Simulation Meets Experiment: Unraveling the Properties of Water in Metal–Organic Frameworks through Vibrational Spectroscopy

Abstract

In nanoporous materials, guest-host interactions affect the properties and function of both adsorbent and adsorbate molecules. Due to their structural and chemical diversity, metal-organic frameworks (MOFs), a common class of nanoporous materials, have been shown to be able to efficiently and, often, selectively adsorb various types of guest molecules. In this study, we characterize the structure and dynamics of water confined in ZIF-90. Through the integration of experimental and computational infrared (IR) spectroscopy, we probe the structure of heavy water (D 2 O) adsorbed in the pores, disentangling the fundamental framework-water and water-water interactions. The experimental IR spectrum of D 2 O in ZIF-90 displays a blue-shifted OD-stretch band compared to liquid D 2 O. The analysis of the IR spectra simulated at both classical and quantum levels indicates that the D 2 O molecules preferentially interact with the carbonyl groups of the framework and highlights the importance of including nuclear quantum effects and taking into account Fermi resonances for a correct interpretation of the OD-stretch band in terms of the underlying hydrogen-bonding motifs. Through a systematic comparison with the experimental spectra, we demonstrate that computational spectroscopy can be used to gain quantitative, molecular-level insights into framework-water interactions that determine the water adsorption capacity of MOFs as well as the spatial arrangements of the water molecules inside the MOF pores which, in turn, are key to the design of MOF-based materials for water harvesting.File list (2) download file view on ChemRxiv water_in_zif90.pdf (4.99 MiB) download file view on ChemRxiv supporting_information.pdf (4.44 MiB)

Water Capture Mechanisms at Zeolitic Imidazolate Framework Interfaces

“…[12][13][14] On the other hand, molecular dynamics (MD) simulations can provide molecularlevel insights into interfacial processes, but these simulations often lack corresponding experimental comparison. 10,11,[15][16][17] In this work, we reveal that the adsorption mechanisms of water in ZIF-90 mechanism 1. This work is feasible by selectively probing the water clustering step, using a spatially-resolved vibrational sum-frequency generation (VSFG) spectroscopy and MB-pol [18][19][20] water-based simulations.…”

“…Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can only probe the molecular details of bulk water adsorption in MOFs. 10,11 Although several studies using diffraction techniques have revealed molecular-level details of water adsorption in a few MOFs, its application for the pore filling mechanism has been limited. [12][13][14] On the other hand, molecular dynamics (MD) simulations can provide molecularlevel insights into interfacial processes, but these simulations often lack corresponding experimental comparison.…”

“…This work is feasible by selectively probing the water clustering step, using a spatially-resolved vibrational sum-frequency generation (VSFG) spectroscopy and MB-pol [18][19][20] water-based simulations. 11 This study emphasizes the importance of interfacial-specific techniques [21][22][23][24][25][26] to decipher the effects of water-water and water-framework interactions on the water adsorption mechanism, which can provide guidance to the rational design of MOFs for water harvesting. Two crucial technical aspects that enable the micron-resolved VSFG to probe adsorbed water at interior MOF surfaces.…”

Water Capture Mechanisms at Zeolitic Imidazolate Framework Interfaces

“…As explained above, VSFG probes D 2 O bound to the hydrophilic linker at the step of water clustering, whereas DRIFTS probes all D 2 O inside the pore, at both water clustering and pore filling steps. Since hydrogen bonding between D 2 O and organic linkers is weaker than that between D 2 O molecules, 11 we expect the VSFG spectra to be more blue-shifted relative to the DRIFTS Figure 4. a) Fitting results for DRIFTS at 43% RH purge.…”

Water Capture Mechanisms at Zeolitic Imidazolate Framework Interfaces

“…As explained above, VSFG probes D2O bound to the hydrophilic linker at the step of water clustering, whereas DRIFTS probes all D2O inside the pore, at both water clustering and pore filling steps. Since hydrogen bonding between D2O and organic linkers is weaker than that between D2O molecules, 11 we expect the VSFG spectra to be more blue-shifted relative to the DRIFTS Figure 4. a) Fitting results for DRIFTS at 43% RH purge.…”