The Nature of One‐Dimensional Carbon: Polyynic versus Cumulenic

AbstractA 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.