Studies on the s-cis-trans isomerism for some furan derivatives through IR and NMR spectroscopies and theoretical calculations

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• 作者：Roberto Rittner^a ; ^{rittner@iqm.unicamp.br} ; Lucas C. Ducati^a ; Clá ; udio F. Tormena^a ; Rodrigo A. Cormanich^a ; Barbara C. Fiorin^b ; Carolyne B. Braga^b ; Raymond J. Abraham^c
• 关键词：Conformational analysis ; S-cis&ndash ; trans isomerism ; Infrared spectroscopy ; NMR spectroscopy ; Theoretical calculations ; Furan derivatives
• 刊名：Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
• 出版年：2013
• 出版时间：15 February, 2013
• 年：2013
• 卷：103
• 期：Complete
• 页码：84-89
• 全文大小：283 K

文摘

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 (CH₂Cl₂ 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 ¹³C NMR spectra in CD₂Cl₂ (ca. ?75 ¡ãC) 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-d₆. This was confirmed by their ¹H 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.