Linear acetylenic carbon

This article is about chains of repeating alkyne units. For other uses of "carbyne", see Carbyne (disambiguation).

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 32.7 TPa – forty times that of diamond, the hardest known material.[3] 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.[4]

History and controversy

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 polyyne carbon.[8] However, these 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 "polyacetylide" 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

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

Structure

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]

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

Properties

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).[18][19][20] Its stiffness of around 109 N·m/kg is also double that of graphene, which is around 4.5×108 N·m/kg.[18][21]

Stretching carbyne as little as 10% alters its electronic band gap from 3.2 to 4.4 eV.[22] 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.[19]

Carbyne chains can take on side molecules that may make the chains suitable for energy[19] and hydrogen[23] storage.

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.[19]

References

  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. ↑ Baughman, R. H. (2006). "CHEMISTRY: Dangerously Seeking Linear Carbon". Science 312 (5776): 1009–1110. doi:10.1126/science.1125999. PMID 16709775.
  3. ↑ 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 117 (45): 7598. 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.
  4. ↑ 4.0 4.1 4.2 4.3 H. Kroto (2010), Carbyne and other myths about carbon. RSC Chemistry World, November 2010.
  5. ↑ V. I. Kasatochkin et al. (1967). Doklady Chemistry 177: 1031. Missing or empty |title= (help) As cited by Kroto(2010).
  6. ↑ Whittaker, A. G. (1978). "Carbon: A New View of Its High-Temperature Behavior". Science 200 (4343): 763. Bibcode:1978Sci...200..763G. doi:10.1126/science.200.4343.763. As cited by Kroto(2010).
  7. ↑ Smith, P. P. K.; Buseck, P. R. (1982). "Carbyne Forms of Carbon: Do They Exist?". Science 216 (4549): 984. Bibcode:1982Sci...216..984S. doi:10.1126/science.216.4549.984. As cited by Kroto(2010).
  8. ↑ E. A. Rohlfing; D. M. Cox; A. J. Kaldor (1984). "Production and characterization of supersonic carbon cluster beams". Journal of Chemical Physics 81: 3332. doi:10.1063/1.447994. As cited by Kroto(2010).
  9. ↑ Yamada, K.; Kunishige, H.; Sawaoka, A. B. (1991). "Formation process of carbyne produced by shock compression". Naturwissenschaften 78 (10): 450. doi:10.1007/BF01134379.
  10. ↑ Lagow, R. J.; Kampa, J. J.; Wei, H. -C.; Battle, S. L.; Genge, J. W.; Laude, D. A.; Harper, C. J.; Bau, R.; Stevens, R. C.; Haw, J. F.; Munson, E. (1995). "Synthesis of Linear Acetylenic Carbon: The "sp" Carbon Allotrope". Science 267 (5196): 362–367. doi:10.1126/science.267.5196.362. PMID 17837484.
  11. ↑ 11.0 11.1 11.2 11.3 Kastner, J.; Kuzmany, H.; Kavan, L.; Dousek, F. P.; Kuerti, J. (1995). "Reductive Preparation of Carbyne with High Yield. An in Situ Raman Scattering Study". Macromolecules 28: 344. doi:10.1021/ma00105a048.
  12. ↑ 12.0 12.1 Cataldo, Franco (1999). "From dicopper acetylide to carbyne". Polymer International 48: 15. doi:10.1002/(SICI)1097-0126(199901)48:1<15::AID-PI85>3.0.CO;2-#.
  13. ↑ Cataldo, Franco (1997). "A study on the structure and electrical properties of the fourth carbon allotrope: Carbyne". Polymer International 44 (2): 191. doi:10.1002/(SICI)1097-0126(199710)44:2<191::AID-PI842>3.0.CO;2-Y.
  14. ↑ Xue, K. H.; Tao, F. F.; Shen, W.; He, C. J.; Chen, Q. L.; Wu, L. J.; Zhu, Y. M. (2004). "Linear carbon allotrope – carbon atom wires prepared by pyrolysis of starch". Chemical Physics Letters 385 (5–6): 477. doi:10.1016/j.cplett.2004.01.007.
  15. ↑ Chalifoux, W. A.; Tykwinski, R. R. (2009). "Synthesis of extended polyynes: Toward carbyne". Comptes Rendus Chimie 12 (3–4): 341. 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. ↑ Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. (2005). "Polyynes as a Model for Carbyne:  Synthesis, Physical Properties, and Nonlinear Optical Response". Journal of the American Chemical Society 127 (8): 2666–2676. doi:10.1021/ja044526l. PMID 15725024.
  18. ↑ 18.0 18.1 Emerging Technology From the arXiv August 15, 2013 (2013-08-15). "New Form of Carbon is Stronger Than Graphene and Diamond | MIT Technology Review". Technologyreview.com. Retrieved 2013-12-24.
  19. ↑ 19.0 19.1 19.2 19.3 "New one-dimensional form of carbon may be the strongest material ever". KurzweilAI. Retrieved 2013-10-11.
  20. ↑ Liu, M.; Artyukhov, V. I.; Lee, H.; Xu, F.; Yakobson, B. I. (2013). "Carbyne from First Principles: Chain of C Atoms, a Nanorod or a Nanorope". ACS Nano 7 (11): 131010092021005. doi:10.1021/nn404177r. PMID 24093753.
  21. ↑ Liu, Mingjie; Artyukhov, Vasilii I.; Lee, Hoonkyung; Xu, Fangbo; Yakobson, Boris I. (2013). "Carbyne from first principles: Chain of C atoms, a nanorod or a nanorope". ACS Nano 7 (11): 10075–82. arXiv:1308.2258. doi:10.1021/nn404177r. PMID 24093753.
  22. ↑ "Carbyne: The new world's strongest material?". Gizmag.com. Retrieved 2013-10-15.
  23. ↑ Sorokin, Pavel B.; Lee, Hoonkyung; Antipina, Lyubov Yu.; Singh, Abhishek K.; Yakobson, Boris I. (2011). "Calcium-decorated carbyne networks as hydrogen storage media". Nano Letters 11 (7): 2660–2665. doi:10.1021/nl200721v.

Further reading