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Abstract:
Carbon-13 MAS−NMR and X-ray diffraction experiments on solid fluorobullvalene are reported. The compound crystallizes in the orthorhombic space group Pnam with four symmetry related molecules per unit cell. The crystal consists entirely of isomer 4 (in which the fluorine is bound to the bridgehead carbon). Temperature dependent 1D and 2D NMR exchange experiments reveal the occurrence of two independent dynamic processes, both preserving the crystal order, but on a completely different time scale. The faster of the two processes involves 3-fold jumps about the molecular (pseudo) C3 symmetry axis. Line shape analysis of dynamic 1D MAS spectra yields an Arrhenius rate equation with a pre-exponential factor, AJ = 6.0 × 1017 s-1, and an activation energy, EJ = 21.7 kcal mol-1. The mechanism of this process was confirmed by a rotor synchronized 2D exchange experiment performed with a mixing time of 20 ms. This spectrum exhibits auto cross peaks between spinning side bands of the same types of carbons, but no hetero cross peaks linking different types of carbons. Two-dimensional exchange spectra recorded on a much longer time scale (of the order of seconds) exhibit, in addition, hetero cross peaks between the main and spinning side bands of different types of carbons. These cross peaks can only result from Cope rearrangement involving other isomers of fluorobullvalene as intermediates. It is argued that the dominant mechanism of this process involves the sequence: isomer 4 → isomer 1 → isomer 3 → isomer 1 → isomer 4, where isomers 1 and 3 serve as transient intermediates. Magnetization transfer experiments provide the following estimates for the kinetic parameters of this process, AC = 4.6 × 109 s-1, EC = 14.5 kcal mol-1
