Diatomic carbon

Diatomic carbon
IUPAC name Diatomic carbon
Systematic name Ethenediylidene (substitutive) Dicarbon(C—C) (additive)
Identifiers
CAS number	12070-15-4 N
PubChem	139247
ChemSpider	122807 Y
ChEBI	CHEBI:30083
Gmelin Reference	196
Jmol-3D images	Image 1
SMILES [C]=[C]
InChI InChI=1S/C2/c1-2 Y Key: LBVWYGNGGJURHQ-UHFFFAOYSA-N Y
Properties
Molecular formula	C2
Molar mass	24.02 g mol−1
Exact mass	24.000000000000 g mol-1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Diatomic carbon is a diatomic molecule of carbon (C₂), which occurs in electric arcs; in comets, stellar atmospheres and the interstellar medium; and in blue hydrocarbon flames.^[1]

Chemistry

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 the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate pi bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons in a C₂ molecule. This is surprising because the MO diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds.

Bond dissociation energies of B₂, C₂, and N₂ show increasing BDE, indicating single, double, and triple bonds, respectively.

C₂ is an unstable molecule; carbon is far more frequently encountered as diamond, graphite, and fullerenes.

Comets

The light of fainter comets mainly originates from the emission of diatomic carbon. There are several lines of C₂ light, mostly in the visible spectrum, forming the Swan bands.^[2]

Properties

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.

Reactions

Diatomic carbon will react with acetone and acetylaldehyde to produce acetylene by two different pathways.^[3]

• 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. ^[3]
• 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. ^[3]
• 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. ^[4]

Charge density

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.^[5]

References

1. ^ Roald Hoffmann (1995). "C2 In All Its Guises". American Scientist 83: 309–311. Bibcode 1995AmSci..83..309H. 
2. ^ Herman Mikuz, Bojan Dintinjana. "CCD Photometry of Comets". http://www.observatorij.org/CCDPhot/iwca5.html. Retrieved 2006-10-26. 
3. ^ ^{a b c} 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. 
4. ^ 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. 
5. ^ 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.