Microwave Synthesis, Characterization and Theoretical Study of para-Toluenesulfonic Acid Threonine Salt and Its Ester Derivatives

Abstract

Some new amino acid ionic liquids (ILs) derived from threonine (Thr) were studied. The para-toluenesulfonic acid threonine and its methyl ester (ThrC1-TsOH) and ethyl ester (ThrC2-TsOH) derivatives were synthesized with one-step microwave synthesis method, and their physicochemical properties, including the IR spectra, melting points, and thermostability, were studied experimentally. Quantum chemistry calculations were also performed to study the influence of the esterification of cation on the melting point. Geometry optimizations on the single molecules of the Thr complex and its methyl and ethyl ester complexes in gas phase were performed with DFT B3LYP/6-311++G** method, and the vibration frequencies of each optimized molecule were analyzed at the same level method. The binding energies between ions were calculated and the basis set superposition error (BSSE) was corrected with counterpoise method (CP). It was found that the esterification of cation would weaken not only the intermolecular hydrogen bond interaction, but also the intramolecular interaction between ions obviously. And so the decrease of the melting point is caused by the two factors, and the weakenness of intramolecular interaction should be the main factor for the titled compound to be an ionic liquid.