Synthesis, DFT computations and antimicrobial activity of a Schiff base derived from 2-hydroxynaphthaldehyde: Remarkable solvent effect

Abstract

A new Schiff base bearing sulfonamide group was synthesized and characterized by spectroscopic techniques. Its spectroscopic properties were investigated through density functional theory (DFT/B3LYP) method with the 6−311++G(d,p) basis set theoretically and the zwitterion ⇌ enol tautomerization was studied using IEF-PCM approximation. The thermodynamic (entropy, enthalpy and Gibbs free energy changes) and energetic quantities of the zwitterion ⇌ enol tautomerization indicate that the migration of single proton between zwitterion ⇌ enol tautomer’s is unfavored for both directions in all phases. During the change from the water phase to the gas phase, the tautomeric energy barrier increases for the proton transfer reaction from the zwitterion form to the enol form, while the barrier decreases from the enol form to the zwitterion form.On the other hand, experimental studies (UV–vis. and NMR spectroscopies) release that there are two tautomeric species which are the zwitterion form in polar and the enol form in apolar solvents for the soluted compound. Furthermore, the antibacterial effect of the compound was investigated in two different solvents (DMSO and THF) to demonstrate the effect of dominant tautomeric species. The results indicate that the solution in which the enol form is dominant than the zwitterionic form has more antimicrobial efficiency for all bacterial species.