When hydrophilic molecules are introduced as guests into the water network, they form H-bonds with water molecules in the host lattice. Theoretical calculations indicated that the formation of extrinsic host-guest H-bonds strongly perturbs the host lattice, results in a local breakage of the intrinsic H-bonds between water molecules, and creates major defects in the water lattice, as exemplified in Figure . Figure shows a standalone empty cage with a standard tetrahedral coordination where each water molecule is presented with three H-bonds while the forth H-bond points outward the cage and is omitted for simplicity. Figures show guest-containing cages where host-guest H-bonds occur. Evidently, the formation of host-guest H-bonds causes the breakage of the intrinsic water-water H-bonds. The affected water molecules are displaced from their tetrahedral positions and are bonded by only one or two H-bonds. These simulations predicted major defects in water lattice induced by host-guest Hbonds. Using liquid-state H 1 magnetic nuclear resonance (called liquid H 1 NMR) enabled the direct probe of the defect formation and evolution in water lattice. While solid-state NMR is widely used for probing ice and gas hydrate, Nguyen et al. showed that liquid H 1 NMR enabled unprecedented insights into the defects of solid water lattices. This technique is sensitive to the presence of mobile water molecules and allows to detect water molecules with H-bonds fluctuating on the microsecond to millisecond time scale. Water molecules with the standard tetrahedral coordination and long-lived Hbonds are observed only as a broad spectral background. 48 Figure shows the liquid H 1 NMR spectra of ice and hydrates at 263 K reported by Nguyen et al. The hydrates contained deuterated guest molecules, namely deuterated tetrahydrofuran (THF-d 8 , C 4 D 8 O) and deuterated cyclopentane-d 10 (C 5 D 10 ). THF-d 8 is polar and forms H-bond with water, while cyclopentane-d 10 is non-polar and does not form H-bonds with water. Having similar molecular sizes, shapes, and weights with different polarities, these two guest molecules form an ideal case for studying the effect of host-guest H-bonds on the host lattice. As guest molecules were deuterated, the H 1 NMR signals carried only the information about the water lattice. Evidently, no liquid H 1 NMR response was observed from ice except for a clear reference signal provided by methylene chloride (Fig ). The absence of liquid H 1 NMR signal of ice demonstrates that all water molecules in ice are tetrahedrally coordinated by standard long-lived H-bonds. These water molecules are prohibited from rotational and translational motions on the microsecond to millisecond time scale. The spectrum of cyclopentane-d 10 hydrate was similar to that of ice, featured by the absence of water signals (Fig 5c). This means that water molecules in the cyclopentane hydrate are tetrahedrally coordinated by standard long-lived Hbonds similar to those in ice. The enclathration of non-polar cyclopentane molecules does not alter the water lattice. In contrast, THF hydrate displayed a strong liquid H 1 NMR signal centered around a chemical shift of 6.5 ppm (Fig ). Experimentally, the 6.5 ppm peak attests the occurrence of mobile water molecules in THF hydrate. The temperature-dependent liquid H 1 NMR spectra allow us to elucidate the origin of mobile water in THF hydrate (Fig ). The 6.5 ppm peak broadens at low temperatures but remains observable at temperatures down to 243 K. This peak manifests major defects of the water lattice induced by THF- The rotational mobility of the affected water molecules intensifies with the increase of the temperature, as evidenced by the growth of the peak at 6.5 ppm (Fig ). At 271 K, a fraction of the affected water molecules gains sufficient thermal energy to undergo translational motions and possess liquid-like behaviors, which is evidenced by the emergence of the second peak at 5.2 ppm (Fig and). This peak grows rapidly with the temperature increase, attesting the increasing fraction of water with liquidlike behaviors. At 277 K, which is the dissociation temperature of THF-d 8 hydrate, the 5.2 ppm peak fully transforms into the signal of liquid water and the 6.5 ppm peak also vanishes, as a result of the full dissociation of the hydrate. Hence, liquid H 1 NMR spectra provide clear and direct evidence of the formation and growth of major defects in the host lattice due to host-guest hydrogen bonding. These defects are controllable by tuning the temperature and the polarity of the guest molecules, opening up a new pathway for defect engineering for manipulating gas encapsulation in water lattice for energy storage or CO 2 sequestration (discussed in the next section). Diffusing defects as path-opening for gas migration: Once defects are created, they diffuse throughout the water lattice. Figure shows a simplified tetrahedral network of water where every water molecule is coordinated by four H-bonds with their neighbours. As discussed above, the incorporation of a H-bonding molecule in the water lattice leads to the formation of host-guest H-bonds (Fig ). The water molecule of interest (labelled #1) rotates about its center of mass to conform demonstrated that all water molecules in their THF hydrate samples experienced rotational motions (i.e. lattice defects) in microsecond to millisecond time scales. We postulate that the diffusing defects might open channels for gas migration into the water lattice and facilitate gas enclathration in ice. In real-world circumstances, the dopant molecule can rotate inside the cage and form H-bond(s) to different water molecules, which leads to a greater perturbation effect and induces more defects in the host lattice.