Dimer (chemistry)  A dimer () ("di-", "two" + "-mer", "parts") is an oligomer consisting of two 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. The reverse of dimerisation is often called dissociation. When two oppositely charged ions associate into dimers, they are referred to as "Bjerrum pairs". 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. 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: 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:  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 occurs as a mercury(I) cation (Hg22+), formally a dimeric ion.