Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The s-cis-trans isomerism of two furan derivatives [2-acetyl- (AF) and 2-acetyl-5-methylfuran, (AMF)] was analyzed, using data from the deconvolution of their carbonyl absorption band in two solvents (CH2-Cl2 and CH3CN). These infrared data showed that the O,O-trans conformers predominate in the less polar solvent (CH2Cl2), but these equilibria change in a more polar solvent (CH3CN) leading to a slight predominance of the O,O-cis conformers, in agreement with the theoretical calculations. The later results were obtained using B3LYP-IEFPCM/6-31++g(3df,3p) level of theory, which taking into account the solvent effects at IEFPCM (Integral Equation Formalism Polarizable Continuum Model). Low temperature C-13 NMR spectra in CD2Cl2 (ca. -75 degrees C) showed pairs of signals for each carbon, due to the known high energy barrier for the cis-trans interconversion leading to a large predominance of the trans isomers, which decreases in acetone-d(6). This was confirmed by their H-1 NMR spectra at the same temperatures. Moreover, despite the larger hyperconjugative interactions for the O,O-cis isomers, obtained from NBO data, these isomers are destabilized by the their Lewis energy. (C) 2012 Elsevier B.V. All rights reserved.1038489Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundacao AraucariaFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)