Reaction Pathways in Solid−State Processes. 2. Carbon−13 NMR and X−ray Crystallography of Cyanobullvalene and Bullvalenecarboxylic acid

Carbon−13 MAS−NMR measurements and the X−ray crystallographic structure of cyanobullvalene (I) and bullvalenecarboxylic acid (II) are reported. The two compounds crystallize as isomer 3 in the triclinic space group P. Two−dimensional solid−state NMR exchange experiments indicate the occurrence of bond shift (Cope) rearrangement in both cases. Analysis of the results shows that the process involves isomer 1 as a transient intermediate. The Arrhenius kinetic parameters were determined from the line broadening in the 1D MAS spectra and magnetization transfer experiments as function of temperature, yielding, AC = 1.0 × 1012 s−1, 2.2 × 1012 s−1 and EC = 13.9 kcal mol−1, 14.1 kcal mol−1 for compounds I and II, respectively. A rearrangement process involving isomer 2 as an intermediate may also occur, but it does not result in permutation of atoms and therefore is NMR−invisible. NMR exchange measurements on solid cyclooctatetraene dimer (III) were also carried out; however, on the time scale of up to 1 min no dynamic effects are observe