Energetically More Stable Singlet Cyclopentane-1,3-diyl Diradical with π‑Single Bonding Character than the Corresponding σ‑Single Bonded Compound

Carbon–carbon σ-single bonds are crucial for constructing molecules like ethane derivatives (R₃C–CR₃), which are composed of tetrahedral four-coordinate carbons. Molecular functions, such as light absorption or emission, originate from the π-bonds existing in ethylene derivatives (R₂CCR₂). In this study, a relatively stable cyclopentane-1,3-diyl species with π-single bonding system (C−π–C) with planar four-coordinate carbons is constructed. This diradicaloid is energetically more stable than the corresponding σ-single bonding system. The π-electron single bonding system provides deeper insights into the chemical bonding and the physical properties derived from the small energy gaps between the bonding and antibonding molecular orbitals.