Diatomic carbon explained

Diatomic carbon (systematically named dicarbon and 2,2λ2-ethene), is a green, gaseous inorganic chemical with the chemical formula C=C (also written [C<sub>2</sub>] or C2). It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation. It occurs in carbon vapor, for example in electric arcs; in comets, stellar atmospheres, and the interstellar medium; and in blue hydrocarbon flames.[1] Diatomic carbon is the second simplest of the allotropes of carbon (after atomic carbon), and is an intermediate participator in the genesis of fullerenes.

Properties

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.

Electromagnetic properties

The electrons in diatomic carbon are distributed among the molecular orbitals according to the Aufbau principle to produce unique quantum states, with corresponding energy levels. The state with the lowest energy level, or ground state, is a singlet state (1Σ), which is systematically named ethene-1,2-diylidene or dicarbon(0•). There are several excited singlet and triplet states that are relatively close in energy to the ground state, which form significant proportions of a sample of dicarbon under ambient conditions. When most of these excited states undergo photochemical relaxation, they emit in the infrared region of the electromagnetic spectrum. However, one state in particular emits in the green region. That state is a triplet state (3Πg), which is systematically named ethene-μ,μ-diyl-μ-ylidene or dicarbon(2•). In addition, there is an excited state somewhat further in energy from the ground state, which only form a significant proportion of a sample of dicarbon under mid-ultraviolet irradiation. Upon relaxation, this excited state fluoresces in the violet region and phosphoresces in the blue region. This state is also a singlet state (1Πg), which is also named ethene-μ,μ-diyl-μ-ylidene or dicarbon(2•).

State Excitation
enthalpy
(kJ mol−1)
Relaxation
transition
Relaxation
wavelength
Relaxation EM-region
X1Σ 0
a3Π 8.5 a3Π→X1Σ 14.0 μm Long-wavelength infrared
b3Σ 77.0 b3Σ→a3Π 1.7 μm Short-wavelength infrared
A1Π 100.4 A1Π→X1Σ
A1Π→b3Σ
1.2 μm
5.1 μm
Near infrared
Mid-wavelength infrared
B1Σ ? B1Σ→A1Π
B1Σ→a3Π
?
?
?
?
c3Σ 159.3 c3Σ→b3Σ
c3Σ→X1Σ
c3Σ→B1Σ
1.5 μm
751.0 nm
?
Short-wavelength infrared
Near infrared
?
d3Π 239.5 d3Π→a3Π
d3Π→c3Σ
d3Π→A1Π
518.0 nm
1.5 μm
860.0 nm
Green
Short-wavelength infrared
Near infrared
C1Π 409.9 C1Π→A1Π
C1Π→a3Π
C1Π→c3Σ
386.6 nm
298.0 nm
477.4 nm
Violet
Mid-ultraviolet
Blue

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 carbon atoms in a C2 molecule. One analysis suggested instead that a quadruple bond exists,[2] an interpretation that was disputed.[3] CASSCF calculations indicate that the quadruple bond based on molecular orbital theory is also reasonable.[4] Bond dissociation energies (BDE) of B2, C2, and N2 show increasing BDE, indicating single, double, and triple bonds, respectively.

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]

Reactions

Diatomic carbon will react with acetone and acetaldehyde to produce acetylene by two different pathways.[6]

History

The light of gas-rich comets mainly originates from the emission of diatomic carbon. An example is C/2014 Q2 (Lovejoy), where there are several lines of C2 light, mostly in the visible spectrum[10], forming the Swan bands.[11] C/2022 E3 (ZTF), visible in early 2023, also exhibits green color due to the presence of diatomic carbon.[12]

See also

Notes and References

  1. Roald. Hoffmann. Roald Hoffmann. Marginalia: C2 In All Its Guises. American Scientist. 1995. 83. 4. 309–311. 1995AmSci..83..309H. 29775475. 2017-07-22. 2023-03-21. https://web.archive.org/web/20230321180249/http://www.roaldhoffmann.com/sites/all/files/c2_in_all-_its_guises.pdf. dead.
  2. Quadruple bonding in C2 and analogous eight-valence electron species. Sason . Shaik. David . Danovich. Wei . Wu. Peifeng . Su. Henry S. . Rzepa . Henry Rzepa. Philippe C.. Hiberty. Nature Chemistry. 2012. 4 . 3. 195–200. 10.1038/nchem.1263 . 22354433 . 2012NatCh...4..195S.
  3. Quantum chemistry: Quadruply bonded carbon. Jörg . Grunenberg. Nature Chemistry. 2012. 4 . 3. 154–155. 10.1038/nchem.1274 . 22354425 . 2012NatCh...4..154G.
  4. Latent harmony in dicarbon between VB and MO theories through orthogonal hybridization of 3σg and 2σu. Ronglin. Zhong. Min. Zhang. Hongliang. Xu. Zhongmin. Su. Chemical Science. 2016. 7. 2. 1028–1032. 10.1039/c5sc03437j. 29896370. 5954846.
  5. Higher-order finite-difference pseudopotential method: An application to diatomic molecules . Chelikowsky . James R. . James R. Chelikowsky . Troullier . N. . Wu . K. . Saad . Y. . . 1994 . 50 . 16 . 11356–11364 . 10.1103/PhysRevB.50.11355 . 9975266 . 1994PhRvB..5011355C.
  6. Chemistry of the singlet and triplet C2 molecules. Mechanism of acetylene formation from reaction with acetone and acetaldehyde . Philip S. . Skell . Philip Skell . James H. . Plonka . . 1970 . 92 . 19 . 5620–5624. 10.1021/ja00722a014.
  7. Reaction of Diatomic Carbon with Alkanes and Ethers/ Trapping of Alkylcarbenes by Vinylidene . Skell . P. S. . Philip Skell . Fagone . F. A. . Klabunde . K. J. . . 1972 . 94 . 22 . 7862–7866. 10.1021/ja00777a032.
  8. Miyamoto. Kazunori. Narita. Shodai. Masumoto. Yui. Hashishin. Takahiro. Osawa. Taisei. Kimura. Mutsumi. Ochiai. Masahito. Uchiyama. Masanobu. 2020-05-01. Room-temperature chemical synthesis of C 2. Nature Communications. en. 11. 1. 2134. 10.1038/s41467-020-16025-x. 32358541. 2041-1723. 7195449. 2020NatCo..11.2134M.
  9. Rzepa. Henry S.. 2021-02-23. A thermodynamic assessment of the reported room-temperature chemical synthesis of C 2. Nature Communications. en. 12. 1. 1241. 10.1038/s41467-021-21433-8. 33623013. 2041-1723. 7902603. 2021NatCo..12.1241R.
  10. Kumar . Venkataramani. Satyesh . Ghetiya. Shashikiran . Ganesh. al. . et. Optical spectroscopy of comet C/2014 Q2 (Lovejoy) from the Mount Abu Infrared Observatory. Monthly Notices of the Royal Astronomical Society. 2016 . 463 . 2 . 2137–2144 . 10.1093/mnras/stw1820 . free. 1607.06682 . 2016MNRAS.463.2137V.
  11. CCD Photometry of Comets. Herman . Mikuz . Bojan . Dintinjana. International Comet Quarterly. 1994 . October 26, 2006.
  12. Web site: Georgiou . Aristos . 2023-01-10 . What makes the green comet green? . live . https://web.archive.org/web/20230125114911/https://www.newsweek.com/green-comet-appear-february-night-sky-never-return-earth-1772429 . 2023-01-25 . 2023-01-25 . . en.