Conformational behaviour of antioxidant chromones. A vibrational spectroscopy study

Chromone and two carboxylic derivatives – chromone-2-carboxylic acid (2CA) and chromone-3-carboxylic acid (3CA) – were investigated as to their conformational preferences by vibrational spectroscopy – Raman, FTIR and INS – combined to density functional theory and plane-wave calculations. INS experiments allowed to clearly detect intramolecular hydrogen-type close contacts in these systems. A natural bond orbital (NBO) analysis was performed, including calculation of the second-order perturbation energies (E(2)) and orbital occupancies, to provide theoretical evidence of the stabilisation due to the πi*→πj* donor–acceptor interaction. A complete assignment of the vibrational spectra was achieved for all compounds, allowing us to determine their most stable conformers and establish their H-bonding profile. 3CA was shown to display a particularly strong intramolecular H-bond, which can explain its singular conformational behaviour and abnormally high pKa value.

Graphical abstractA full vibrational analysis was performed for chromone and two carboxylic acid derivatives – IR, Raman and INS spectroscopies, coupled to quantum mechanical calculations, with particular emphasis on the H-bond interactions ruling the conformational behaviour of these potentially cancer chemopreventive phytochemicals.Figure optionsDownload full-size imageDownload as PowerPoint slide