| 词条 | Dimer (chemistry) |
| 释义 |
A dimer ({{IPAc-en|ˈ|d|aɪ|m|ər}}) (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. 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.[1] Noncovalent dimersCarboxylic 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 acid]ity of the boron center. [Excimer]s 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 dimersMolecular 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. 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. Other metals may form a proportion of dimers in their vapour. Known metallic dimers include Li2, Na2, K2, Rb2 and Cs2. Many small organic molecules, most notably formaldehyde, easily form dimers. The dimer of formaldehyde (CH2O) is dioxetane (C2H4O2). Polymer chemistryIn 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 dimersPyrimidine 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{{Commons|Dimers}}
References
1. ^{{Cite journal|last=Adar|first=Ram M.|last2=Markovich|first2=Tomer|last3=Andelman|first3=David|date=2017-05-17|title=Bjerrum pairs in ionic solutions: A Poisson-Boltzmann approach|url=http://aip.scitation.org/doi/10.1063/1.4982885|journal=The Journal of Chemical Physics|volume=146|issue=19|pages=194904|doi=10.1063/1.4982885|issn=0021-9606|arxiv=1702.04853|bibcode=2017JChPh.146s4904A}} 2 : Chemical compounds|Dimers (chemistry) |
| 随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识，基本涵盖了大多数领域的百科知识，是一部内容自由、开放的电子版国际百科全书。