On Air

Investment

Buy this Domain?
Do you interesting about this domain and the running project?
Feel free to send your offer to webmaster.
pay with Paypal

Advertising

Dimer (chemistry)

s are often found in vapour phase.]] A dimer () (, "two" + , "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. When two oppositely charged ions associate into dimers, they are referred to as Bjerrum pairs .

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

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 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 DNA mutations, which can be carcinogenic, causing skin cancers.

See also

References

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Dimer_(chemistry), the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Dimer_(chemistry)&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany