Cycloncarbons─molecular rings composed exclusively of sp-hybridized carbon atoms─represent a uniquely demanding frontier in carbon allotrope chemistry owing to their extreme structural sensitivity and unconventional π-electron behavior. The recent on-surface synthesis of cyclo25 carbon (C25), the largest odd-membered ring realized to date, provides an unprecedented opportunity to interrogate the fundamental properties of electron-deficient, symmetry-broken carbon nanostructures. Here we combine extensive first-principles calculations─spanning ground-state electronic structure, magnetic response, and thermodynamic stability─to elucidate the bonding and antiaromatic character of C25. We show that C25 adopts a planar Cs-symmetric triplet ground state featuring pronounced bond-length and bond-angle alternation, reflecting a delicate interplay between Peierls distortion and electron delocalization. π-Electron analysis reveals a striking departure from even-membered cyclocarbons: the πin subsystem supports simultaneous clockwise and counterclockwise induced ring currents under an external magnetic field. This counterintuitive duality originates from the intrinsic phase discontinuity of odd-membered rings, wherein the πin wave function cannot achieve periodic phase closure and undergoes a π phase inversion between the inner and outer regions of the ring. ACID, NICS and ICSSZZ analyses collectively confirm that C25 is globally antiaromatic, with its unusual current patterns arising from phase-frustrated π conjugation rather than conventional Hückel-type electron counting. NICSZZ analysis of structures sampled from AIMD trajectories confirms the thermally robust weak antiaromaticity of C25 without aromaticity inversion. Excited-state calculations further show that the dominant absorption features stem from strongly localized π → π* transitions involving mixed contributions from both πin and πout manifolds. These results identify C25 as a phase-frustrated, open-shell cyclocarbon with structural and magnetic properties fundamentally distinct from those of even-membered homologues. The insights obtained here provide a basis for understanding antiaromaticity and excited-state behavior in odd-membered sp-carbon rings and may facilitate future studies of larger or functionalized cyclocarbons.
Wu et al. (Wed,) studied this question.