The C-13 spectra of cyclohexene oxide (1) show decoalescence of the peak at lowest frequency, with slow exchange at -187.7 degrees C and a coalescence temperature slightly above -178.2 degrees C. The low-temperature NMR results are interpreted in terms of two enantiomeric half-chair conformations, la and Ib, which could interconvert by way of either the endo-boat (1c) or exo-boat (1d) conformation. Ab initio calculations indicate that the endo-boat is significantly lower in energy than the exo-boat. Both boat conformations are shallow energy minima, as evidenced by the absence of imaginary frequencies. Relative free energies for the three conformations at -187.7 degrees C obtained from Allinger's MM3 program are in reasonable agreement with the ab initio results for 25 degrees C. A possible explanation for the greater stability of the endo-boat in terms of less eclipsing for the CH hydrogens of the three-membered ring with the CH2 hydrogens on the adjacent carbons is supported by calculated geometries. The experimental rate constant and free-energy barrier for interconversion of 1a and 1b were 227 s(-1) and 4.3 +/- 0.2 kcal/mol at -178.2 degrees C, and the corresponding parameters for the conversion of the half-chair to the endo-boat were 454 s(-1) and 4.2 kcal/mol at this temperature. Estimates of the free energy at 25 degrees C of the transition state leading to the ring inversion were obtained at the HF/6-311G* and MP2/6-311G* levels by using the STQN method and were found to be 1.09 and 0.88 kcal/mol, respectively, above the local endo-boat minima. The corresponding calculated half-chair to endo-boat free-energy barriers at 25 OC were 4.87 and 4.96 kcal/mol, in reasonable agreement with the experimental value at -178.2 degrees C. Chemical shifts for the carbons of 1a were calculated at the HF/6-311G* and HF/6-311+G(2d,p) levels, using the GIAO method, to assign peaks to specific carbons.