Conformational energetics and low-frequency vibrations of cyclohexene and its oxygen analogs

Ab initio and DFT calculations with MP2/cc-pVTZ and B3LYP/cc-pVTZ basis sets have been carried out for cyclohexene and four of its oxygen analogs. All of the molecules possess a twisted structure while the bent forms represent saddle points in two-dimensional surfaces. The structures, relative energies, and frequencies for the lowest energy vibrations of the twisted, bent, and planar forms were calculated and compared to experimental results. The calculated results agree very well with the microwave data but the computed barriers are somewhat less than those based on low-frequency infrared data.