title: Structure and vibrational spectra of some 8-oxa[5]helicenes.
abstract: 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.
authors.affiliation: Department of Physics, University of Lucknow, Lucknow, UP, India.
authors.forename: Pallavi
authors.initials: P
authors.lastname: Thul
authors.forename: V P
authors.initials: VP
authors.lastname: Gupta
authors.forename: Soni
authors.initials: S
authors.lastname: Mishra
authors.forename: Poonam
authors.initials: P
authors.lastname: Tandon
mesh_terms: D015394:Molecular Structure; D011083:Polycyclic Compounds / Q000737:chemistry*; D013059:Spectrum Analysis, Raman / Q000379:methods*; D014732:Vibration
chemical_list: D011083:Polycyclic Compounds; C031660:helicenes