VIBRATIONAL-SPECTRA AND CONFORMATIONS OF 1,4-CYCLOHEXADIENE AND ITS OXYGEN ANALOGS - AB-INITIO AND DENSITY-FUNCTIONAL CALCULATIONS

The vibrational spectra and ring-puckering potential energy functions of 1,4-cyclohexadiene, 4H-pyran and 1,4-dioxin have been examined usin g a density functional theory (DFT) method as well as the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) methods. The calculated vi brational frequencies and potential energy functions of those molecule s have been compared with previously reported experimental data and MM 3 results. For all three molecules, the DFT method using Becke's three -parameter functional (B3LYP) has led to the prediction of more accura te vibrational frequencies than the HF and MP2 methods. The enlargemen t of the basis set at the B3LYP levels has improved the accuracy of ca lculated vibrational frequencies. In particular, the C-O-C=C torsional force field parameters obtained from the B3LYP method have correctly predicted the ring-puckering potential energy functions of the oxygen- containing analogues, 4H-pyran and 1,4-dioxin, which could not be done by the MM3 method. (C) 1998 Elsevier Science B.V. All rights reserved .