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Cyclic nitrogen compound with formula HN5 From Wikipedia, the free encyclopedia
Pentazole is an aromatic molecule consisting of a five-membered ring with all nitrogen atoms, one of which is bonded to a hydrogen atom. It has the molecular formula HN5. Although strictly speaking a homocyclic, inorganic compound, pentazole has historically been classed as the last in a series of heterocyclic azole compounds containing one to five nitrogen atoms. This set contains pyrrole, imidazole, pyrazole, triazoles, tetrazole, and pentazole.
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Names | |||
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Systematic IUPAC name
1H-Pentazole[1] | |||
Identifiers | |||
3D model (JSmol) |
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PubChem CID |
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CompTox Dashboard (EPA) |
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Properties | |||
N 5H | |||
Molar mass | 71.0414 g/mol | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Substituted analogs of pentazole are collectively known as pentazoles. As a class, they are unstable and often highly explosive compounds. The first pentazole synthesized was phenylpentazole, where the pentazole ring is highly stabilized by conjugation with the phenyl ring. The derivative 4-dimethylaminophenylpentazole is among the most stable pentazole compounds known, although it still decomposes at temperatures over 50 °C. It is known that electron-donating groups stabilize aryl pentazole compounds.[2]
The cyclic pentazolium cation (N+
5) is not known due to its probable antiaromatic character; whereas the open-chained pentazenium cation (N+
5) is known. Butler et al. first demonstrated the presence of the cyclic N−
5 in solution through the decomposition of substituted aryl pentazoles at low temperature. The presence of N
5H and N−
5 (held in solution through the interaction with zinc ions) was proven primarily using 15N NMR techniques of the decomposition products.[3] These results were initially challenged by some authors,[4] but subsequent experiments involving the detailed analysis of the decomposition products, complemented by computational studies, bore out the initial conclusion.[5][6][7] The pentazolide anion is not expected to last longer than a few seconds in aqueous solution without the aid of complexing agents. The discovery of pentazoles spurred attempts to create all-nitrogen salts such as N+
5N−
5, which should be highly potent propellants for space travel.
In 2002, the pentazolate anion was first detected with electrospray ionization mass spectrometry[8] In 2016, the ion was also detected in solution.[9] In 2017, white cubic crystals of the pentazolate salt, (N5)6(H3O)3(NH4)4Cl were announced. In this salt, the N−
5 rings are planar. The bond lengths in the ring are 1.309 Å, 1.310 Å, 1.310 Å, 1.324 Å, and 1.324 Å.[10] When heated, the salt is stable up to 117 °C, and over this temperature it decomposes to ammonium azide.[10] Under extreme pressure conditions, the pentazolate ion was also synthesized. It was first obtained in 2016 in the form of the CsN5 salt by compressing and laser-heating a mixture of CsN3 embedded in molecular N2 at 60 GPa. Following the pressure release, it was found metastable down to 18 GPa.[11] In 2018, another team reported the high pressure synthesis of LiN5 above 45 GPa from a pure lithium surrounded by molecular nitrogen. This compound could be retained down to ambient conditions after the complete release of pressure.[12]
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