Phase transitions of L-histidinium hydrogen oxalate crystal under high pressures investigated by Raman spectroscopy

L-histidinium hydrogen oxalate (L-HisH)(HC2O4) crystal is formed from amino acid. L-histidine with oxalic acid whose vibrational high pressures behavior have not yet been investigated in the literature. Here we synthesized (L-HisH)(HC2O4) crystal by slow solvent evaporation method in a 1:1 ratio of L-histidine and oxalic acid. In addition, a vibrational study of (L-HisH)(HC2O4) crystal as a function of pressure was performed via Raman spectroscopy in the pressure range of 0.0-7.3 GPa. From analysis of the behavior of the bands within 1.5-2.8 GPa, characterized by the disappearance of lattice modes, the occurrence of a conformational phase transition was noted. A second phase transition, now from structural type, close to 5.1 GPa was observed due to the incidence of considerable changes in lattice and internal modes, mainly in vibrational modes related to imidazole ring motions.

Automated summary

In this study, L-histidinium hydrogen oxalate crystal was successfully synthesized using the slow solvent evaporation method, and its vibrational behavior under different pressures was investigated by Raman spectroscopy. A conformational phase transition was observed from the analysis of the band behavior within the pressure range of 1.5-2.8 GPa, characterized by the disappearance of lattice modes. A second phase transition, now of structural type, close to 5.1 GPa was also observed.