Abstract
The mixed homonuclear and heteronuclear hydrogen bonds in ammonia hydrates have been of interest for several decades. In this manuscript, a neutron powder diffraction study is presented to investigate the structure of ammonia monohydrate IV at 170 K at an elevated pressure of 3–5 GPa. The most plausible structure that accounts for all features in the experimental pattern was found in the P21/c space group and has the lattice parameters a=5.487(3) Å, b=19.068(4) Å, c=5.989(3) Å, and β=99.537(16) deg. While the data quality limits discussion to a proton-ordered structure, the structure presented here sheds light on an important part of the ammonia–water phase diagram.
Highlights
The water–ammonia system is of interest for a variety of reasons
First and foremost, it is a model system in which the behaviour of materials that contain homonuclear and heteronuclear hydrogen bonds can be observed; H2O readily forms H-bonds with itself and with NH3, while NH3 tends to donate H-bonds to H2O. These N-H· · · O and N· · · H-O hydrogen bonds are highly relevant as proxies for the hydrogen bonds in proteins and other biomolecules, where these H-bonds play an important role in folding, replication, and overall functionality [1]
In this body of work, we investigate the structure of ammonia monohydrate IV
Summary
The water–ammonia system is of interest for a variety of reasons. First and foremost, it is a model system in which the behaviour of materials that contain homonuclear and heteronuclear hydrogen bonds can be observed; H2O readily forms H-bonds with itself and with NH3, while NH3 tends to donate H-bonds to H2O. Of equal importance is the abundance of ammonia (15%) and water (45%) in the outer solar system With results based on a neutron diffraction study carried out at the ISIS neutron source, paired with density functional theory (DFT) calculations, we try to propose a possible structure solution for this—so far, unsolved—polymorph
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