Phase transition and proton exchange in 1,3-diazinium hydrogen chloranilate monohydrate.

In the hydrate crystal of 1:1 salt with 1,3-diazine and chloranilic acid (Hca), (1,3-diazineH)·HO·Hca, an unique hydrogen-bonded molecular aggregate is formed. There exists proton disorder in the N-H...O hydrogen bond between 1,3-diazinium ion and water (HO) of crystallization. In order to reveal dynamic aspect of this disorder, Cl NQR measurements were conducted. Two resonance lines observed at 35.973 and 35.449 MHz at 321 K split into four lines below T = 198 K clearly showing occurrence of a solid-solid phase transition; 36.565, 36.357, 36.011, 35.974 MHz at 77 K. Temperature dependence of spin-lattice relaxation time T in high-temperature phase was observed to obey an Arrhenius-type relation with the activation energy of 8.5 kJ mol. This result leads to the conclusion that proton exchange in the N-H...O hydrogen bond takes place in the high-temperature phase. Specific heat measurements by DSC resulted in the transition entropy ΔS = 1.3 J K per 1 mole [(1,3-diazineH)·HO·Hca] which is far less than 2 R ln2 = 11.5 J K mol. It is expected that proton exchange in the two hydrogen bonds within the aggregate does not occur independently but concertedly with strong correlation in the high-temperature phase. [ABSTRACT FROM AUTHOR]