What is the structure of the Carbyne molecule? 

In organic chemistry, carbyne is often a 'general term' for any compound whose molecular structure includes an electrically neutral carbon atom with three non-bonded electrons, connected to another atom by a single bond. The chain of carbon atoms is held together by either double or alternating single and triple atomic bonds. That makes it a true one-dimensional material, unlike atom-thin sheets of graphene. 

Carbon chains are traditionally classified as cumulene (monatomic chains with double bonds, = C = C = ) or polyyne (dimerized chains with alternating single and triple bonds, −C≡C−).

A polyyne is any organic compounds with alternating single and triple bonds; that is, (−C≡C−) n with n greater than 1. The simplest example is diacetylene or buta-1,3-diyne, H−C≡C−C≡C−H. Linear acetylenic carbon (also referred to as carbyne) is an allotrope of carbon that has the chemical structure (−C≡C−) n , with alternating single and triple bonds would thus be the ultimate member of the polyyne family. 

A cumulene is a hydrocarbon with three or more cumulative (consecutive) double bonds. A member of this compound class is butatriene (which is also called simply cumulene), H2C=C=C=CH2. Unlike most alkanes and alkenes, cumulenes tend to be rigid, which makes them appealing for molecular nanotechnology. Cumulenes are found in regions of space where hydrogen is rare.

JPEG image of 12 carbon chain carbyne molecule containing alternating carbon single and triple bonds. The image was created using the free software package ArgusLab.

JPEG image of 12 carbon chain carbyne molecule containing all carbon double bonds. The image was created using the free software package ArgusLab.

Note: All molecules on this page were built using Argus Lab 4.01 (see free molecular modeling software). The structures were Geometry Optimized in Argus Lab using the semiempirical method (AM1 Hamiltonian).

BOND LENGTHS IN CARBON CHAINS

The bond lengths of carbon chains characterizes the type of bond between the carbons, For example alkanes have a bond length of 1.51 ˚A , alkenes 1.33 ˚A and alkynes 1.19 ˚A. This however is not the case with carbynes (See below)

What is the difference between Graphene and Carbyne?

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes, and fullerenes. The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov "for groundbreaking experiments regarding the two-dimensional material graphene". For a more detailed description of graphene see our graphene 3D molecule page.

Image of graphene -- a two dimensional plane of carbon atoms.

Carbyne (Linear acetylenic carbon) has been in the news lately as a structure that is possibly the worlds strongest material that has great potential if anyone can make it in bulk. Carbyne is basically a chain of single carbon atoms, but having twice the tensile strength of graphene , and three times the tensile stiffness of diamond (1,2). The chemical structure is repeating chain with alternating single and triple bonds --- ...Because it is just a single strand of molecules it is often referred to as "one-dimensional" structure.

Readings and References

All-carbon sp-sp2 hybrid structures: Geometrical properties, current rectification, and current amplification --Scientific Reports Sept. 2013

Gaining knowledge of single carbon chains --Radboud University Nijmegen Sept. 2011

The [18] all-carbon molecule: cumulene or polyacetylene --J. Am. Chem. Soc., 1991.