Dimer (chemistry)

Dimers of carboxylic acids are often found in vapour phase.

A dimer (/ˈdaɪmər/) (di-, "two" + -mer, "parts") is an oligomer consisting of two structurally similar monomers joined by bonds that can be either strong or weak, covalent or intermolecular. 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.

Noncovalent dimers

Carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when anhydrous. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers, e.g. the water dimer.

Borane ("BH₃") occurs as the dimer diborane (B₂H₆), due to the high Lewis acidity of the boron center.

Excimers and exciplexes are excited structures with a short lifetime. For example, noble gases do not form stable dimers, but do form the excimers Ar₂*, Kr₂* and Xe₂* under high pressure and electrical stimulation.

Covalent dimers

The dimer of cyclopentadiene although this might not be readily apparent on initial inspection

Molecular dimers are often formed by the reaction of two identical compounds e.g.: 2A → A-A. In this example, monomer "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. Mercury occurs as a mercury(I) cation (Hg₂²⁺), formally a heterodimer.

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 particularly 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.

Biochemical dimers

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

See also

• Monomer
• Trimer
• Polymer
• Protein dimer