Probing the Mechanochemistry of Metal–Organic Frameworks with Low-Frequency Vibrational Spectroscopy

Abstract

The identification and characterization of low-frequency vibrational motions of metalorganic frameworks (MOFs) allows for a better understanding of their mechanical and structural response upon perturbation by external stimuli such as temperature, pressure, and adsorption. Here, we describe the combination of an experimental temperatureand pressure-dependent terahertz spectroscopy system with quantum mechanical simulations to measure and assign specific low-frequency vibrational modes that directly drive the mechanochemical properties of this important class of porous materials. More specifically, those intense spectral features in the terahertz region of the vibrational spectrum of ZIF-8 are identified, which are directly connected to its mechanochemical response. In particular, the mechanical compressibility of pristine ZIF-8 is found to follow a peculiar non-linear trend upon pressure: its bulk modulus initially increases up to 0.1 GPa and decreases at higher pressures, which is simultaneously reflected in the terahertz vibrational spectra. This work highlights the interplay between structural, vibrational, and mechanochemical phenomena, all of which are key to the effective exploitation of MOFs. The importance of terahertz vibrational motions on the function of MOFs is demonstrated, and a method presented for their measurement and interpretation, which can be applied widely to any supramolecular material.

Quasi‐Harmonic Lattice Dynamics of a Prototypical Metal–Organic Framework

“…The stability condition is no longer satisfied for pressures above about 0.45 GPa, which corresponds to a compression of the structure of about 0.8%. We note that this type of shear mechanical instability is common to many zeolites and MOFs . We followed the methodology outlined in refs.…”

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

“…Terahertz vibrational spectroscopy is a powerful technique for studying intermolecular interactions in the condensed phase, having been demonstrated to be useful for a wide-array of applications. [1][2][3][4][5][6] At terahertz frequencies (0.1 -10 THz or 3 -333 cm −1 ) large-amplitude complex motions of entire molecules are excited and sampled, and thus terahertz spectroscopy is a highly-sensitive probe of weak, often non-covalent, forces. The unique combination of sampling weak forces while probing long-range collective dynamics has spurred the growth of two, often related, lines of study.…”

“…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.…”

Assessing the Performance of Density Functional Theory Methods on the Prediction of Low-Frequency Vibrational Spectra

“…Terahertz spectroscopy is a powerful method for the evaluation of crystalline phonons, which are increasingly being shown to be related to the bulk properties of the solids. [1][2][3][4] Low-frequency vibrations in molecular crystals often involve motions of entire molecules, for example hindered translations, making the vibrational dynamics dependent upon inherently weak long-range forces. [1,3,[5][6][7][8] This ultimately results in each molecular crystal, including crystals consisting of the same molecule but with different bulk packing (e.g.…”

“…FTIR and Raman), terahertz vibrations are much more complex and depend on many factors, making assignment difficult. [1,2,4,[11][12][13][14][15] Computational methods have been successfully used to assign the predict vibrational spectra with success. However, the simulation of low-frequency vibrations presents a greater challenge compared to mid-infrared motions, due to the incredibly weak forces that dictate terahertz motions.…”