The Nature of One‐Dimensional Carbon: Polyynic versus Cumulenic

Abstract

A question of both fundamental as well as practical importance is the nature of one-dimensional carbon, in particular whether a one-dimensional carbon allotrope is polyynic or cumulenic, that is, whether bond-length alternation occurs or not. By combining the concept of aromaticity and antiaromaticity with the rule of Peierls distortion, the occurrence and magnitude of bond-length alternation in carbon chains with periodic boundary conditions and corresponding carbon rings as a function of the chain or ring length can be explained. The electronic properties of one-dimensional carbon depend crucially on the bond-length alternation. Whereas it is generally accepted that carbon chains in the limit of infinite length have a polyynic structure at the minimum of the potential energy surface with bond-length alternation, we show here that zero-point vibrations lead to an effective equalization of all carbon-carbon bond lengths and thus to a cumulenic structure.

Carbyne: The Molecular Approach

“…To examine the potential for polyyne bending, the group in Milan again figured prominently, and the polyynes Ad[n] ( Figure 5) were studied by IR and Raman spectroscopies, in both the solution and solid states. The solid-state IR spectra in Figure 9b show that mutual exclusion between Raman and IR lines is not upheld (see arrows), particularly for the longer polyynes, such as Ad [6]. This is not especially surprising, since inversion symmetry is Figure 9c).…”

“…combining the concept of aromaticity and antiaromaticity with the rule of Peierls distortion". [6] These last [a] two points perhaps help to justify the inclusion of this perspective in a topical issue on aromaticity dedicated to Professor Nozoe.…”

“…[75] Polyynes have often revealed nonlinear structures in the solid state, such as that observed for Ad [6] (Figure 9a). In solution, however, the influence of endgroups and "crystal packing effects" would be absent.…”

“…Notably, the solid-state structure of tBu [10] was the first reported for a decayne ( Figure 7). In hindsight, it was perhaps not so surprising that the tBu[n] series so readily produced nice crystals, since Bohlmann had foreshadowed our success decades ago by describing, for example, the crystallinity of tBu [6] and tBu [7] as "gelbe Nadeln", [57] while Johnson and Walton reported that tBu[8] ". .…”

Cu-metalated carbyne acting as a promising molecular wire

“…Although the Peierls distortion makes the polyynic structure being at the minimum of the potential energy and stretching enhances the Peierls transition, 12 zeropoint atomic vibrations eliminate the distortion and restore the cumulenic structure. 13 Electrical transport measurements of finite monatomic carbon chains have confirmed that the measurement conditions, including strain, have a decisive influence on their conductivity. 14,15 Furthermore, even when an ohmic behavior is observed in unstrained carbon chains, the measured conductivity is still much lower than the predicted theoretical value, illustrating the key role of the contacting leads.…”