Near-infrared Fourier transform Raman, surface-enhanced Raman scattering and Fourier transform infrared spectra and ab initio calculations of the natural product nodakenetin angelate

Abstract

Near-infrared Fourier transform Raman and Fourier transform infrared spectra of nodakenetin angelate (C19H20O5), extracted from seeds of Heracleum candolleaum, were recorded and analysed. The root extract of this plant is used as an antiarthritic and nerve tonic. Ab initio SCF Hartree-Fock computations were performed employing the 6-31G basis set for geometry optimization for the prediction of IR and Raman spectral activities and wavenumber calculations. Parameters initially optimized using AM1 calculations were used as the input for ab initio computations. The computed results were used for the interpretation of the vibrational spectra. Important thermodynamic parameters were also provided. The strong band at 1712 cm -1 and medium-intensity band at 1731 cm-1 resulting from ester and lactone carbonyl vibrations, respectively, are identified in the Raman spectrum. The C=O stretching band in IR is broadened around 1717 cm -1 owing to the overlapping of ester and lactone carbonyl vibrations. The lowering of the carbonyl stretching vibrations is due to conjugation. The computed values indicate a larger degree of conjugation for the ester group. The characteristic vibrations of the furanocoumarin ring which is part of the molecule were identified. The CH stretching and bending vibrations of the CH3 group of the ester part indicate the presence of hyperconjugation. Vibrational analysis indicates the presence of blue-shifting H-bonds resulting from interaction of the methyl group The large enhancement of in-plane ring stretching and ring breathing modes in the surface-enhanced Raman scattering spectrum indicates that the molecule is adsorbed on the silver surface in a 'vertical' configuration, with the lactone ring perpendicular to the silver surface and probably on the opposite side of the lactonic C=O group. Copyright © 2004 John Wiley & Sons, Ltd.