Structural conformations and electronic interactions of the natural product, oroxylin: a vibrational spectroscopic study

The oroxylin, 5,7-dihydroxy 6-methoxy flavone is a potent natural product extracted from 'Vitex peduncularis'. Density functional theory (DFT) at B3LYP/6-311G(d,p) level has been used to compute energies of different conformers of oroxylin to find out their stability, the optimized geometry of the most stable conformer and its vibrational spectrum. The conformer ORLN-1 with torsion angles 0, 180,180 and 0 degrees, respectively, for H-13-O-12-C-6 -C-5, H-14-O-10-C-4-C-5, H-13-O-12-C-6-C-5 and H-14-O-10-C-4-C-5 is found to be most stable. The optimized geometry reveals that the dihedral angle p between phenyl ring B and the chrome part of the molecule in -19.21 degrees is due to the repulsive force due to steric interaction between the ortho-hydrogen atom H-29 of the B ring and H-18 of the ring C (H-29 ... H-18 = 2.198 angstrom). A vibrational analysis based on the near-infrared Fourier transform(NIR-FT) Raman, Fourier transform-infrared (FT-IR) and the computed spectrum reveals that the methoxy group is influenced by the oxygen lone pair-aryl p, orbital by back donation. Hence the stretching and bending vibrational modes of the methoxy group possess the lowest wavenumber from the normal values of methyl group. The carbonyl stretching vibrations have been lowered due to conjugation and hydrogen bonding in the molecules. The intramolecular H-bonding and nonbonded intramolecular interactions shift the band position of O-10-H-14 and O-12-H-13 stretching modes, which is justified by DFT results. Copyright (C) 2008 John Wiley & Sons, Ltd.