Synthesis, characterization, conformational equilibrium and intramolecular hydrogen bond analysis of Novel Azocalix[4]arenes including acryloyl moiety using DFT studies

Abstract Azocalix[4]arenes were synthesized via diazo-coupling reaction with aniline in 1:1, 1:2, 1:3 and 1:4 molar ratio. Novel Azocalix[4]arenes having Acryloyl moiety were prepared esterification reaction with metallic sodium and Acryloyl chloride with the same molar ratio of Azocalix[4]arene derivatives. The spectroscopic characterizations of the compounds were made by using IR, UV–Vis, 1H NMR and 13C NMR spectroscopies. In our study, the validation of the exact conformation of Azocalix[4]arene existing in a Cone, Partial Cone, 1,2-Alternate and 1,3-Alternate was realized by spectroscopic and DFT analyses. These results reveal that the compounds exist in the Cone conformation because of the existence of Intramolecular Hydrogen Bonds (IHBs) in the calixarene core. According to the conformational analysis, Cone conformation corresponds to the minimum energy for compounds 6, 7 and 8, as well. Both DFT and NMR results confirmed that a conformational rearrangement of compound 9 is 1,3-Alternate conformer. The experimental results were correlated with the theoretical calculations. In addition, Natural Bond Orbital (NBO) calculations were used to examine the electronic characteristics of the intramolecular hydrogen bonds. NBO energies show that the main contributions to energy stabilization correspond to LP → σ* interactions for IHBs, O⋅⋅⋅O H and the delocalisation LP → π* for O C O.