Proton dynamics in the hydrogen bonds of 4-amino-3,5-dihalogenobenzoic acid

•4-Amino-3,5-dichlorobenzoic acid has a symmetric dimer structure.•The compound undergoes a phase transition at 138 K.•The symmetry breaking of the dimer was revealed by 35Cl NQR.•The proton dynamics was analyzed by coherent and incoherent tunneling models.

On the polycrystalline sample of 4-amino-3,5-dihalogenobenzoic acid, 4-NH2-3,5-X2C6H2COOH, which has a symmetric dimer structure in the crystal, the proton tunneling in the hydrogen bonds has been investigated by NQR and NMR spin-lattice relaxation times T1 measurements. Two 35Cl NQR lines of the X = Cl derivative show the existence of two crystallographically inequivalent chlorine atoms in the high-temperature phase, in consistency with the reported crystal structure. Below 138 K, each splits into a doublet indicating the symmetry breaking of the benzoic acid dimer. The proton dynamics was analyzed by a coherent and incoherent tunneling models, for the high- and low-temperature phases, respectively. The temperature dependence of the correlation time of proton translation was estimated. As for the X = I derivative, the proton dynamics was discussed similarly by 1H NMR T1 data by assuming occurrence of a phase transition at low-temperature.

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