Protons involved in the H-bond system in 1,2-diazine-chloranilic acid (2 : 1) are assumed to be in jumping motion in the double-minimum potential corresponding to the two extreme electronic states of O-H...N and O-...H-N+. 14N nuclear quadrupole coupling constants were determined by 1H-14N nuclear quadrupole double resonance. Assuming that the observed coupling constants are result of a fast exchange of the two extreme electronic states, the coupling constants for each state were estimated by use of the equilibrium populations of the two extreme states determined from multi-temperature X-ray single-crystal diffraction. It was suggested that not only the population but also the electron distribution of the extreme electronic states itself changes with temperature.

The phase transition of tetramethylpyrazine-chloranilic acid (1:1) was studied by 35 Cl NQR measurements. The frequency splits into a doublet below T c = 83.0 ± 0.5 K. The averaged frequency of 36.98 MHz at 77 K suggests an electrically neutral electronic state of the chloranilic acid molecule, that is, no proton transfer between the acid and base molecules. An enhancement of the relaxation rate was observed at T c : it is presumably due to critical slowing down of a hydrogen motion in the one-dimensional hydrogen bond network. The excitation of a hightemperature hydrogen motion at T > 250 K is also suggested by the steep decrease of T 1 observed above 250 K. From the slope of the Arrhenius plots, an activation energy of 35 kJ mol −1 is calculated for the motion.

The proton dynamics in one-dimensional hydrogen-bonding system in molecular co-crystals of tetramethylpyrazine (TMP) with chloranilic acid (H 2 ca), as well as 2,6-dimethylpyrazine (DMP) with H 2 ca is studied by 35 Cl NQR and 2 H NMR spin-lattice relaxation measurements. No transfer motion of proton between the acid and base molecules is observed in DMP-H 2 ca, while the motion of the acid proton is excited in TMP-H 2 ca and the activation energy for the motion increases from 35 kJ mol −1 to 50 kJ mol −1 by the deuteration.

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