Halogen and hydrogen host-guest bonding in cathrate hydrates

Abstract

Clathrate hydrates with low melting points (often below \u201320\ub0C) are difficult subjects for single crystal data collection. A high level of guest molecule disorder inside the high symmetry cages causes difficulty for structure determination of such crystals as well. Recent advances in single crystal X-ray diffraction have allowed this technique to be used as a valuable tool for the analysis of hydrate structure and composition. With detailed analysis of guest and water molecules disorder, not only the guest positions are clearly defined, but also it becomes possible to find interactions between guest and water molecules. For the first time, single crystal x-ray crystallography is used to detect the presence of guest host hydrogen and halogen bonding in structure I, II and structure H clathrate hydrates. Clathrates studied are the tert- butylamine (tBA) sII clathrate with H2S and Xe help gases, the pinacolone + H2S binary sH clathrate, 1,3-Dioxolane hydrate, chlorine, bromine, and mixed chlorine with bromine hydrates. X-ray structural analysis shows that the tBA nitrogen atom has a distance of 2.64 \uc5 from the closest large cage oxygen atom. This water molecule is pulled inwards toward the tBA guest (cage center) and the structure of the large cage is substantially distorted in comparison to the ideal cage structure. The pinacolone oxygen atom is determined to have a distance of 2.96 \uc5 from the closest large cage oxygen atom.Peer reviewed: YesNRC publication: Ye