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8714611 
Journal Article 
Conformational study of cyclohexene oxide by dynamic NMR spectroscopy and ab initio molecular orbital calculations 
Pawar, DM; Noe, EA 
1998 
Yes 
Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 
120 
7 
1485-1488 
English 
The 13B spectra of cyclohexene oxide (1) show decoalescence of the peak at lowest frequency, with slow exchange at - 187.7 °C and a coalescence temperature slightly above - 178.2 °C. The low-temperature NMR results are interpreted in terms of two enantiomeric half-chair conformation, 1a and 1b, 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 °C obtained from Allinger's MM3 program are in reasonable agreement with the ab initio results for 25 °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 °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 of 25 °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 °C were 4.87 and 4.96 kcal/mol, in reasonable agreement with the experimental value at - 178.2 °C. Chemical shifts for the carbons of 1a were calculated at the HF/6-311* and HF/6-311G* and HF/6-311+G(2d,p) levels, using the GIAO method, to assign peaks to specific carbons.