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

Abstract

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 138K, 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.