The single carbon-carbon bond is among the most familiar covalent connections in organic compounds. But a new study suggests that chemists have yet to fully explore the limits of this basic bond.

Scientists have previously found that these ubiquitous bonds, which typically measure about 1.54 Å in length, can be elongated through the use of bulky substituents to endow molecules with special properties. Now, researchers led by Takanori Suzuki and Yusuke Ishigaki at Hokkaido University have synthesized a molecule they claim has the longest reported C–C bond among neutral hydrocarbons, measuring 1.806 Å in length (Chem 2018, DOI: 10.1016/j.chempr.2018.01.011).

The team set out to break the record by designing a stable dihydropyracylene compound with a highly strained core and two spirocyclic units that are forced to face each other, helping stretch out the central carbon-carbon bond. The chemists confirmed the presence of the C–C bond by observing its stretching vibration through Raman spectroscopy, and they measured the molecule’s record bond length using X-ray crystallography.

The bond breaks the theoretical limit of 1.803 Å, previously calculated for caged dimer compounds. This limit is set at the point at which the bond has a dissociation energy of zero and would thus dissociate. Chemists calculated the limit by assuming a linear relationship between bond length and bond dissociation energy. The Hokkaido researchers suggest that molecules like theirs with extralong bonds may deviate from this linear relationship.

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Structures with extremely long or short bonds help us refine our understanding of chemical bonding, Peter R. Schreiner of Justus Liebig University Giessen says. He thinks work like this, which pushes the limits of bonding, is worth careful consideration. “It also urges us to keep asking the question, When is a bond a bond?”