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

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.