Proton Dynamics in Hydrogen-Bonded Molecular Crystals

Solitoic mechanism concerning the intermolecular proton transfer in a one-dimensional (1D) hydrogen-boned chain has been theoretically proposed. Recently, (pseudo) 1D hydrogen-bonded crystals composed of squaric acid derivatives were prepared and their hydrogen-bonded scheme was elucidated by low temperature X-ray crystallography. A characteristic temperature- and frequency-dependent dielectric response exhibited by the hydrogen-bonded crystal of bisquaric acid was interpreted by the solitonic migration of ionic defects generated by the local proton transfer between bisquaric acid molecules. The dielectric response observed in a hydrated crystal of bis (squaric acid) biphenyl turned out to be even more pronounced. The dielectric behavior was relationarized by a collective proton-relay along the hybrid hydrogenbonded 1D-chain composed of deprotonated squarate ions and partially protonated water molecules. These experimental results are expected to provide a clue to understand the active proton transport observed in a hydrogen-bonded channel composed of water pools and amino acid residues in membrane proteins.