“Liquid-like” Water in Clathrates Induced by Host–Guest Hydrogen Bonding
Ngoc N. Nguyen, Rüdiger Berger, Manfred Wagner, Jürgen Thiel, Hans‐Jürgen Butt, Robert Graf
Abstract
Clathrate structures are sustained by a host lattice formed by hydrogen-bonding water molecules, which encapsulates guest molecules. Up to now, all water molecules in the host lattice are considered ice-like crystallized. Here, we discovered the occurrence of “liquid-like” water molecules and the resulting defects in (polar) tetrahydrofuran clathrates by liquid-state 1H NMR experiments. The liquid-like water molecules start occurring at 271 K, well below the apparent dissociation point (277 K) of the clathrate matrix, via extracting water molecules from the host lattice by host–guest H-bonding. We found an intriguing two-stage dissociation of the clathrate: Partial dissociation at 271 K converting one-third of water molecules into liquid-like followed by complete dissociation at 277 K. The clathrate structure is molecularly heterogeneous in the region between 271 K and 277 K. No liquid-like water exists in (nonpolar) cyclopentane clathrates. This work uncovers the essentiality of host–guest interaction for clathrate structures and the ability to tune their stability using polar molecules.