Collective librations of water molecules in the crystal lattice of rubidium bromide: experiment and simulation

Abstract

Terahertz spectroscopy of RbBr reveals four prominent absorption lines at room temperature and a further 15 lines at 10 K. Via density-functional-theory (DFT) numerical modelling using the PBE0 hybrid GGA functional, all the absorptions are identified as correlated librations of water molecules in the RbBr lattice. Each libration mode is a combination of rocking, wagging and twisting motions of the water molecules. The number of libration lines and numerical modelling show that the C2v symmetry of water in RbBr is broken. Our modelling shows that the distribution of libration amplitudes and phases for different water molecules in the RbBr unit cell varies greatly between the different modes. All librational lines red-shift with increasing temperature. The rate of change for most lines is in the range 60-90 MHz K(-1) (or (2-3) × 10(-3) cm(-1) K(-1)). Two lines shift more rapidly with temperature, at rates of 240 and 300 MHz K(-1) (or (8 and 10) × 10(-3) cm(-1) K(-1)), respectively. Furthermore, the temperature dependence of the linewidth distinguishes two groups of lines. For one group, with weak linear temperature dependence of linewidth, cubic anharmonic terms in the RbBr crystal field are significant. This group is mainly associated with fully symmetric correlated librations. For the second group, with strong non-linear temperature dependence of the linewidth, quartic anharmonic terms in the RbBr crystal field are significant. However, the distribution of libration amplitudes, as well as the type of libration modes, influence the temperature dependence of the red shift, the linewidth, and the intensity, as well. Our combined experimental and theoretical investigation confirms the necessity of obtaining low-temperature data to observe all the calculated modes; moreover, the richness of detail in the temperature dependence of the data invites further modelling spanning a range of temperatures.

Elucidating low-frequency vibrational dynamics in calcite and water with time-resolved third-harmonic generation spectroscopy

“…These results based on our new experimental data demonstrate the resolving power of TRTHG in capturing equilibrium skeletal motions of liquid samples such as water. In comparison to terahertz (THz) spectroscopy that is a powerful technique studying low-frequency modes in molecular systems, 11,12,59,60 TRTHG has a simpler optical setup (e.g., no nonlinear crystal for THz pulse generation), higher time resolution (e.g., exposing multiple mode lifetimes on the fs timescale), and specificity in observing microscopic molecular dynamics corresponding to the higher-frequency (typically 4100 cm À1 ) processes in dielectric spectrum in the THz domain. 61 To further confirm the observed low-frequency Raman-active modes below 500 cm À1 , we use our newly developed tunable FSRS setup in the same laboratory to measure the ground-state Raman modes of pure water (see Experimental section).…”

Terahertz Spectroscopy of 2,4-Dinitrotoluene over a Wide Temperature Range (7–245 K)

“…The sample was cooled from 245 to 7 K, with a step size of 14 K. The spectra were collected with a resolution of 0.5 cm –1 (0.015 THz). More details of the experimental conditions can be found elsewhere . Data analysis was via the IGOR software package.…”

“…More details of the experimental conditions can be found elsewhere. 19 Data analysis was via the IGOR software package. The absorption peaks obtained experimentally were fitted with Lorentzian curves to determine the line position, width, intensity, and their corresponding uncertainties.…”

On the influence of water on THz vibrational spectral features of molecular crystals

“…A wide range of experimental techniques can be used to address the role of water in organic molecular structures: X-ray and neutron diffraction 2 , nuclear magnetic resonance (NMR) 3,4 and various optical spectroscopic techniques 5,6 . Computational studies (most commonly, Density Functional Theory (DFT) calculations and Molecular Dynamics (MD) simulation) can complement the knowledge gained through experiments and offer a description of the macromolecule or the molecular crystal and solvent as well as their dynamics at the atomic scale [7][8][9][10][11] .…”