Terahertz time-domain spectroscopy of l-histidine hydrochloride monohydrate

Abstract The low-frequency absorption spectrum of l -histidine hydrochloride monohydrate (LHHM) over 0.5–4.0 THz was measured by terahertz time-domain spectroscopy (THz-TDS). The comparison between the THz absorption spectra of LHHM and l -histidine (LHis) was made. The two distinctive THz spectra of LHHM and LHis indicate that THz spectroscopy is sensitive to different specimens. The low-temperature THz spectral features of LHHM presented the blue-shift and splitting as the temperature decreased. Solid-state density functional theory (DFT) calculation of the vibrational modes of LHHM was performed for better understanding the THz characteristic spectrum, and the validity of the calculation modes was confirmed by comparing with the experimental observations. Powder X-ray diffraction (PXRD) was also carried out to check the structures of LHHM and LHis.