|Jmol-3D images||Image 1|
|Molar mass||24.02 g mol−1|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
Diatomic carbon (systematically named ethynylidene and dicarbon(C—C)), also called dicarbon, is an inorganic chemical with the chemical formula C≡C (also written [C
2] or C
2). It is a gas that only exists above 3,642 °C (6,588 °F)), below which it aggregates into graphite or other fullerenes. It occurs in carbon vapor, for example in electric arcs; in comets, stellar atmospheres and the interstellar medium; and in blue hydrocarbon flames.
Ethynylidene is viewed as acetylene with two hydrogen atoms removed. By default, this name pays no regard to the radicality of the diatomic carbon. Although in context, it can also specifically refer to the non-radical singlet excited state, whereas the non-radical ground-state is named ethenediylidene.
Valence bond theory predicts a quadruple bond as the only way to satisfy the octet rule for carbon. However, molecular orbital theory shows that there are two sets of paired electrons in a degenerate pi bonding set of orbitals. This gives a bond order of 2, meaning that there should exist a double bond between the two carbons in a C2 molecule.
C2 is a component of carbon vapor. One paper estimates that carbon vapor is around 28% diatomic, but theoretically this depends on the temperature and pressure.
Cohesive energy (eV): 6.32
Bond length (Angstrom): 1.24
Vibrational Mode (cm-1): 1855
The triplet state has a longer bond length than the singlet state.
- Triplet C2 molecules will react through an intermolecular pathway, which is shown to exhibit radical character. The intermediate for this pathway is the ethylene radical. Its abstraction is correlated with bond energies.
- Singlet C2 molecules will react through an intramolecular, nonradical pathway in which two hydrogen atoms will be taken away from one molecule. The intermediate for this pathway is singlet vinylidene. The singlet reaction can happen through a 1,1-diabstraction or a 1,2-diabstraction. This reaction is insensitive to isotope substitution. The different abstractions are possibly due to the spatial orientations of the collisions rather than the bond energies.
- Singlet C2 will also react with alkenes. Acetylene is a major product; however, it appears C2 will insert into carbon-hydrogen bonds.
- C2 is 2.5 times more likely to insert into a methyl group as into methylene groups.
In certain forms of crystalline carbon, such as diamond and graphite, a saddle point or “hump” occurs at the bond site in the charge density. The triplet state of C2 does follow this trend. However, the singlet state of C2 acts more like silicon or germanium; that is, the charge density has a maximum at the bond site.
- Roald Hoffmann (1995). "C2 In All Its Guises". American Scientist 83: 309–311. Bibcode:1995AmSci..83..309H.
- Skell, P. S.; Plonka, J. H. (1970). "Chemistry of the Singlet and Triplet C2 Molecules. Mechanism of Acetylene Formation from Reaction with Acetone and Acetaldehyde". Journal of the American Chemical Society 92 (19): 5620–5624. doi:10.1021/ja00722a014.
- Herman Mikuz, Bojan Dintinjana. "CCD Photometry of Comets". Retrieved 2006-10-26.
- Skell, P. S.; Fagone, F. A.; Klabunde, K. J. (1972). "Reaction of Diatomic Carbon with Alkanes and Ethers/ Trapping of Alkylcarbenes by Vinylidene". Journal of the American Chemical Society 94 (22): 7862–7866. doi:10.1021/ja00777a032.
- Chelikowsky, J. R.; Troullier, N.; Wu, K.; Saad, Y. (1994). "Higher-order finite-difference pseudopotential method: An application to diatomic molecules". Physical Review B 50: 11356–11364. Bibcode:1994PhRvB..5011355C. doi:10.1103/PhysRevB.50.11355.