Towards the Quadruple Carbon-Carbon Bond

Posted on 24 August 2017

New theoretical studies support the view that C2 has partial quadruple bond character.

C2 has partial quadruple bond character yet is weaker than a triple bond

All chemists are well-acquainted with alkanes, alkenes and alkynes, which have single, double and triple carbon-carbon bonds respectively. But what happens in dicarbon C2, a molecule which only exists at high temperature in the gas phase? Should the bond be considered as a double bond, a triple bond or perhaps even a unique quadruple bond? This intriguing question has been the subject of much speculation in the field of quantum chemistry.

Recent research from the theoretical chemistry group of Dr Peter B Karadakov uses magnetic shielding studies of C2 and C2H2 to provide new insight into this problem. Off-nucleus shielding calculations indicate that the carbon–carbon bond in C2 is more shielded than the triple carbon–carbon bond in the alkyne C2H2, and, therefore, has higher than triple bond multiplicity. 

Intriguingly, however, the authors find the carbon–carbon bond in C2 is bulkier than in C2H2, and, although it has partial quadruple bond character, it is actually weaker than the corresponding triple bond in C2H2.

This study makes use of a new approach developed in Karadakov’s group based on the analysis of changes in the off-nucleus magnetic shielding tensor within the space surrounding a molecule, which has been shown to reveal more information about chemical bonding than simple variations in electron density.

Notes to editors:

The paper, 'Magnetic Shielding Studies of Cand C2H2 Support Higher than Triple Bond Multiplicity in C2' is published in Chemistry A European Journal, where it was selected by reviewers and editors as a Hot Paper