Vibrational analysis and NMR properties based on ab initio and DFT calculations of two naturally occurring xanthones: 1,5-dihydroxy-2,3-dimethoxyxanthone and 1-hydroxy-5-methoxy-2,3-methylenedioxyxanthone

This work reports a investigation concerning the NMR and vibrational spectra of 1,5-dihydroxy-2,3-dimethoxyxanthone (1) and 1-hydroxy-5-methoxy-2,3-methylenedioxyxanthone (2). The Hartree-Fock (HF) and the hybrid DFT/HF B3-LYP methods rendered planar structures, with the hydroxyl groups forming hydrogen bonding with the carbonyl group, and the methoxy groups in the ring plane, exception made for the methoxy group at position 2 of compound (1) and the methoxy group at position 5 in compound (2). The calculated NMR chemical shifts fitted very well to experimental values previously reported, except for the hydroxyl proton, because of the hydrogen bonding. The importance of theoretical methods in the assignment of quaternary carbons (4a, 4b, 8a and 8b) chemical shifts in the 13C NMR spectra was demonstrated since their high relaxation time preclude their observation in the spectra, as suggested by the absent signal of the C-8a in the 13C NMR spectra. The infrared and micro FT-Raman spectra were obtained and an assignment based on the HF and B3-LYP methods was performed, presenting the normal mode descriptions for the first time. The comparison between experimental and calculated values was very good. It was observed an extensive coupling between ring and substituent group modes. The B3-LYP method overestimates the hydrogen bonding formed between the hydroxyl group at position 1 and the carbonyl group, as revealed by the shorter distance and the lower wavenumber obtained for the ν(OH), if compared to HF method.