An exotic 3-center/4-electron carbon–carbon pi long-bond: Is it tangible?

Centrosymmetric species with AB2 boding motif where central atom (A) is relatively more electronegative than the end ligands (B atoms) have been predicted to exhibit an unusual form of three-center/four-electron (3c/4e) long-bonding bonding/antibonding interactions occur between two (B–B) bypassing albeit via sharing latter’s lone pair. Such a long-bond sigma- and pi-type has so far as single bonds mainly linear (B–A–B) bonding motifs. This work, through computations based on density-functional theory (DFT) natural bond orbital (NBO) analysis, reveals carbon atoms separated by 2 angstroms in gas-phase chain species, namely CnX (X = O,N; n 2,4,6,8). Notably, this DFT NBO not only successfully predicts nonlinear (mainly those X atom) but also able reveal multiple pi long-bonds particularly triplet-C2O well isomer C8N N atom. However, preliminary using complete active space self-consistent field (CASSCF) multi-reference method performed work could confirm antibonding orbital, whereas validity proposed be ascertained CASSCF.