Carbyne

Carbyne is the linear allotrope of carbon, consisting of a chain of carbon atoms connected by sp-hybridized bonds. In chemical discussions, two structural variants are typically considered: cumulene-type chains with consecutive double bonds (-C=C=C=...), and polyyne-type chains with alternating single and triple bonds (-C≡C-C≡C-). Both forms represent highly anisotropic, one-dimensional systems.

The bonding in carbyne leads to unique electronic and vibrational properties. In theory, cumulene chains have equalized bond characters along the length, while polyyne chains show alternating bond lengths due to Peierls-type distortion. The exact ground-state arrangement can depend on chain length, end groups, and stabilizing surroundings. The linear, sp-hybridized nature gives rise to high stiffness along the chain and low bending flexibility.

Predicted properties for carbyne include extremely high tensile strength and stiffness, as well as notable electronic conductivity along the chain. However, stability is a major practical challenge: bare carbyne is highly reactive and tends to crosslink or transform into other carbon forms, making the isolation of long free-standing chains difficult. As a result, researchers have focused on stabilized or encapsulated forms.

Synthesis and observation have historically yielded short chains. Long polyyne and cumulene sequences have been prepared as molecular species or stabilized within hosts such as carbon nanotubes or other matrices, enabling study of their properties. Carbyne remains primarily a subject of theoretical interest and laboratory investigation, with ongoing research aimed at understanding its bonding, stability, and potential applications in materials science and nanoelectronics.