Critique of two inorganic chemistry scientific papers

Inorganic Chemistry

Linear sp Carbon Allotropes in Question

While modern day discoveries and characterizations of allotropes are reported, the definition of allotropy remains ambiguous as the question of what constitutes an allotrope is under debate. Lagow et al. reported the synthesis of a terminally capped linear acetylenic carbon with alternating single and triple binds, claiming it to be a stable sp carbon allotrope (1994), a subject of debate as the classification of acetylenic carbon as an allotrope continues to be determined, and the stability of such a compound is in question. The proposed structure and stability of a linear sp carbon of such a proposed length and with alternating single and triple bonds is disputed by Hirsch et al. Thus, the stability of long-chain carbon allotropes and the characterization of the linear sp carbon synthesized by Lagow et al., given its terminal end design, remains in question as to the significance of the claim.

The IUPAC Red Book defines allotropes as "different structural modifications of [an] element," with allotropic transition considered the "transition of a pure element, at a defined temperature and pressure, from one crystal structure to another which contains the same atoms but which has different properties" (Golden Age, p.2). This definition excludes ozone and oxygen, and does not discuss the classification of carbon allotropes, which can be examined on the basis of the hybridization of their valence orbitals. More modern definitions include the classification of tin where one allotrope is a covalently bonded solid and the other a metal, and differentiate between allotropic molecules, such as dioxygen, infinite covalent solids, like diamond, and infinite, covalently bonded layers with weak intermolecular forces, as seen with graphite. There are also materials that crystallize where the covalent bonding between the elements is unchanged, which are termed polymorphs rather than true allotropes.

The report that a series of acetylenic carbon species with sp hybridized carbon atom linear chains and stabilized terminal groups has added to the debate over the definition of allotropes because of their incorporation of heteroatoms at their terminal ends, as well as the experimental determination of their overall stability. For reference, acetylene is a molecular structure where carbon is bound to two other atoms via two double bonds or one single and one triple bond, with linear hybridization (180o between bonds):

The controversy arises because, while they do not directly fit the definition of allotropes, non-molecular allotropes like diamond and graphite will terminate with other elements (so no longer a pure element) to become stable. It is argued that the stoichiometry of the terminating groups of the acetylenic carbon species is defined as compared to that for diamond or graphite, so are less considered to belong to the allotropes. Also, allotropes have generally been isolated through reducing conditions rather than synthesized, as is the case with the acetylenic carbon species.

The linear carbon allotrope with sp hybridization proposed by Lagow et al. was synthesized using non-reactive terminal end groups for stabilization, with claims that the chain length was a minimum of 300 carbon atoms long. They synthesized a (t-Bu) C8(t-Bu) model, producing a crystalline structure of alternating long and short bonds, demonstrated to be thermally stable at 130oC and at pressures up to 60 kbar. In a second synthesis carbon was produced as a by-product. Through modification of this process they were able to prepare acetylene carbon chains with phenyl caps, with stability shown to 130oC. During the study, they also determined that terminating the carbon chains with delocalized electrons rather than end groups showed instability, and the addition of free radicals to the end caps suppresses fullerenes. Lagow et al. concede that the question of characterization of their synthesized carbon compound as a carbon allotrope arises as the fullerenes are the only molecular allotropes absent of end groups, with diamond and graphite containing terminal groups. Hirsch et al., however, dispute the discovery based on theoretical evidence, contrary to Lagow's claims based on observation.

Hirsch et al. reported on the isolation and characterization of new rod-shaped homologues 1-5, as well as the following homologue, C18N2, extrapolating increases in chain length to predict the spectroscopic characterization of the hypothetical carbyne sp-C?. It was found during stability experiments that polyenes become less stable with increases in chain length. 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.