Dissecting the single-electron C–C bond: NBO and AIM perspectives

Abstract

In this work, we present a comprehensive electronic structure analysis of the highly praised oxidation product of tricyclic compound spiro-dibenzocycloheptatriene (1p), with emphasis on characterizing the nature of the non-covalent interaction between ipso carbon atoms (C1 and C2), which has been characterized as an allegedly single electron sigma C–C bond. Our NCI analysis reveals that the interaction between the tricyclic moieties is weak and predominantly van der Waals in character (regardless of the counterion). AIM theory identifies a persistent bond critical point between C1 and C2 across all structures, albeit with low electron density indicative of a weak interaction. A comparison of the Laplacian density contour between C1–C2 and a previously reported B–B single-electron σ-bond (herein labeled as 1p-B), shows charge depletion between C1–C2 whereas in B1–B2 there is charge accumulation, characteristic of a σ-bond. The spin density population shows that half the radical is distributed among the two tricyclic structures. Our NBO and NRT analyses indicate that a single-electron σ-bond is present in some resonance structures, although its overall contribution is minimal. The calculated natural bond order for C1–C2 in the cationic form is only 0.066, suggesting limited bond character. Finally, NBO deletion analysis quantifies the interaction energy between the rings, showing that the C1–C2 interaction contributes only ∼9.5% to the total π–π interaction energy, primarily through donor–acceptor interactions between bonding orbitals and Rydberg orbitals. These results converge to show that the C1–C2 interaction is a weak, highly delocalized interaction governed by subtle electronic effects rather than a single electron σ-bond.