Low-frequency vibrations of crystalline molecules are very sensitive to the local environment in which the molecules, for example, hydrated ions captured in crystals, find themselves. We present lowtemperature X-ray crystallographic measurements on the harvested thiamine crystal containing hydrated ions and its temperature-dependent terahertz spectra and synchrotron infrared microspectra. It is found from the X-ray structure that the hydrated ions and hydration water are in a similar environment to liquid, although those are captured in crystals. The vibrationally resolved THz spectra of two states in the present organic crystals containing hydrated ions are well explained by the difference in the hydrogen-bonded pattern. Peak assignments were performed based on highly accurate first-principles calculations incorporating relativistic effects and dispersion corrections. The temperature dependences are observed for the vibrations around the chloride ions and hydration water due to the loose binding of chloride ions, the bond elongation with increasing temperature, and the cleavage of weak hydrogen bonds.

Terahertz resonances are capable of relaxing through multiphonon scattering because of the long lifetimes of excited vibrational states compared with the periods of lattice vibrations of a crystal. In this study, we examine the temperature-dependent frequency shifts in the frequency ranges of 20–90 and 200–600 cm−1 of thiamin crystals containing hydrated ions and provide evidence for multiphonon scattering. One-, two-, and three-phonon scattering are clearly observed in the form of discrete linear temperature coefficients of vibrational frequencies. Higher transition frequencies tend to cause more phonon scattering in the frequency range of 20–90 cm−1, while higher transition frequencies tend to cause less phonon scattering in the frequency range of 200–600 cm−1. The characteristic temperature parameter of the effective phonon taking part in the scattering processes is estimated to be 44.6 K, which is shared by different vibrational transitions, and the corresponding characteristic frequency is estimated to be 31.0 cm−1. Highly accurate solid-state first-principles calculations incorporating relativistic effects and dispersion corrections are performed, and the results show that the characteristic frequency is close to the lower limit of the calculated optical phonon frequencies, lying between the lowest two. The ratio of the vibrational transition frequency to the characteristic frequency is 1.6 for single-phonon scattering, 2.0 and 2.2 for two-phonon scattering, and 2.5 for three-phonon scattering in the frequency range of 20–90 cm−1. In the frequency range of 200–600 cm−1, the ratio is much higher than that in the frequency range of 20–90 cm−1 and is 9.4 to 16.4.