Vibrational Potential Energy Surfaces of 3, 4-Dihydro-2H-pyran, 3, 6-Dihydro-2H-pyran, 2, 3-Dihydro-1, 4-dioxin, and 4H-1, 3-Dioxin

The far-infrared and Raman spectra of several oxygen analogues of cyclohexene have been reported. The data have been reanalyzed for each molecule in terms of two-dimensional vibrational potential energy surfaces for the ring-bending and ring-twisting vibrations. Kinetic energy functions for each vibration were determined using vectorial methods. Coefficients for quartic-quadratic potential energy functions with fourth-order cross terms were calculated. In each case, the twisted (half-chair) conformation has the lowest energy. The bent (boat) conformations were determined to be saddle points in the surfaces with energies 3120 to 3830 cm⁻¹(37.2 to 45.6 kJ/mol) above the energy minima. The barriers to planarity range from 3500 to 4130 cm⁻¹(41.8 to 49.4 kJ/mol). The angles determined for the twisting of the ring are in satisfactory agreement with microwave results.