Assignment of Terahertz Modes in Hydroquinone Clathrates

Terahertz Vibrational Motions Mediate Gas Uptake in Organic Clathrates

“…In order to study the role of low-frequency dynamics on these adsorption processes, the atomic-level dynamics are explored using computational means. The experimental THz-TDS spectra of the two studied crystals are shown in Figure , along with the previously published DFT-predicted spectra . The vibrational motions occurring at terahertz frequencies in α-HQ represent predominantly external librations of the HQ molecules, with the large number of symmetry-independent atoms yielding numerous degrees of freedom and thus a complex set of vibrational motions involving many molecules simultaneously.…”

“…In order to further investigate this, we perform ab initio molecular dynamics (AIMD) simulations using CP2k on a β-HQ supercell (3 × 3 × 3) with only one CO 2 site occupied in the central unit cell and three-dimensional periodic boundary conditions applied. The PBE-D3 functional was again used, with a triple-ζ basis set, in order to be similar to the static-DFT simulations . The simulations were performed with a time step of 0.5 fs in the NVT ensemble, with the temperature set to 300 K. The generated IR spectrum was obtained by taking the Fourier transform of the dipole moment autocorrelation function, where the dipoles were calculated using the localized Wannier function approach. , The results (Figure e, red curve) show that a single enclathrated CO 2 molecule does indeed exhibit transitions that mirror those observed in the bulk extended crystal.…”

“…Given the distinct differences between the respective structures of the two crystals and the corresponding differences to the THz-TDS spectra, 30 it is possible to track the transition from one solid to the other as pressure is applied to the sample using THz-TDS alone (Figure 2). By monitoring the THz spectral change, we can track the kinetics of the reaction from α-HQ to β-HQ-CO 2 .…”

“…The experimental THz-TDS spectra of the two studied crystals are shown in Figure 3, along with the previously published DFT-predicted spectra. 30 The vibrational motions occurring at terahertz frequencies in α-HQ represent predominantly external librations of the HQ molecules, with the large number of symmetry-independent atoms yielding numerous degrees of freedom and thus a complex set of vibrational motions involving many molecules simultaneously. In the case of β-HQ-CO 2 , the mode structure is much less complex; the first two peaks correspond to hindered translations and rotations of the CO 2 molecules alone within the pore, while the third feature at the highest frequency corresponds to a coupled motion between the CO 2 guest molecule and the HQ host.…”

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…As mentioned, each individual solid exhibits a unique terahertz spectrum, governed by a complex combination of forces, which ultimately does not result in any functional/structural-motif-specific vibrations or trends, unlike in the mid-IR. For example, except for the relatively-rare case of isomorphic solids (crystals with a different chemical composition but the same overall structure), 92,93 it is not possible to compare terahertz spectra from two different materials and compare them using common mid-IR methods, such as spectral peak shifting, as each solid has a completely unique spectral fingerprint. Therefore, the assignment of terahertz vibrational spectra requires additional analysis, which is most often accomplished through the use of simulation methods.…”