Complexes Based on Calix[4]arene Sulfonic Acid with Acetic Acid and Its Derivatives: NMR Analysis

Abstract

Calix[4]arene sulfonic acid complexes with acetic acid CH₃COOH and its derivatives CCl₃COOH, CF₃COOH were studied by nuclear magnetic resonance (NMR). The formation of guest–host type complexes was shown by magic angle spinning (MAS) NMR and high-resolution NMR techniques. Increase in calixarene molecule conformational flexibility with moisture content rising was observed by ¹³C MAS NMR. Consequently, CCl₃COOH, CF₃COOH molecules vacate calixarene cavity at relative humidity RH > 33%. While the complex with CH₃COOH remains stable up to RH = 64%. The self-diffusion processes were investigated by ¹H pulsed field gradient technique. Spin-echo attenuation curves show non-exponential shape indicating the occurrence of several phases with a different translational mobility. The existence CCl₃COOH, CF₃COOH acid molecules inside the calixarene cavity leads to average self-diffusion coefficient D_s^aver reduction compared with pure calix[4]arene sulfonic acid. The values of D_s^aver for the CH₃COOH complex are close to average self-diffusion coefficient of calix[4]arene sulfonic acid. From D_s^aver proton conductivities σ_calc on the basis of the Nernst–Einstein equation were calculated. Calculated σ_calc and experimental σ_exp conductivities are agreed well in the RH range from 33% to 75%. At low water content (RH = 10%) σ_exp is about one order of magnitude less compare with σ_calc in calixarene and its complexes with CH₃COOH and CCl₃COOH. It may be explained by low conductivity boundary formation at these conditions between grains in process of target sample preparation for conductivity measurement.