Linear acetylenic carbon
Linear acetylenic carbon (LAC), 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. 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.
The first claims of detection of this allotrope were made by V. I. Kasatochkin, Y. P. Kudryavtsev and others in 1960 and repeated in 1978. 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. Absence of carbyne crystalline rendered the direct observation of a pure carbyne-assembled solid still a major challenge, because carbyne crystals with well-defined structures and sufficient sizes are not available to date. This is indeed the major obstacle to general acceptance of carbyne as a true carbon allotrope. The mysterious carbyne still attracted scientists with its possible extraordinary properties.
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 1991, carbyne was allegedly detected among various other allotropes of carbon in samples of amorphous carbon black vaporized and quenched by shock waves produced by shaped explosive charges.
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.
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