Conformational preferences for some 5-substituted 2-acetylthiophenes through infrared spectroscopy and theoretical calculations

Abstract

The s- cis– trans isomerisms of some derivatives of thiophene (2-acetyl, AT; 2-acetyl-5-bromo, ABT and 2-acetyl-5-chloro, ACT) were analyzed, using data from deconvolution of their carbonyl absorption bands in two solvents (CCl 4 and CHCl 3). These infrared data showed that the O,S- cis conformer largely predominates in the studied solvents and that the same occurs in the gas phase, as observed from theoretical calculations. The latter results were obtained using B3LYP/6-311++G(3df,3p) and MP2/6-311++G(3df,3p) levels of theory, with zero-point energy correction. Moreover, the use of the IEFPCM (Integral Equation Formalism Polarizable Continuum Model) to take into account the solvent effects, using the same levels of theory, confirmed the results observed from infrared data. Low temperature 13C NMR spectra in CS 2/CD 2Cl 2 (−90 °C) and in acetone- d 6 (−80 °C) did not show pairs of signals for each carbon, due to the known low energy barrier (∼8 kcal mol −1) for the cis– trans interconversion. Data from NBO calculations show that the n O ( 2 ) → σ S-C 5 * and n O ( 2 ) → σ C 2 – C 3 * interactions occur only in the O,S -cis isomer and can explain its conformational preference.