Uncovering the Connection Between Low-Frequency Dynamics and Phase Transformation Phenomena in Molecular Solids

Abstract

The low-frequency motions of molecules in the condensed phase have been shown to be vital to a large number of physical properties and processes. However, in the case of disordered systems, it is often difficult to elucidate the atomic-level details surrounding these phenomena. In this work, we have performed an extensive experimental and computational study on the molecular solid camphor, which exhibits a rich and complex structure-dynamics relationship, and undergoes an order-disorder transition near ambient conditions. The combination of x-ray diffraction, variable temperature and pressure terahertz time-domain spectroscopy, ab initio molecular dynamics, and periodic density functional theory calculations enables a complete picture of the phase transition to be obtained, inclusive of mechanistic, structural, and thermodynamic phenomena. Additionally, the low-frequency vibrations of a disordered solid are characterized for the first time with atomic-level precision, uncovering a clear link between such motions and the phase transformation. Overall, this combination of methods allows for significant details to be obtained for disordered solids and the associated transformations, providing a framework that can be directly applied for a wide range of similar systems.

Exploring the Dynamics of Bound Water in Nylon Polymers with Terahertz Spectroscopy

“…Advances in computational resources have permitted increasingly large calculations to be performed, which has recently proven to be valuable for a wide-range of applications, including phase-transformation phenomena, organic semiconducting efficiency, and pharmaceutical stability, to name a few. [23][24][25] In those studies, the structural information obtained by THz spectroscopy is in good agreement with those obtained by conventional methods, such as differential scanning calorimetry (DSC), NMR, and X-ray diffraction measurements. Moreover, THz spectroscopy shows great advantages for in situ spectroscopy, such as time-lapse monitoring of the isothermal crystallization 17,26 or the imaging of the spatial distribution of higher order structures in the samples.…”

Vibrational Response of Felodipine in the THz Domain: Optical and Neutron Spectroscopy Versus Plane-Wave DFT Modeling

“…Applicability of Born-Oppenheimer molecular dynamics (BOMD) simulations in the modeling of the THz spectra was discussed, e.g., in [123]. Recently, Ruggiero and Zeitler have reported successful attempts to combine CRYSTAL and CP2K to account for the anharmonicity and elucidate the nature of phase transitions in several solids [87,91,142]. In this work, we present a similar strategy, combining CASTEP and CP2K to predict INS and THz-TDs/IR spectra of the title system.…”

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

“…This has made terahertz spectroscopy a powerful complement to traditional structural methods (e.g., X-ray diffraction) for the identification of different packing geometries, as observed in crystalline polymorphs, for example. [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-persurface to a level of accuracy that enables a proper determination of the weak forces responsible for terahertz dynamics. Over the past two decades there have been a number of advances that have dramatically improved the description of terahertz dynamics, 29 including the determination of IR intensities in periodic simulations, 30 the incorporation of dispersion forces, 31,32 more advanced basis sets, 33,34 force fields, 35 and density functionals, 36 and so on.…”