Linear acetylenic carbon
Linear acetylenic carbon, also called carbyne, is an allotrope of carbon that has the chemical structure (−C≡C−)n as a repeating chain, with alternating single and triple bonds. It would thus be the ultimate member of the polyyne family.
This type of carbyne is of considerable interest to nanotechnology as its Young's modulus is 32.7 TPa – forty times that of diamond, the hardest known material. It has also been identified in interstellar space, however, its existence in condensed phases has been contested recently, as such chains would crosslink exothermically (and perhaps explosively) if they approached each other.
History and controversy
The first claims of detection of this allotrope were made by V. I. Kasatochkin and others in 1967 and repeated in 1978. However, in 1982 P. P. K. Smith and P. R. Buseck re-examined samples from several previous reports and showed that the signals attributed to carbyne were in fact due to silicate impurities in the samples.
In 1984, a group at Exxon reported the detection of clusters with even numbers of carbons, between 30 and 180, in carbon evaporation experiments, and attributed them to polyyne carbon. However, these clusters later were identified as fullerenes.
In 1995, the preparation of carbyne chains with over 300 carbons was reported. They were claimed to be reasonably stable, even against moisture and oxygen, as long as the terminal alkynes on the chain are capped with inert groups (such as tert-butyl or trifluoromethyl) rather than hydrogen atoms. The study claimed that the data specifically indicated a carbyne-like structures rather than fullerene-like ones. However, according to H. Kroto, the properties and synthetic methods used in those studies are consistent with generation of fullerenes.
Another 1995 report claimed detection of carbyne chains of indeterminate length in a layer of carbonized material, about 180 nm thick, resulting from the reaction of solid polytetrafluoroethylene (PTFE, Teflon) immersed in alkali metal amalgam at ambient temperature (with no hydrogen-bearing species present). The assumed reaction was
2−)n + 4 M → (−C≡C−)n + 4 MF,
In 1999, F. Cataldo observed that copper(I) acetylide ((Cu+
2) after partial oxidation by exposure to air or copper(II) ions releases polyynes H(−C≡C−)nH, with n from 2 to 6, when decomposed by hydrochloric acid, and leaves a "carbonaceous" residue with the spectral signature of (−C≡C−)n chains. He conjectured that the oxidation causes polymerization of the acetylide anions C2−
2 into carbyne-type anions C(≡C−C≡)nC2− or cumulene-type anions C(=C=C=)mC4−. Also, thermal decomposition of copper acetylide in vacuum yielded a fluffy deposit of fine carbon powder on the walls of the flask, which, on the basis of spectral data, was claimed to be carbyne rather than graphite. Finally, the oxidation of copper acetylide in ammoniacal solution (Glaser's reaction) produces a carbonaceous residue that was claimed to consist of "polyacetylide" anions capped with residual copper(I) ions,
On the basis of the residual amount of copper, the mean number of units n was estimated to be around 230.
In 2004, an analysis of a synthesized linear carbon allotrope found it to have a cumulene electronic structure—sequential double bonds along an sp-hybridized carbon chain—rather than the alternating triple–single pattern of linear carbyne.
While the existence of "carbyne" chains in pure neutral carbon material is still disputed, short (−C≡C−)n chains are well established as substructures of larger molecules (polyynes) and are even synthesized by several living organisms. As of 2010, the longest such chain in a stable molecule had 22 acetylenic units (44 atoms), stabilized by rather bulky end groups.
Other possible configurations for a chain of carbon atoms include polycumulene (polyethylene-diylidene) chains with double bonds only (128.2 pm). This chain is expected to have slightly higher energy, with a Peierls gap of 2 to 5 eV. For short C
n Cn molecules, however, the polycumulene structure seems favored. When n is even, two ground configurations, very close in energy, may coexist, one linear and one cyclic (rhombic).
The limits of flexibility of the carbyne chain are illustrated by a synthetic polyyne with a backbone of 8 acetylenic units, whose chain was found to be bent by 25 degrees or more (about 3 degrees at each carbon) in the solid state, to accommodate the bulky end groups of adjacent molecules.
The highly symmetric carbyne chain is expected to have only one Raman-active mode with Σg symmetry, due to stretching of bonds in each single-double pair, with frequency typically between 1950 and 2300 cm−1.
Carbyne chains have been claimed to be the strongest material known. Calculations indicate that carbyne’s tensile strength of 6.0–7.5×107 N·m/kg beats graphene (4.7–5.5×107 N·m/kg), carbon nanotubes (4.3–5.0×107 N·m/kg), and diamond (2.5–6.5×107 N·m/kg). Its stiffness of around 9 N·m/kg is also double that of graphene, which is around 104.5×108 N·m/kg.
Stretching carbyne as little as 10% alters its electronic band gap from 3.2 to 4.4 eV. Outfitted with molecular handles at chain's ends, it can also be twisted to alter its band gap. With a 90-degree end-to-end rotation, it becomes a magnetic semiconductor just by stretching the material by 10% and finally, when twisted by 90°, carbyne also turns into a magnetic semiconductor.
The material is stable at room temperature, largely resisting crosslinks with nearby chains. The rods' stiffness prevents them from coming together in a second location, at least at room temperature.
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