Protonic motion in one-dimensional hydrogen-bonded molecular crystal [4,6-dmpH][Hca] as revealed by 35Cl NQR

A protonic motion in the 1-D hydrogen-bonded zigzag chain in 4,6-dimethylpyrimidinium hydrogen chloranilate, [4,6-dmpH][Hca], was studied from the measurements of 35Cl nuclear quadrupole resonance (NQR) spin-lattice relaxation time (T1). The four NQR frequencies were observed at 36.846, 36.767, 35.945, and 35.905 MHz at 77 K. The temperature dependences of these frequencies were smooth and continuous in 77–360 K. The absence of solid–solid phase transition and the metastable phase was confirmed by heat capacity measurements. Temperature dependence of the NQR T1 suggests an excitation of a motion above ca. 200 K with the activation energy of 11.5 kJ mol−1. In the stable phase too, the excitation of proton transfer is expected to occur as suggested for the metastable phase of as-grown crystals. Such transfer motion may be possible at the kink structure of the 1-D ferroelectric zigzag chain, and the kink can migrate in the chain accordingly.