Structure and vibrational spectra of some 8-oxa[5]helicenes

A systematic study has been conducted on the geometrical and electronic structure, heliomeric conformations of a series of 8-oxa[5]helicenes based on density functional theory (DFT) computations. A complete vibrational analysis has also been attempted for one of the 8-oxa[5]helicenes (molecule 5) on the basis of experimental infrared spectra in the far and mid infrared regions (60–3100 cm−1) and density functional theory computations using B3LYP/6-31G** method characteristic bands of the molecule identified. The approximate mean plane angle between the terminal rings A and E in the presently studied molecules are found to have values between 48.64° and 59.46°. This angle is much larger than the corresponding angle between the terminal rings in benzo[c]phenanthrene (∼27°) and partially reduced benzo[c]phenanthrene (34.6–46.0°) and indicates that the presence of oxygen-containing six-membered ring provides a greater helicity to the molecules. Detailed quantum chemical study on molecule 4 shows the existence of two enantiomeric forms M- and P- of almost equal energy separated by a potential barrier of 15.55 kcal/mol. It is expected that similar 8-oxa[5]helicines (molecules 3, 5 and 6) may also exist in two enantiomeric forms.