Investigating the prototropic tautomerism in (E)-2-[(4-fluorophenyl)iminomethyl]-5-methoxyphenol compound for solid state and solvent media by experimental and quantum computational tools

Abstract

Abstract The tautomerism in o-Hydroxy Schiff bases appears as a result of intramolecular proton transfer between the phenolic oxygen atom and nitrogen atom. This process results in two tautomeric forms (phenol-imine and keto-amine). The preference of a particular form by the compound changes in solid and solvent media. In the current study, the proton tautomerism in (E)-2-[(4-fluorophenyl)iminomethyl]-5-methoxyphenol (an o-Hydroxy Schiff basis) was investigated for solid and solvent media by experimental (IR and UV–vis) and computational methods (DFT and TD-DFT). Density functional theory (DFT) at B3LYP/6-31G(d) level was used for gas phase geometry optimization. For geometry optimizations in solvent media the polarizable continuum model (PCM) was used. The excitation energies of the title compound were calculated for gas and solution phase with time-depended density functional theory (TD-DFT) approach. Experimental FT-IR studies for the solid phase of the title compound show the preference of enol form, as supported by X-ray analysis results. Experimental and computational UV–vis studies of proton transfer process were carried out for various organic solvents of different polarities (EtOH, DMSO, Benzene and CHCl3). It was found that the stability of the molecule increases with the increase of solvent polarity. In addition, the existence of a new absorption band only in EtOH was attributed to keto form by considering TD-DFT calculations and previous studies. The proton transfer process was further investigated by considering a TS structure in DFT (B3LYP) calculations for gas phase and solvent media. The results for gas phase revealed that the enol form is more stable than keto form. The activation energy for proton transfer reaction was calculated as 6.1 kcal/mol for gas phase. For the proton transfer process in solvent media, it is found that the enol form in Benzene is more stable tautomeric form while the keto form is more stable one in CHCl3, EtOH and DMSO. In addition, the energy gap between enol and keto forms was found to decrease from gas phase to solution phase, proposing that the tautomerization in a polar solvent occurs easier than an apolar solvent.