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 Norberto Sanches Gonc ¸alves a, * , Rodrigo Cristiano b , Moacir Geraldo Pizzolatti b , Fabio da Silva Miranda a a Laborato ´rio de Estrutura e Espectroscopia Molecular, Departamento de Quı ´mica, Universidade Federal de Santa Catarina, Campus Universitario Trindade, CP476, 88040-900 Floriano ´polis, SC, Brazil b Laborato ´rio de Produtos Naturais, Departamento de Quı ´mica, Universidade Federal de Santa Catarina, CP476, 88040-900 Floriano ´polis, SC, Brazil Received 17 June 2004; revised 28 July 2004; accepted 30 July 2004 Available online 11 September 2004 Abstract 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 13 C 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 13 C 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 n(OH), if compared to HF method. q 2004 Elsevier B.V. All rights reserved. Keywords: Xanthones; Hartree–Fock; B3-LYP; Vibrational spectroscopy; NMR spectroscopy 1. Introduction Xanthones are secondary metabolites commonly occur- ring in a few higher plant families, as well as in fungi and lichens [1]. Their basic structure is depicted in Fig. 1. They posses a broad spectrum of biological activities, including in vitro cytotoxicity, in vivo antitumour activity, antifungal, antibacterial and anti-inflammatory properties [2]. The compounds (1) (1,5-dihydroxy-2,3-dimethoxyxanthone) and (2) (1-hydroxy-5-methoxy-2,3-methylenedioxy- xanthone) were previously isolated by our group from Polygala paniculata [3]. Even though many xanthones have been isolated from Polygala species, only few spectral data were discussed in the literature [4] and [5]. Infrared spectroscopy in xanthone chemistry is generally used to detect the vibrational modes of carbonyl group (n(CaO)) [6], as well as other functional groups, such as unchelated hydroxyl and methyl group, while 1 H and 13 C 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.07.038 Journal of Molecular Structure 733 (2005) 53–61 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: C55 44 3319219; fax: C55 11 30913890. E-mail address: norberto@qmc.ufsc.br (N.S. Gonc ¸alves).