Conformational study of oxacyclooctan-2-one by dynamic NMR spectroscopy and computational methods

The slow-exchange C-13 NMR spectrum of the eight-membered lactone, oxacyclo octan-2-one (1), at -154.2 degrees C shows the presence of two conformation s 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 c arbon peak (-128.6 degrees C). The H-1 NMR spectrum of the CH2O protons spl its into two multiplets of equal intensity by -100 degrees C, and the corre sponding free-energy barrier of 9.16 kcal/mol at -85.2 degrees C was sugges ted to be associated with exchange of the geminal protons. Relative free en ergies and populations at +25 and -160 degrees C were estimated by using Al linger's MM3 program, and free energies and dipole moments were obtained fo r three conformations using ab initio calculations at the HF/6-311G* and MP 2/6-311G* levels. The spectra were interpreted in terms of two conformation s having the E configuration of the lactone group and resembling the boat-c hair conformation of cyclooctane.