Conformational study of 1,2-cycloundecadiene by dynamic NMR spectroscopy and computational methods

Solutions of 1,2-cycloundecadiene in propane were studied by low-temperature (13)C NMR spectroscopy. A total of 17 peaks were observed at -166.7 degrees C, corresponding to two conformations of similar populations, one of C(1) symmetry (11 peaks) and the other of C(2) symmetry. The line shapes show that the predominant pathway for exchange of the topomers (C(1) and C(1)') of the C(1) conformation does not include the C(2) conformation. From the (13)C spectra, free-energy barriers of 8.38 +/- 0.15, 9.45 +/- 0.15, and 9.35 +/- 0.15 kcal/mol were determined for the C(1) to C(1)', (C(1) + C(1)') to C(2), and C(2) to (C(1) + C(1)') conversions, respectively, at -72.2 degrees 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.