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Fig. 24: The host frameworks of sTr (a), sO (b), sO-II (c) and sT (d) structures viewed as close packing of space- filling polyhedra. Black lines outline the unit cell.

Fig. 25: Ordered and disordered acetone molecules in the cages of sO (a), sO-II (b) and sT (c) structures. The dashed grey lines represent the closest oxygen of the acetone and the oxygen of the host framework. Grey and red spheres represent carbon and oxygen atoms, respectively.

demonstrated that high-pressure clathrates hosting polar molecules can be stabilised without promoters, despite strong host−guest interactions. Moreover, it is likely that hydrogen bonding plays an important role in stabilising a clathrate crystal structure.

In this study, clathrate hydrates were probed using in- situ HP-LT SCXRD techniques, demonstrating its ability to provide detailed information on structurally and chemically complex non-stochiometric compounds. Further studies will be aimed at elucidating host−guest hydrogen bonding phenomenon and the associated chemical and physical properties of clathrates.

PRINCIPAL PUBLICATION AND AUTHORS

Host−Guest Hydrogen Bonding in High-Pressure Acetone Clathrate Hydrates: In Situ Single-Crystal X-ray Diffraction Study, A. Pakhomova (a,b), I.E. Collings (b), B. Journaux (c), S. Petitgirard (d), T. Boffa Ballaran (e), D. Huang (d), J. Ott (c), A. Kurnosov (e), M. Hanfland (b), G. Garbarino (b), D. Comboni (b), J. Phys. Chem. Lett. 13, 1833-1838 (2022); https:/doi.org/10.1021/acs.jpclett.1c03911 (a) Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany) (b) ESRF (c) Department of Earth and Space Science, University of Washington, Seattle (USA) (d) Institute of Geochemistry and Petrology, ETH Zürich (Switzerland) (e) Bayerisches Geoinstitut, University of Bayreuth (Germany)

In contrast, the sO and sT are constructed from uniform cages 445466 (T3) and 425864 (T4), respectively. Previously unknown clathrate structure sO-II is based on both T3 and T4 cages. Noteworthy, only the sO structure was known for acetone clathrates prior to this study.

In addition to the architecture of host frameworks, the SC-XRD technique allowed to determine the orientation, disorder and occupancy of guest acetone molecules. Structural analysis shows that acetone oxygens lie at distances of 2.73−2.95 Å from the closest water oxygen atoms of the host frameworks of sO, sO-II and sT clathrates (Figure 25), evidencing host−guest hydrogen bonding. The high quality of the diffraction data allowed to define the hydrogen bonding scheme of the crystal structure of sT. The oxygen (O3) of the acetone molecule is refined at the distance of 2.24(9) Å to the hydrogen atom (H4) of the framework water, while the DHA angle of the O2-H4-O3 hydrogen bond is at 118(8)°. The present study