Synthesis, isomerization, and DFT studies of five-membered heterocyclic ketoximes

A homologous series of five-membered heterocyclic ketoximes is synthesized by condensation reactions of 2-acetylpyrrole, 2-acetylthiophene, and 2-acetylfuran with hydroxylamine hydrochloride. These compounds were fully characterized by X-ray single-crystal diffraction, elemental analysis, H-1 NMR, IR, UV-Vis, and fluorescence spectra. Interesting isomerization phenomena are observed for these heterocyclic ketoximes by different characterization methods. By means of H-1 NMR and X-ray single-crystal diffraction, it was found that 2-acetylpyrrole oxime (1) exists as the Z-type geometric isomer, 2-acetylfuran oxime (3) exists as the E-isomer, while 2-acetylthiophene oxime (2) is identified as a mixture of Z- and E-isomers. To study the above phenomena further, quantum chemical calculations of total energy, dipole moment, E-HOMO, E-LUMO, and other parameters of each heterocyclic ketoxime were carried out by Density functional theory (DFT). The theoretical calculations were in good accordance with the experimental spectroscopic results.