2H NMR study of phase transition and hydrogen dynamics in hydrogen bonded organic antiferroelectric 55DMBP-H2ca

Hydrogen dynamics in one-dimensional hydrogen bonded organic antiferroelectric, co-crystal of 5,5’-dimethyl-2,2’-bipyridine (55DMBP) and chloranilic acid (H₂ca), was investigated by use of ²H high resolution solid-state NMR. The two types of hydrogen bonds O-H …N and N⁺-H …O^? in the antiferroelectric phase were clearly observed as the splitting of the side band of the ²H MAS NMR spectra of the acid-proton deuterated compound 55DMBP-D ₂ca. The temperature dependence of the spin-lattice relaxation time was measured of the N⁺-H and O-H deuterons, respectively. It was suggested that the motion of the O-H deuteron is already in the antiferroelectric phase in the fast-motion regime in the NMR time scale, while that of the N⁺-H deuteron is a slow motion. In the high-temperature paraelectric phase, the both deuterons become equivalent and the fast motion of the deuterons in the NMR time scale is taking place with the activation energy of 7.9 kJ mol^?1.