Probing the acid-base equilibrium in acid-benzimidazole complexes by 1H NMR spectra and density functional theory calculations.

The acid-base equilibrium in acid (phenylphosphonic acid, methylsulfonic acid, phosphoric acid, hydrochloric acid, acetic acid, and sulfuric acid)-benzimidazole (BIm) complexes was studied by (1)H NMR spectra, FTIR spectra, and density functional theory (DFT) calculations. The DFT optimized structures of the acid-BIm complexes, the vibrational frequencies of the acidic protons, and potential profiles were applied to study the equilibrium between the acids and BIm. In gas phase, it was shown that the strong sulfuric acid could completely protonate BIm and the other acids could only incompletely protonate BIm. When the polarizable continuum model of DMSO was applied, it was shown that all the acids could completely protonate BIm, except the weak acetic acid and phenylphosphonic acid. Furthermore, the potential profile for the PPoA-BIm shows double-well structure facilitating the free movement of the acidic proton between PPoA and BIm. If an explicit solvent molecule of DMSO was included into an acid-BIm complex, significant changes in equilibrium between acid and BIm are observed. The potential profiles for the MSA-BIm and HCl-BIm show very flat bottom wells, facilitating the free movement of proton between the acid and BIm. These calculations were consistent with the (1)H NMR chemical shifts of the immobile protons of the benzimidazole ring in DMSO-d(6) and the FTIR spectra of the acid-BIm complexes.