Tricarbon monoxide
Preferred IUPAC name
Other names
3D model (JSmol)
  • InChI=1S/C3O/c1-2-3-4
  • [C-]#CC#[O+]
Molar mass 52.032 g·mol−1
Appearance Gas
Related compounds
Related oxides
carbon monoxide
dicarbon monoxide
tetracarbon monoxide
Related compounds
tricarbon monosulfide
carbon subnitride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Tricarbon monoxide C3O is a reactive radical oxocarbon molecule found in space, and which can be made as a transient substance in the laboratory. It can be trapped in an inert gas matrix or made as a short lived gas. C3O can be classified as a ketene or an oxocumulene a kind of heterocumulene.[3]

Natural occurrence

C3O has been detected by its microwave spectrum in the dark cold Taurus Molecular Cloud One[4] and also in the protostar Elias 18.[5]

The route to produce this is speculated to be:[6]

+ CO2 → HC3O+ + CO
HC3O+ → C3O + H+


C2 + CO → C3O which is more favourable at lower temperatures.

The related C3S is more abundant in dark molecular clouds, even though oxygen is 20 times more common than sulfur. The difference is due to the higher rate of formation and that C3S is less polar.[5]


C3O can be produced by heating Meldrum's acid. This also produces acetone, carbon monoxide and carbon dioxide.[7]

R. L. DeKock and W. Waltner were the first to identify C3O by reacting atomic carbon with carbon monoxide in an argon matrix. They observed an infrared absorption line at 2241 cm−1.[7] They produced carbon atoms by heating graphite inside a thin tantalum tube.[8]

M. E. Jacox photolysed C3O2 in an argon matrix to produce C3O with an IR absorption line at 2244 cm−1, however he did not recognise what was produced.[8]

By heating diazocyclopentanetrione or a similar acid anhydride, (2,4-azo-3-oxo-dipentanoic anhydride), C3O is produced. Also the action of light on tetracarbon dioxide yields C3O and CO.[9]

Heating fumaryl chloride also yields C3O.[3] Heating Lead 2,4-dinitroresorcinate also produces C3O along with C2O, CO and carbon suboxide.[10] An electric discharge in carbon suboxide produces about 11 ppm C3O.[11]

Roger Brown heated 3,5-dimethyl-1-propynolpyrazole to over 700 °C to make C3O.[12] Also pyrolysis of 5,5'-bis(2,2-dimethyl-4,6-dioxo-1,3-dioxanylidene or di-isopropylidene ethylenetetracarboxylate yields C3O.[12]

Irradiating carbon monoxide ice with electrons yields a mixture of carbon oxides, including C3O. This process could happen on icy bodies in space.[13]


C3O can be stabilised as a ligand in the pentacarbonyls of group 6 elements as in Cr(CO)5CCCO. This is formed from [n-Bu4N][CrI(CO)5] and the silver acetylide derivative of sodium propiolate (AgC≡CCOONa), and then thiophosgene. AgC≡CCOONa in turn is made from silver ions and sodium propiolate.[14] The blue black solid complex is called pentacarbony1(3-oxopropadienylidene)chromium(0). It is quite volatile and decomposes at 32 °C. Its infrared spectrum shows a band at 2028 cm−1 due to CCCO. The complex can dissolve in hexane, however it slowly decomposes, losing dicarbon (C2) which goes on to form acetylenes and cumulenes in the solvent. Dimethyl sulfoxide oxidises the CCCO ligand to carbon suboxide./[15]

C3O deposits a reddish-black film on glass.[12]

The reaction of C3O and urea is predicted to form uracil.[16] The pathway for this, is that firstly the two molecules react to form isocyanuric acid and propiolamide, the NH then reacts to bond with the triple bond, with the NH2 group moving back. Then a final cyclisation occurs to make uracil.[17]


The C3O molecules do not last long. At the low pressure of 1 pascal, they survive about one second.[18] The force constants for the bonds are:C1-O 14.94, C1-C2 1.39 C2-C3 6.02 mdyn/Å.[8] The bond lengths are C-O 1.149, C1-C2 1.300, C2-C3 1.273 Å. The molecule is linear.[6]

bond atom 1 atom 2 length
force constant
IR bands
CCC-O C1 O 1.149 14.94
CC-CO C2 C1 1.300 1.39
C-CCCO C3 C2 1.273 6.02

Proton affinity is 885 kJmol−1.[6] The dipole moment is 2.391 D.[14] The oxygen end has a positive charge, and the carbon end the negative charge.[6] The molecule behaves as if there are triple bonds at each end, and a single bond in the middle. This is isoelectronic to cyanogen.[19]

Molecular constants used in determining the microwave spectrum are rotational constant B0=4810.8862 MHz centrifugal distortion constant D0=0.00077 MHz. Known microwave spectral lines vary from 9621.76 for J=1←0 to 182792.35 MHz for J=19←18.[11]


  1. ^ Brown, Ronald D.; Rice, E. H. (October 1984). "Tricarbon monoxide - a theoretical study". Journal of the American Chemical Society. 106 (22): 6475–6478. doi:10.1021/ja00334a002.
  2. ^ Cooksy, A. L.; Watson, J. K. G.; Gottlieb, C. A.; Thaddeus, P. (February 1992). "The rotational spectrum of the carbon chain radical HCCCO". The Astrophysical Journal. 386: L27. Bibcode:1992ApJ...386L..27C. doi:10.1086/186284.
  3. ^ a b Ruppel, Raimund (1999). "Neue Heterokumulene und Carbene" (PDF) (in German). Gießen: Justus-Liebig-Universität: 13. Retrieved 10 November 2016. ((cite journal)): Cite journal requires |journal= (help)
  4. ^ Matthews, H. E.; Irvine, W. M.; Friberg, P; Brown, R. D.; Godfrey, P. D. (12 July 1984). "A new interstellar molecule: tricarbon monoxide". Nature. 310 (5973): 125–126. Bibcode:1984Natur.310..125M. doi:10.1038/310125a0. PMID 11541993. S2CID 4335136.
  5. ^ a b c Abbas, Haider (6 February 2014). "Neutral-neutral reactions for the formation of C3O and C3S". Astrophysics and Space Science. 351 (1): 53–57. Bibcode:2014Ap&SS.351...53A. doi:10.1007/s10509-014-1809-y. S2CID 124813337.
  6. ^ a b c d e Botschwina, Peter (1989). "A theoretical investigation of the astrophysically important molecules C3O and HC3O+". The Journal of Chemical Physics. 90 (8): 4301–4313. Bibcode:1989JChPh..90.4301B. doi:10.1063/1.455787.
  7. ^ a b Brown, Ronald D.; Eastwood, Frank W.; Elmes, Patricia S.; Godfrey, Peter D. (October 1983). "Tricarbon monoxide". Journal of the American Chemical Society. 105 (21): 6496–6497. doi:10.1021/ja00359a026. image of the tricarbon monoxide research team
  8. ^ a b c d DeKock, R. L.; Weltner, W. (December 1971). "C2O, CN2, and C3O molecules". Journal of the American Chemical Society. 93 (25): 7106–7107. doi:10.1021/ja00754a081.
  9. ^ Maier, Günther; Reisenauer, Hans Peter; Balli, Heinz; Brandt, Willy; Janoschek, Rudolf (August 1990). "C4O2(1,2,3-Butatriene-1,4-dione), the First Dioxide of Carbon with an Even Number of C Atoms". Angewandte Chemie International Edition in English. 29 (8): 905–908. doi:10.1002/anie.199009051.
  10. ^ Tang T.B. (1 February 1985). "Tricarbon monoxide and dicarbon monoxide: Addendum to "decomposition of lead (ii) 2,4-dinitroresorcinate"". Thermochimica Acta. 83 (2): 397–398. doi:10.1016/0040-6031(85)87024-6.
  11. ^ a b Tang, Tong B.; Inokuchi, Hiroo; Saito, Shuji; Yamada, Chikashi; Hirota, Eizi (April 1985). "CCCO: Generation by dc glow discharge in carbon suboxide, and microwave spectrum". Chemical Physics Letters. 116 (1): 83–85. Bibcode:1985CPL...116...83T. doi:10.1016/0009-2614(85)80130-5.
  12. ^ a b c Brown, Roger F.C.; Godfrey, Peter D.; Lee, Swee Choo (1985). "Flash vacuum pyrolysis of 1-propynoylpyrazoles: a new precursor of tricarbon monoxide". Tetrahedron Letters. 26 (51): 6373–6376. doi:10.1016/S0040-4039(01)84602-5.
  13. ^ Jamieson, Corey S.; Mebel, Alexander M.; Kaiser, Ralf I. (March 2006). "Understanding the Kinetics and Dynamics of Radiation‐induced Reaction Pathways in Carbon Monoxide Ice at 10 K". The Astrophysical Journal Supplement Series. 163 (1): 184–206. Bibcode:2006ApJS..163..184J. CiteSeerX doi:10.1086/499245.
  14. ^ a b Baceiredo, Antoine (2010). Transition Metal Complexes of Neutral Eta1-Carbon Ligands. Springer Science & Business Media. pp. 247–248. ISBN 9783642047213.
  15. ^ Berke, Heinz; Härter, Peter (March 1980). "Complex Stabilization of 3-Oxopropadienylidene(C3O) with Pentacarbonylchromium(0)". Angewandte Chemie International Edition in English. 19 (3): 225–226. doi:10.1002/anie.198002251.
  16. ^ Wang, Tianfang; Bowie, John H. (November 2011). "Studies of cyclization reactions of linear cumulenes and heterocumulenes using the neutralization-reionization procedure and/or ab initio calculations". Mass Spectrometry Reviews. 30 (6): 1225–1241. Bibcode:2011MSRv...30.1225W. doi:10.1002/mas.20328. PMID 21400561.
  17. ^ Wang, Tianfang; Bowie, John H. (2012). "Can cytosine, thymine and uracil be formed in interstellar regions? A theoretical study". Org. Biomol. Chem. 10 (3): 652–662. doi:10.1039/C1OB06352A. PMID 22120518.
  18. ^ Information, Reed Business (9 May 1985). "Theory predicts a new oxide of carbon". New Scientist (1455): 21. Retrieved 10 November 2016.
  19. ^ Brown, Ronald D.; Pullin, David E.; Rice, Edward H. N.; Rodler, Martin (December 1985). "The infrared spectrum and force field of tricarbon monoxide". Journal of the American Chemical Society. 107 (26): 7877–7880. doi:10.1021/ja00312a013.