Real-space X-ray pair distribution function analysis and molecular- dynamics modelling of diflunisal channel hydrates Anuradha Pallipurath 1,2,3 , Francesco Civati 1 , Jonathan Skelton 4 , Dean Keeble 5 , Clare Crowley 6 , Mary-Ellen Crowley 7 , Patrick McArdle 1 and Andrea Erxleben 1* 1 National University of Ireland, Ireland, 2 University of Leeds, UK, 3Centre for Innovative Manufacturing in Continuous Manufacturing and Advanced Crystallisation Research Complex at Harwell, UK, 4 University of Manchester, UK, 5 Diamond Light Source, UK, 6 University of Limerick, Ireland, 7 University College Cork, Ireland Understanding the dynamics of the host-guest interactions in porous materials is vital for several applications including gas storage, catalysis and drug delivery. While it may be possible to collect a well-defined crystal structure that describes the host material, subtle structural changes due to the host-guest interactions are usually spatially averaged. Synchrotron X-ray pair distribution (XPDF) analysis is fast becoming an excellent technique to study porous materials such as metal-organic frameworks and their guest molecular uptake. PDF analysis provides information about interactions in materials between a chosen central atom with other atoms at a distance r. This makes it a versatile technique to study solutions and amorphous materials, as well as host-guest interactions, with sub- second time resolution. We combine X-ray pair-distribution function (XPDF) analysis with first-principles molecular-dynamics simulations to investigate the host-guest interactions in the model hydrate of diflunisal. We find that the initial hydrate obtained from solution undergoes systematic changes in hydration level on drying and when stored under controlled humidity. The structure of the guest molecules in the solvent channels varies from positional disorder in the reported 1:1 hydrate to a crystalline H-bonded network in the fully-occupied 1:2 stoichiometry obtained under high humidity. The simulations indicate cooperative binding of the guest water molecules, leading to a characteristic stepwise release of water molecules on heating and a prominent hysteresis in the absorption/desorption behaviour. Our results highlight the utility of first-principles modelling and PDF analysis for studying the host-guest interactions and structural dynamics of solvates and other mesoporous solids, with implication for understanding and controlling the capture and delivery of molecules in framework structures
SM07
© The Author(s), 2023
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