The pathways of the combined Cope rearrangement — molecular reorientation process in solid bullvalene: a deuterium 2D exchange NMR study on a single crystal

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Abstract
Deuterium 2D exchange nuclear magnetic resonance (NMR) spectroscopy was used to elucidate the detailed mechanisms of the dynamic processes in solid bullvalene. The measurements were performed in the temperature range of -73 to 10°C on a deuterated single crystal with the magnetic field parallel to the monoclinic plane where the four molecules in the unit cell are magnetically equivalent. In agreement with earlier carbon-13 and deuterium NMR studies, two independent processes were identified: symmetric threefold jump and valence bond isomerization (Cope rearrangement) combined with molecular reorientation. The latter process can proceed in the crystal by nine different pathways, characterized by specific cross peak patterns in the 2D spectrum. Due to the low symmetry at the crystallographic sites of the bullvalene molecules the Cope rearrangement/reorientation process may proceed with different rates along the different pathways. Analyses of the experimental results show that only four pathways are actually active with approximately equal rates. Combining the present results with those obtained in earlier work yields the following rate equations for respectively the threefold jump and the overall Cope rearrangement/reorientation processes: kJ = 1·6 × 1018 exp (-80·5/RT) and kC = 7·0 × 1014 × exp (-65·0/RT), where R is in kJoule mol-1 deg-1 and the k are in s-1.