Cope rearrangements in shapeshifting molecules re-examined by means of high-level CCSDT(Q) composite ab initio methods

Benchmark reaction barrier heights for the degenerate Cope rearrangements in the highly fluxional bullvalene and semibullvalene hydrocarbon cages are obtained at the CCSDT(Q) level close to the one-particle basis set limit and include inner-shell, scalar-relativistic, and Born-Oppenheimer corrections. Our best theoretical CCSDT(Q) Gibbs free reaction barrier height for semibullvalene (ΔG₂₉₈^‡ = 27.9 kJ mol^-1) is in good agreement with the most recent experimental value of 25.9 kJ mol^-1. However, our CCSDT(Q) reaction barrier height for bullvalene (ΔG₂₉₈^‡ = 62.2 kJ mol^-1) indicates that the most recent gas-phase experimental value of 54.8 ± 0.8 should be revised upward.