Terahertz Vibrational Motions Mediate Gas Uptake in Organic Clathrates

Abstract

Clathrates, supramolecular materials that contain guest molecules trapped within a framework of a host lattice, are important in many areas of science and technology. However, despite much research, fundamental questions concerning the mechanism and kinetics associated with the enclathration process remain unanswered. Here, through a combination of terahertz spectroscopy, density functional theory, and molecular dynamics simulations, we demonstrate that the gas-capture process is mediated by a single terahertz vibrational mode and then use this mode to track the uptake of gas. This result provides a critical molecular-level understanding of this key kinetic process in clathrate science.

Identifying Ordered OH/F Anions in Hydroxyfluorides by a Terahertz Spectroscopic Approach

“…Unlike solid-state NMR, IR, and Raman spectroscopy, which reflect local structural information under the influence of intramolecular magnetic fields or intramolecular interactions, THz spectroscopy can probe structures on an extended scale. − THz radiation resonates with optical phonon modes governed by intermolecular interactions. − Ligand substitution may not cause significant changes in the intramolecular magnetic field or intramolecular interaction, but the intermolecular interaction may be significantly altered . As evidence, OH – /F – substitutions at the ortho position of benzene have been found to significantly alter the melting point, ranging from −34 °C for 1,2-difluorobenzene to 16.1 °C for 2-fluorophenol and 103–106 °C for pyrocatechol .…”

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

“…[7][8][9][10] On the other hand, the complex and largeamplitude motions occurring at terahertz frequencies have recently been related to a wide-variety of bulk phenomena, for example phase transformations, 11,12 mechanical responses, 5,13,14 electron-phonon coupling, 15,16 and gas-capture in porous mate-rials. 17,18 Unfortunately, the factors that make terahertz spectroscopy a valuable analytical technique simultaneously lead to one of its biggest challenges -the interpretation of experimental results. Unlike mid-infrared (mid-IR) methods, such as Fourier transform infrared spectroscopy (FTIR) and mid-IR Raman scattering, there do not exist any functional-group specific transitions at terahertz frequencies.…”

Advances in Low-Frequency Vibrational Spectroscopy and Applications in Crystal Engineering

“…22,23 Even adsorption of gas molecules by porous organic crystals has been suggested to be dictated by terahertz dynamics. 24,25 Over the years, the intimate link between solid-state phase transitions and terahertz normal modes has been validated numerous times, and it is now becoming common practice not only to characterize phase transitions using low-frequency vibrational spectroscopy but also to promote such processes using terahertz radiation alonedemonstrated for example in polymers, 26 disordered solids, 27 and inorganic crystals. 28 This area of the terahertz sciences is highly active and represents some of the most cutting-edge work currently being performed.…”

“…As the field has evolved, the community has learned to leverage the actual terahertz vibrational motions themselves to understand material properties from a new perspective. Some of the first examples in this respect were the discoveries in which many solid-state phase transformation processes follow a mechanism that is often correlated to a single low-frequency vibrational mode. , Even adsorption of gas molecules by porous organic crystals has been suggested to be dictated by terahertz dynamics. , Over the years, the intimate link between solid-state phase transitions and terahertz normal modes has been validated numerous times, and it is now becoming common practice not only to characterize phase transitions using low-frequency vibrational spectroscopy but also to promote such processes using terahertz radiation alonedemonstrated for example in polymers, disordered solids, and inorganic crystals . This area of the terahertz sciences is highly active and represents some of the most cutting-edge work currently being performed. , …”