THE PATHWAYS OF THE COMBINED COPE REARRANGEMENT - MOLECULAR-REORIENTATION PROCESS IN SOLID BULLVALENCE - A DEUTERIUM-2D EXCHANGE NMR-STUDY ON A SINGLE-CRYSTAL

Deuterium 2D exchange nuclear magnetic resonance (NMR) spectroscopy wa s used to elucidate the detailed mechanisms of the dynamic processes i n solid bullvalene. The measurements were performed in the temperature range of -73 to 10-degrees-C on a deuterated single crystal with the magnetic field parallel to the monoclinic plane where the four molecul es in the unit cell are magnetically equivalent. In agreement with ear lier carbon-13 and deuterium NMR studies, two independent processes we re identified: symmetric threefold jump and valence bond isomerization (Cope rearrangement) combined with molecular reorientation. The latte r process can proceed in the crystal by nine different pathways, chara cterized by specific cross peak patterns in the 2D spectrum. Due to th e low symmetry at the crystallographic sites of the bullvalene molecul es the Cope rearrangement/reorientation process may proceed with diffe rent rates along the different pathways. Analyses of the experimental results show that only four pathways are actually active with approxim ately equal rates. Combining the present results with those obtained i n earlier work yields the following rate equations for respectively th e threefold jump and the overall Cope rearrangement/reorientation proc esses: k(J) = 1.6 x 10(18) exp 80.5/RT) and k(C) = 7.0 x 10(14) x exp (-65.0/RT), where R is in kJoule mol-1 deg-1 and the k are in s-1.