Conformational Study of Oxacyclooctan-2-one by Dynamic NMR Spectroscopy and Computational Methods

The slow-exchange (13)C NMR spectrum of the eight-membered lactone, oxacyclooctan-2-one (1), at -154.2 degrees C shows the presence of two conformations with populations of 25.6 and 74.4%. Free-energy barriers of 6.98 and 7.23 kcal/mol were determined at the coalescence temperature for the carbonyl carbon peak (-128.6 degrees C). The (1)H NMR spectrum of the CH(2)O protons splits into two multiplets of equal intensity by -100 degrees C, and the corresponding free-energy barrier of 9.16 kcal/mol at -85.2 degrees C was suggested to be associated with exchange of the geminal protons. Relative free energies and populations at +25 and -160 degrees C were estimated by using Allinger's MM3 program, and free energies and dipole moments were obtained for three conformations using ab initio calculations at the HF/6-311G and MP2/6-311G levels. The spectra were interpreted in terms of two conformations having the E configuration of the lactone group and resembling the boat-chair conformation of cyclooctane.