Conformational Study of 1,2-Cycloundecadiene by Dynamic NMR Spectroscopy and Computational Methods

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Abstract
Solutions of 1,2-cycloundecadiene in propane were studied by low-temperature 13C NMR spectroscopy. A total of 17 peaks were observed at −166.7 °C, corresponding to two conformations of similar populations, one of C1 symmetry (11 peaks) and the other of C2 symmetry. The line shapes show that the predominant pathway for exchange of the topomers (C1 and C1‘) of the C1 conformation does not include the C2 conformation. From the 13C spectra, free-energy barriers of 8.38 ± 0.15, 9.45 ± 0.15, and 9.35 ± 0.15 kcal/mol were determined for the C1 to C1‘, (C1 + C1‘) to C2, and C2 to (C1 + C1‘) conversions, respectively, at −72.2 °C. The NMR results for this compound are discussed in terms of the conformations predicted by molecular mechanics calculations obtained with Allinger's MM3 program. Ab initio calculations of free energies are also reported at the HF/ 6-311G* level for 25 conformations.