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A dimer () ('' di-'', "two" + ''-mer'', "parts") is an
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few similar or identical repeating units which could be derived, actually or conceptually, from copies of a smaller molecule, its monomer. Quote: ''Oligomer molecule: A m ...
consisting of two
monomerA monomer ( ; ''mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Monomers can be clas ...
s joined by bonds that can be either strong or weak,
covalent A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
or
intermolecular Intermolecular forces (IMF) (or secondary forces) are the forces which mediate interaction between molecules, including forces of attraction or repulsion which act between atoms and other types of neighboring particles, e.g. atoms or ions. Intermole ...
. The term ''homodimer'' is used when the two molecules are identical (e.g. A–A) and ''heterodimer'' when they are not (e.g. A–B). The reverse of dimerisation is often called
dissociation Dissociation, in the wide sense of the word, is an act of disuniting or separating a complex object into parts. Dissociation may also refer to: * Dissociation (chemistry), general process in which molecules or ionic compounds (complexes, or salts) s ...
. When two oppositely charged ions associate into dimers, they are referred to as ''Bjerrum pairs''.


Noncovalent dimers

Carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic a ...
s form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when
anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achiev ...
. For example,
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
forms a dimer in the gas phase, where the monomer units are held together by
hydrogen bond#REDIRECT Hydrogen bond#REDIRECT Hydrogen bond {{R from other capitalisation ...
{{R from other capitalisation ...
s. Under special conditions, most OH-containing molecules form dimers, e.g. the
water dimer Ball-and-stick model of the linear water dimer The water dimer consists of two water molecules loosely bound by a hydrogen bond. It is the smallest water cluster. Because it is the simplest model system for studying hydrogen bonding in water, it h ...
. Borane ("BH3") occurs as the dimer
diborane Diborane(6), generally known as diborane, is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless, pyrophoric gas with a repulsively sweet odor. Synonyms include boroethane, boron hydride, and diboron hex ...
(B2H6), due to the high
Lewis acidity A Lewis acid is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a filled orbital containing an electron pair whi ...
of the
boron Boron is a chemical element with the symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low-abundance element in the Solar System and in the Earth's cru ...
center.
Excimers An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has a valence shell completely filled with electrons (for example, noble gases). In this case, formati ...
and exciplex, exciplexes are Excited state, excited structures with a short lifetime. For example, noble gases do not form stable dimers, but they (dimers) do form the excimers Ar2*, Kr2* and Xe2* under high pressure and electrical stimulation.


Covalent dimers

Molecular dimers are often formed by the reaction of two identical compounds e.g.: 2A → A-A. In this example,
monomerA monomer ( ; ''mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Monomers can be clas ...
"A" is said to dimerise to give the dimer "A-A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene: :2 C(NR2)2 → (R2N)2C=C(NR2)2 Carbenes are highly reactive and readily form bonds. Dicyclopentadiene is an asymmetrical dimer of two cyclopentadiene molecules that have reacted in a Diels-Alder reaction to give the product. Upon heating, it "cracks" (undergoes a retro-Diels-Alder reaction) to give identical monomers: :C10H12 → 2 C5H6 Many nonmetallic elements occur as dimers: hydrogen, nitrogen, oxygen, the halogens, i.e. fluorine, chlorine, bromine and iodine. Noble gases can form dimers linked by van der Waals bonds, for example dihelium or diargon. Mercury (element), Mercury occurs as a mercury(I) cation (Hg22+), formally a dimeric ion. Other metals may form a proportion of dimers in their vapour. Known metallic dimers include dilithium, Li2, disodium, Na2, dipotassium, K2, dirubidium, Rb2 and dicaesium, Cs2. Many small organic molecules, most notably formaldehyde, easily form dimers. The dimer of formaldehyde (CH2O) is dioxetane (C2H4O2).


Polymer chemistry

In the context of polymers, "dimer" also refers to the degree of polymerization 2, regardless of the stoichiometry or condensation reactions. This is applicable to disaccharides. For example, cellobiose is a dimer of glucose, even though the formation reaction produces water: : 2C6H12O6 → C12H22O11 + H2O Here, the dimer has a stoichiometry different from the pair of monomers. Amino acids can also form dimers, which are called dipeptides. An example is glycylglycine, consisting of two glycine molecules joined by a peptide bond. Other examples are aspartame and carnosine.


Biochemical dimers

Pyrimidine dimers are formed by a photochemical reaction from pyrimidine DNA bases. This cross-linking causes Mutation, DNA mutations, which can be carcinogenic, causing skin cancers.


See also

*Monomer *trimer (chemistry), Trimer *Polymer *Protein dimer


References

* {{cite web , url=http://goldbook.iupac.org/D01744.html , title=IUPAC "Gold Book" definition , access-date=2009-04-30 Chemical compounds Dimers (chemistry),