| Abstract
| - Carbon-13 MAS−NMR and X-ray diffraction experimentson solid fluorobullvalene are reported. Thecompound crystallizes in the orthorhombic space group Pnamwith four symmetry related molecules per unit cell.The crystal consists entirely of isomer 4 (in which the fluorineis bound to the bridgehead carbon). Temperaturedependent 1D and 2D NMR exchange experiments reveal the occurrence oftwo independent dynamic processes,both preserving the crystal order, but on a completely different timescale. The faster of the two processes involves3-fold jumps about the molecular (pseudo) C3symmetry axis. Line shape analysis of dynamic 1D MASspectrayields an Arrhenius rate equation with a pre-exponential factor,AJ = 6.0 × 1017s-1, and an activation energy,EJ= 21.7 kcal mol-1. The mechanism of this processwas confirmed by a rotor synchronized 2D exchangeexperimentperformed with a mixing time of 20 ms. This spectrum exhibits autocross peaks between spinning side bands ofthe same types of carbons, but no hetero cross peaks linking differenttypes of carbons. Two-dimensional exchangespectra recorded on a much longer time scale (of the order of seconds)exhibit, in addition, hetero cross peaksbetween the main and spinning side bands of different types of carbons.These cross peaks can only result fromCope rearrangement involving other isomers of fluorobullvalene asintermediates. It is argued that the dominantmechanism of this process involves the sequence: isomer 4 → isomer1 → isomer 3 → isomer 1 → isomer 4, whereisomers 1 and 3 serve as transient intermediates. Magnetizationtransfer experiments provide the following estimatesfor the kinetic parameters of this process, AC= 4.6 × 109 s-1,EC = 14.5 kcalmol-1.
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