High-pressure Raman spectra of l-histidine hydrochloride monohydrate crystal

Single-crystals of l-histidine hydrochloride monohydrate, C₆H₉N₃O₂·HCl·H₂O, were studied by Raman spectroscopy as a function of pressure in a diamond anvil cell up to 7.5 GPa at room temperature over the spectral range 3450-30 cm⁻¹. The effect of changing pressure on the vibrational spectrum is discussed. From the analysis of results we inferred that the crystal undergoes a reversible structural phase transition between 2.7 and 3.1 GPa. This transition is characterized by the splitting of a band related to torsion of CO₂⁻, the disappearance and appearance of modes related with stretching of OH⁻ and deformation of CO₂⁻, as well as with bands of low wavenumber which are assigned as lattice modes, and by the discontinuities of the curves of wavenumber versus pressure. Pressure coefficients for all modes observed in this work are also given.