warwick in collaboration with IBM synthesized and imaged
triangulene Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid. It exists as a biradical with the chemical formula . It was first hypothesized by Czech chemist Erich Clar in 1953. Its first confirmed synthesi ...
. In 2019, larger homologues of triangulene, consisting of ten ( riangulene) and fifteen fused six-membered rings ( riangulene) were synthesized in 2019. In 2021, synthesis of the hitherto largest triangulene homologue, consisting of twenty-eight fused six-membered rings ( riangulene) was achieved. Scanning tunneling microscopy experiments on triangulene spin chains have revealed the clearest proof yet of the existence of Haldane gap and fractional edge states predicted for spin-1 Heisenberg chain. A related class of biradicals are para-benzynes. Other studied biradicals are those based on pleiadene, extended viologens,
corannulene Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20 H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is irculene. It is of scientific interest because it is a geodesi ...
s, nitronyl-nitroxide, bis(phenalenyl)s and teranthenes. Pleiadene has been synthesised from
acenaphthylene Acenaphthylene, a polycyclic aromatic hydrocarbon is an ortho- and peri-fused tricyclic hydrocarbon. The molecule resembles naphthalene with positions 1 and 8 connected by a -CH=CH- unit. It is a yellow solid. Unlike many polycyclic aromatic hy ...
and
anthranilic acid Anthranilic acid is an aromatic acid with the formula C6H4(NH2)(CO2H) and has a sweetish taste. The molecule consists of a benzene ring, ''ortho''-substituted with a carboxylic acid and an amine. As a result of containing both acidic and basic f ...
/
amyl nitrite Amyl nitrite is a chemical compound with the formula C5H11ONO. A variety of isomers are known, but they all feature an amyl group attached to the nitrite functional group. The alkyl group (the amyl in this case) is unreactive and the chemical ...
:


Oxyallyl

The oxyallyl diradical (OXA) is a
trimethylenemethane Trimethylenemethane (often abbreviated TMM) is a chemical compound with molecular formula, formula . It is a Charge (chemistry), neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, ...
molecule with one
methylene group A methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—m ...
replaced by
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. This
reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...
is postulated to occur in ring opening of
cyclopropanone Cyclopropanone is an organic compound with molecular formula (CH2)2CO consisting of a cyclopropane carbon framework with a ketone functional group. The parent compound is labile, being highly sensitive toward even weak nucleophiles. Surrogates o ...
s,
allene oxide In organic chemistry, an allene oxide is an epoxide of an allene. The parent allene oxide is CH2=C(O)CH2 (CAS registry number, RN 40079-14-9), a rare and reactive species of only theoretical interest. Typical allene oxides require steric protecti ...
s and in the
Favorskii rearrangement The Favorskii rearrangement is principally a rearrangement of cyclopropanones and α-halo ketones that leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorskii rearrangement constitutes a ring contraction. This ...
. The intermediate has been produced by reaction of oxygen radical anions with
acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...
and studied by
photoelectron spectroscopy Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in t ...
. The experimental
electron affinity The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy This differs by si ...
of OXA is 1.94 eV.


Classification

Non-Kekulé molecules with two formal radical centers (non-Kekulé diradicals) can be classified into ''non-disjoint'' and ''disjoint'' by the shape of their two non-bonding
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 (NBMOs). Both NBMOs of molecules with ''non-disjoint'' characteristics such as trimethylenemethane have
electron density Electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial variables and is typical ...
at the same
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
. According to Hund's rule, each orbital is filled with one electron with parallel spin, avoiding the
Coulomb repulsion Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called the ''electrostatic f ...
by filling one orbital with two electrons. Therefore, such molecules with ''non-disjoint'' NBMOs are expected to prefer a triplet
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; 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 ...
. In contrast, the NBMOs of the molecules with ''disjoint'' characteristics such as tetramethyleneethane can be described without having electron density at the same atom. With such MOs, the destabilization factor by the Coulomb repulsion becomes much smaller than with ''non-disjoint'' type molecules, and therefore the relative stability of the singlet ground state to the triplet ground state will be nearly equal, or even reversed because of
exchange interaction In chemistry and physics, the exchange interaction is a quantum mechanical constraint on the states of indistinguishable particles. While sometimes called an exchange force, or, in the case of fermions, Pauli repulsion, its consequences cannot alw ...
.


References

IUPAC Gold Book The International Union of Pure and Applied Chemistry (IUPAC) publishes many books which contain its complete list of definitions. The definitions are divided initially into seven IUPAC Colour Books: Gold, Green, Blue, Purple, Orange, White, and R ...
definitions o
''biradical''
an

/ref> Robert A. Moss ed. (2004), "Reactive Intermediate Chemistry" (Book) Wiley-Interscience. M. Baumgarten (2003/2004), "High spin molecules directed towards molecular magnets", chapter 12 in "EPR of free radicals in solids, Trends in methods and application", A. Lund, M. Shiotani (eds), Kluwer, pages 491-528
{{DEFAULTSORT:Non-Kekulé molecule Organic chemistry Free radicals