Molecular dynamics study of water confined in MIL-101 metal-organic frameworks.

Molecular dynamics simulations of water adsorbed in Material Institute Lavoisier MIL-101(Cr) metal-organic frameworks are performed to analyze the kinetic properties of water molecules confined in the framework at 298.15 K and under different vapor pressures and clarify the water adsorption mechanism in MIL-101(Cr). The terahertz frequency-domain spectra (THz-FDS) of water are calculated by applying fast Fourier transform to the configurational data of water molecules. According to the characteristic frequencies in the THz-FDS, the dominant motions of water molecules in MIL-101(Cr) can be categorized into three types: (1) low-frequency translational motion (0-0.5 THz), (2) medium-frequency vibrational motion (2-2.5 THz), and (3) high-frequency vibrational motion (>6 THz). Each type of water motion is confirmed by visualizing the water configuration in MIL-101(Cr). The ratio of the number of water molecules with low-frequency translational motion to the total number of water molecules increases with the increase in vapor pressure. In contrast, that with medium-frequency vibrational motion is found to decrease with vapor pressure, exhibiting a pronounced decrease after water condensation has started in the cavities. That with the high-frequency vibrational motion is almost independent of the vapor pressure. The interactions between different types of water molecules affect the THz-FDS. Furthermore, the self-diffusion coefficient and the velocity auto-correlation function are calculated to clarify the adsorption state of the water confined in MIL-101(Cr). To confirm that the general trend of the THz-FDS does not depend on the water model, the simulations are performed using three water models, namely, rigid SPC/E, flexible SPC/E, and rigid TIP5PEw.