Direct Evidence for a Carbon-Carbon One-Electron σ-Bond, or a Weak Carbon-Carbon Interaction?

Shimajiri et al. (Nature, 2024, 634, 347-351) reported for the first time on direct evidence for a carbon-carbon one-electron σ bond in a recently synthesized stable radical cation. Single-crystal X-ray diffraction analysis, electron density maps, and RAM spectroscopy provided analytical results supporting the derived picture. However, the theoretical counterpart at the density functional theory level tends to support a different viewpoint, namely two sp² carbon atoms at a rather high distance of 2.921(3) Å undergoing a weak non-covalent interaction. In this contribution, we wish to highlight the discrepancies observed in relation to laboratory measurements as well as the similarities with a recent research by L. I. Lugo-Fuentes (Phys. Chem. Chem. Phys., 2026, 28, 683-691) with the aim of promoting new theoretical studies-going beyond the Kohn-Sham (KS) framework in the field of wavefunction correlated methods-on what could become a new benchmark system for the coming years. Taken together, the available data do not unequivocally demonstrate the existence of a discrete C─C one-electron σ-bond but instead highlight the difficulty of distinguishing extremely weak bonding interactions from non-bonded or delocalized radical states.