Linear acetylenic carbon

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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.[1][2] 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 forty times that of diamond, the hardest known material.[3] It has also been identified in interstellar space. However its existence has been contested recently, as it has become clear that such chains would be extremely unstable in condensed form, as they would crosslink exothermically (explosively indeed) if they approached each other.[4]

Contents

History and controversy[edit]

The first claims of detection of this allotrope were made by V. I. Kasatochkin and others in 1967[4][5] and repeated in 1978.[6] 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.[7]

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 polyyine carbon.[8] However those clusters later were identified as fullerenes.[4]

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.[9]

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.[10] However, according to H. Kroto, the properties and synthetic methods used in those studies are consistent with generation of fullerenes.[4]

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).[11] The assumed reaction was

(-CF
2−CF
2-)n + 4 M → (−C≡C−)n + 4 MF

where M is either lithium, sodium, or potassium. The authors conjectured that nanocrystals of the metal fluoride between the chains prevented their polymerization.

In 1999, F. Cataldo observed that copper(I) acetylide ((Cu+
)2C2−
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−.[12] 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.[12] Finally, the oxidation of copper acetylide in ammoniacal solution (Glaser's reaction) produces a carbonaceous residue that was claimed to consist of "polyacetylyde" anions capped with residual copper(I) ions,

Cu+
 −C(≡C−C≡)nC− Cu+

On the basis of the residual amount of copper, the mean number of units n was estimated to be around 230.[13]

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.[14]

Polyynes[edit]

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),[15] 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), being stabilized by rather bulky end-groups.[16]

Structure[edit]

The carbon atoms in this form are each linear in geometry with sp orbital hybridisation. The estimated length of the bonds is 120.7 pm (triple) and 137.9 pm (single).[11]

There are other possible configurations for a chain of carbon atoms. The main alternative are polycumulene (polyethylene-diylidene) chain with double bonds only (128.2 pm).[11] This chain is expected to have slightly higher energy, with a Peierls gap of 2 to 5 eV.[11] For short Cn molecules, however, the polycumulene structure seems favored.[11] When n is even, two ground configurations, very close in energy, may coexist, one linear and one cyclic (rhombic).[11]

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.[17]

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, which frequency typically between 1950 and 2300 cm−1.[11]

See also[edit]

References[edit]

  1. ^ R.B. Heimann, S.E. Evsyukov, L. Kavan, eds. (1999), Carbyne and carbynoid structures (book), page 452. Volume 21 in the series Physics and Chemistry of Materials with Low-Dimensional Structures ISBN 0-7923-5323-4
  2. ^ R. H. Baughman (2006), Dangerously Seeking Linear Carbon. Science, volume 312, pages 1009–1110 doi:10.1126/science.1125999
  3. ^ L. Itzhaki, E. Altus, H. Basch, S. Hoz (2005), Harder than diamond: Determining the cross-sectional area and Young's modulus of molecular rods. Angewandte Chemie Int. Edition, volume 44, pages 7432–7435 doi:10.1002/ange.200502448 Itzhaki, L.; Altus, E.; Basch, H.; Hoz, S. (2005). "Harder than Diamond: Determining the Cross-Sectional Area and Young's Modulus of Molecular Rods". Angewandte Chemie International Edition 44 (45): 7432–7435. doi:10.1002/anie.200502448. PMID 16240306.  edit
  4. ^ a b c d H. Kroto (2010), Carbyne and other myths about carbon. RSC Chemistry World, November 2010.
  5. ^ V. I. Kasatochkin and others (1967). Dokl. Chem., volume 177, page 1031. As cited by Kroto(2010).
  6. ^ A. G. Whittaker (1978). Science, volume 200, page 763. As cited by Kroto(2010).
  7. ^ P. P. K. Smith and P. R. Busek (1982), Science, volume 216, page 984. As cited by Kroto(2010).
  8. ^ E. A. Rohlfing, D. M. Cox, and A. J. Kaldor (1984), Journal of Chem. Phys., volume 81, page 3332. As cited by Kroto(2010).
  9. ^ K. Yamada, H. Kunishige, and A. B. Sawaoka (1991) Formation process of carbyne produced by shock compression Naturwissenschaften volume 78, pages 450-452 doi:10.1007/BF01134379
  10. ^ R. J. Lagow, J. J. Kampa, Han-Chao Wei, Scott L. Battle, John W. Genge, D. A. Laude, C. J. Harper, R. Bau, R. C. Stevens, J. F. Haw., E. Munson (1995), Synthesis of linear acetylenic carbon: The "sp" carbon allotrope. Science, volume 267, issue 5196, pages 362–367. Bibcode 1995Sci...267..362L; doi:10.1126/science.267.5196.362
  11. ^ a b c d e f g J. Kastner, H. Kuzmany, L. Kavan, F. P. Dousek, and J. Kürti (1995) Reductive preparation of carbyne with high yield: An in situ raman scattering study. Macromolecules, volume 28, pages 344-353. doi:10.1021/ma00105a048
  12. ^ a b Franco Cataldo (1999), From dicopper acetylide to carbyne.Polymer International, volume 48, issue 1, pages 15-22. doi:10.1002/(SICI)1097-0126(199901)48:1
  13. ^ Franco Cataldo (1999), ' 'A study on the structure and electrical properties of the fourth carbon allotrope: carbyne. Polymer International, volume 44, issue 2, pages 191–200. doi:10.1002/(SICI)1097-0126(199710)44:2
  14. ^ K.-H. Xue, F.-F. Tao, W. Shen, C.-J. He, Q.-L. Chen, L.-J. Wu, Y.-M. Zhu (2004), Linear carbon allotrope: Carbon atom wires prepared by pyrolysis of starch. Chemical Physics Letters, volume 385, issue 5–6, pages 477–480 Bibcode 2004CPL...385..477X; doi:10.1016/j.cplett.2004.01.007
  15. ^ Wesley A. Chalifoux and Rik R. Tykwinski (2009), Synthesis of extended polyynes: Toward carbyne. (A review of polyyne sythesis.) Comptes Rendus Chimie, volume 12, issue 3, pages 341–358 doi:10.1016/j.crci.2008.10.004
  16. ^ Simon Hadlington (2010), One dimensional carbon chains get longer. Report on Wesley A. Chalifoux and Rik R. Tykwinski's announcement. RSC Chemistry World, September 2010.
  17. ^ Sara Eisler, Aaron D. Slepkov, Erin Elliott, Thanh Luu, Robert McDonald, Frank A. Hegmann, and Rik R. Tykwinski (2005), Polyynes as a model for carbyne: Synthesis, physical properties, and nonlinear optical response. Journal of the American Chemical Society, volume 127, issue 8, pages 2666–2676. doi:10.1021/ja044526l
sp3 forms
sp2 forms
sp forms
  • Linear acetylenic carbon
mixed sp3/sp2 forms
other forms
hypothetical forms
related
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