Hydrogen Bond and Proton Transport in Acid–Base Complexes and Amphoteric Molecules by Density Functional Theory Calculations and 1H and 31P Nuclear Magnetic Resonance Spectroscopy

Intermolecular and intramolecular hydrogen bond (H-bond) and proton transport in acid–base complexes and amphoteric molecules consisting of phosphonic acid groups and nitrogenous heterocyclic rings are investigated by density functional theory calculations and 1H NMR and 31P NMR spectroscopy. It is concluded that a phosphonic acid group can act both as H-bond donor and H-bond acceptor, while an imine nitrogen atom can only act as H-bond acceptor and an amine group as H-bond donor. And the intramolecular H-bond is weaker than the intermolecular H-bond attributing to configurational restriction. In addition, the strongest H-bond interaction is observed between a phosphonic acid and a 1H-indazole because of the formation of double H-bonds. The 1H NMR and 31P NMR chemical shifts for the acid–base complexes are consistent with the density functional theory calculations. From the 1H NMR chemical shifts, fast proton exchange is observed between a phosphonic acid and 1H-benzimidazole or 1H-indazole. Finally, it is proposed that polymeric material tethered with 1H-benzimidazole or 1H-indazole rings is a favorable component for high-temperature proton exchange membranes based on acid–base complexes or acid–base amphoteric molecules.