Probing the Fluxional Bonding Nature of Rapid Cope rearrangements in Bullvalene C10H10 and Its Analogs C8H8, C9H10, and C8BH9

Bullvalene C10H10 and its analogs semibullvalene C8H8, barbaralane C9H10, and 9-Borabarbaralane C8BH9 are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional pi-bonds (2 2c-2e pi -> 2 3c-2e pi -> 2 2c-2e pi) and one fluxional sigma-bond (1 2c-2e sigma -> 1 4c-2e sigma -> 1 2c-2e sigma) in their ground states and transition states, unveiling the universal pi + sigma double fluxional bonding nature of these fluctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fluxional bonds dominating the dynamics of the systems. The C-13-NMR and H-1-NMR shielding tensors and chemical shifts of the model compound C8BH9 are computationally predicted to facilitate future experiments.