Gas-phase NMR study of the degenerate cope rearrangement of bullvalene

Gas-phase ¹H NMR spectra of tricyclo(3.3.2.0)deca-2,7,9-triene (bullvalene) obtained at temperatures above 340 K, display a single broad resonance 4.271 ppm down field from gaseous TMS whose line width decreases with increasing temperature and pressure. Spin-lattice relaxation does not contribute significantly to the line width over the pressure and temperature ranges studied and the observed broadening can be attributed to proton exchange associated with the degenerate Cope rearrangement. Infinite pressure, unimolecular rate constants for the Cope rearrangement of gaseous bullvalene are ca. 15% lower than those observed in solutions of bullvalene in CS₂ at the same temperatures. Gas-phase kinetic parameters for the rearrangement are E_act = 13.8 (0.2) kcal mol^-1, A_∞ = 2.1 (1.5) × 10¹³ s^-1, ΔG^‡₂₉₈ = 13.1 (0.2) kcal mol^-1, ΔH^‡ = 13.2 (0.3) kcal mol^-1, and ΔS^‡ = 0.4 (1.2) cal mol^-1 K^-1. The pressure dependence of the rate constants indicates that intramolecular vibrational redistribution in critically energized bullvalene molecules is statistical or nearly so.