Proton Conducting Compound of Benzimidazole with Sebacic Acid: Structure, Molecular Dynamics, and Proton Conductivity

Abstract

Benzimidazole salt of sebacic acid, a new proton conductor from the family of benzimidazole compounds of dicarboxylic acids, was crystallized to search for the factors which determine the hydrogen bond motif and the structure of the crystals. The molecular structure of benzimidazole-sebacic acid salt was solved by using X-ray diffractions and confirmed by 1H and 13C MAS NMR experiments combined with DFT calculations. The salt of sebacic acid, with 10 carbon atoms in the chain, was found to exhibit an undulated layer-type structure with banana-shaped acid molecules linked by O–H···O bonds into rectangular-type wavy chains and flat base molecules attached to the carboxylic groups by N–H···O bonds. The undulated layers are not linked with hydrogen bonds. Comparison of the architecture of benzimidazole salts with weak dicarboxylic acids of shorter carbon chains, studied by us earlier, points at the role of the acid chain length in the formation of structural and hydrogen bond motifs. The dynamics of protons in the ordered crystalline phase and disordered surface layers was characterized by NMR spin–lattice relaxation measurements, whereas complex impedance studies yielded information on the activation energy of proton diffusion in the both phases.