Acid Dissociation Constants of the Benzimidazole Unit in the Polybenzimidazole Chain: Configuration Effects

The acid dissociation constant of three benzimidazoles, namely 2,2'-bibenzo[d]imidazole, 2,5'-bibenzo[d]imidazole, and 5,5'-bibenzo[d]imidazole, have been investigated by means of density functional theory calculations in gas phase and in aqueous solution. The theoretical approach was validated by the comparing of predicted and experimentally determined pK_a values in imidazole, benzimidazole, and 2-phenylbenzimidazole. From the studied compounds, 2,2'-bibenzo[d]imidazole was found to be the most acidic, which made it a valuable candidate as a material for polymer electrolyte membrane fuel cells.