Probing the Fluxional Bonding Nature of Rapid Cope rearrangements in Bullvalene C(10)H(10) and Its Analogs C(8)H(8), C(9)H(10), and C(8)BH(9)

Bullvalene C(10)H(10) and its analogs semibullvalene C(8)H(8), barbaralane C(9)H(10), and 9-Borabarbaralane C(8)BH(9) are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional π-bonds (2 2c-2e π → 2 3c-2e π → 2 2c-2e π) and one fluxional σ-bond (1 2c-2e σ → 1 4c-2e σ → 1 2c-2e σ) in their ground states and transition states, unveiling the universal π + σ 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 (13)C-NMR and (1)H-NMR shielding tensors and chemical shifts of the model compound C(8)BH(9) are computationally predicted to facilitate future experiments.