Vibrational spectra study of p-sulfonatocalix[4]arene containing azobenzene groups

Abstract

The FTIR and FT Raman spectra of p-sulfonatocalix[4]arene acid, having azobenzene groups (FA), and its tetrasodium salt (S) were studied. Vibrational spectra of acid and its salts were calculated using the DFT method. The energy differences between E- and Z-forms of azobenzene groups in FA and S are 53.2 and 30.8 kcal/mol, respectively.The p-sulfonatocalix[4]arene is in cone conformation with a cyclic system of hydrogen bonds. The strength of the hydrogen bonds is lower in the salt molecule. The orientation of the aromatic units in the acid and salt is different. The assignment of the bands in the experimental vibrational spectra of the calixarenes was carried out by calculating the contributions of the internal coordinates to the potential energy. We identified characteristic bands for different conformations of the azobenzene groups. The analysis of molecular orbitals shows that HOMO is localized on aromatic units and that LUMO is associated with sulphonate groups.