Probing the Fluxional Bonding Nature of Rapid Cope rearrangements in Bullvalene CH and Its Analogs CH, CH, and CBH.

Bullvalene CH and its analogs semibullvalene CH, barbaralane CH, and 9-Borabarbaralane CBH 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 C-NMR and H-NMR shielding tensors and chemical shifts of the model compound CBH are computationally predicted to facilitate future experiments.