Vibrational spectroscopy and conformational studies of 1,4-benzodioxan: ab initio and density functional calculations

Abstract

The gas-phase infrared and pure-liquid Raman spectra of 1,4-benzodioxan have been recorded and analyzed. The infrared vibrational frequencies, absolute intensities, and ring-inversion potential energy profile of the molecule have been also predicted using Becke's three-parameter hybrid (B3LYP) method in the density functional theory (DFT) method, as well as the Hartree–Fock (HF) and molecular mechanics (MM3) methods. The vibrational frequencies calculated at the B3LYP levels agree much better with the observed frequencies than those predicted by the HF or MM3 methods. The enlargement of basis sets at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. The ring-inversion process of 1,4-benzodioxan between the twist and bent conformers has also been investigated using the DFT, HF, and MM3 methods. The calculated results at the B3LYP/6-31G* level indicate that the twisted conformer has a lower energy than the bent conformer and that the energy difference between the two forms is 7.5 kcal/mol. This value is 1.3 kcal/mol lower than the barrier of nonbenzene-fused ring 1,4-dioxene.