Term Paper: Inorganic Chemistry Linear Sp Carbon

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[. . .] Additionally, when the polyenes are end-capped with organic or organometallic groups, the instability becomes likewise proportionately affected. It was cited from previous studies that shorter interchain distances are favorable for achieving stability, and the addition of bulky terminating groups do impact the molecular stability. Extending the carbon chains beyond the homologues presented by Hirsch et al. is considered to demonstrate unstable states (they found even homologue 6 to be greatly unstable), causing them to question the plausibility of Lagow's claim of an sp-carbon allotrope of length greater than 300 carbons. Also presented by Hirsch et al., molecular arrangements with alternating single and triple bonds show a decrease in bond length from the ends to the center, which increases with increasing chain length. Extrapolating for the infinite chain length scenario, bond lengths will approach a given limit with no bond length alternation towards the center of the chain. Hirsch et al. conclude that the short-chain model compounds 1-6 that they synthesized for the evaluation of a hypothetical sp-C? model demonstrate the unstable characteristics of any carbon allotrope of a length claimed by Lagow.

Other theories also exist as to why Lagow's carbon allotrope is not feasible. Demishev et al. maintain the assertion that linear sp polymeric carbon molecule segments alternate with sp2 hybridized carbon atoms, and thus the carbon chains form complex globular structures to overcome the otherwise weak van der Waals forces. Thus carbyne, an sp allotropic carbon with a linear chain structure "cannot be synthesized as a perfect crystal because its chains contain 'built in' disorder, probably due to the instability of large linear carbon clusters" (Demishev et al., 2002, p. 585). Carbyne was not included in the IUPAC of 1995 description of carbon as a solid. Recent use of modern analytical electron microscopy (AEM) is currently providing evidence of carbyne existence with observations of its sp hybridization features, and was utilized by Li et al. To indicate the existence of carbyne as a member of the carbon family. They reported the unambiguous identification of unique sp hybridization features in combination with the atomic structure and electronic properties previously observed, and concluded that, "The sp hybridization of carbynes is distinct from the other carbon allotropes" (Li et al., p.1), accounting for its inclusion as a carbon allotrope, but with a given nature.

It is generally considered that long chain carbon allotropes are unstable, but Lagow et al. assert that by incorporating non-reactive terminal end caps the molecular stability improves. The dispute made by Hirsch et al. cites the instability of any sp carbon allotrope species of such a proposed length with alternating single and triple bonds, and terminal end groups. The Hirsch argument of theoretical implausibility thus rivals the success claimed by Lagow, based more on observation of molecular stability rather than experimentation. Scientific extrapolation of known factors is essential to determining the validity of experimental results. While factors that may only be realized through actual synthesis of such a proposed carbon allotrope can present during such work, the potential for skewed observation rather than the presentation of objective results does raise question over the validity of Lagow's claim. Future scientific experimentation in this area is required to determine a true carbon sp allotrope of usable stability, if one exists.

Bibliography

Demishev, SV, Pronin, AA, Sluchanko, NE, Samarin, NA, Glushkov, VV, Lyapin, AG, Kondrin,

MV, Brazhkin, VV, Varfolomeeva, TD, Popova, SV, & H. Ohta. (2002). "New nanocluster carbyne-based material synthesized under high pressure." General Physics Institute, Russian Academy of Sciences. Russia: Moscow. Vol. 44, No. 4, pp. 585-588. [Online]. Available at http://www.ioffe.rssi.ru/journals/ftt/2002/04/p585-588.pdf

Lagow, RJ, Kampa, JJ, Wei, HC, Battle, SL, Genge, JW, Laude, DA, Harper, CJ, Bau, R,

Stevens, RC, Haw, JF, & E. Munson. (20 Jan. 1995) "Synthesis of Linear Acetylenic Carbon: The 'sp' Carbon Allotrope." Science, New Series. Vol. 267, No. 5196, pp. 362-367.

Li, SH, et al. (2000) "Does Carbyne Really Exist?" Dr. Shu-You Li. [Online] Available at http://www.uni-mainz.de/~syli/papers/published/Carbyne_CARBON/CARBON.html

Schermann, G, Grosser, T, Hampel, F, & A Hirsch. (1997). "Dicyanopolyynes: A Homologous

Series of End-Capped Linear sp Carbon." Chem, Eur, J. Vol. 3, No. 7.

The University of Sydney Australia. The Golden Age of Allotropes. [Online] Available at http://notes.chem.usyd.edu.au/course/masters/Chem1/Special%20Topics/Allotropes.pdf [END OF PREVIEW]

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