Krypton hydrate

CasHeoOao O S Kr-5.78 HaO Cyclomaltohexaose - krypton, hydrate (complex I) CYDXKR10 37 408... 

Physics and Chemistry of Ice showing the recent results of krypton hydrate, nitrogen... 

As for krypton hydrate, Desgreniers et aU studied the pressure-induced phase transformations of krypton hydrate at room temperature by using X-ray diffraction measurements. They found that the initial cubic sll of krypton hydrate (KH-I) successively transformed to the cubic si (KH-II), the hexagonal structure (KH-III), and the sO (KH-IV) at 0.3 GPa, 0.6 GPa, and 1.8 GPa, respectively. They also found that the sO phase decomposed at pressures above 3.8 GPa. 

Figure 4 Comparisons of phase transformation pressures between the X-ray diffraction (XRD) and neutron diffraction (ND) studies and the present results by microscopic observations and in-situ Raman scattering measurements for krypton hydrate, nitrogen hydrate, and methane hydrate.

Handa et al. (1990) have used MD to study pressure-induced phase transitions in ethylene oxide and krypton hydrates. The MD results suggest that under pressure the water molecules in the hydrates collapse around the guest molecules, and the repulsive forces between the guest and the water molecules are mainly responsible for the reversible transition to the original structure when the pressure is released. 

To prepare [Si46]K6.g, isotype of the krypton hydrate or of the Melanoi ogite [(Si02)46l ... 

For guest molecules small enough that l t 12 Structure II is the more stable unless approaches zero. We must therefore conclude that > C- 2 is a stability requirement for most type I hydrates. This contradicts much of the conventional wisdom about small guest molecules in which nearly equal occupancies of the 12-and 14-hedral cages are commonly found (e.g., references [2], [3] and [4]) from model calculations of dissociation pressures. Most of these calculations, in addition to erroneously taking argon and krypton hydrates as model type I structures for fixing molecular interaction parameters, appear to under-estimate the effective size of the water molecule. 

In their study of krypton hydration, Durell and Wallqvist also reported a calculation of the enthalpy of hydration evaluated by the direct method of Eq. [31]." Both constant volume and constant pressure enthalpies were determined by varying the volume of the krypton solution. Their results are displayed in Table 1. The enthalpy of hydration in the constant volume case (—6.3 1.3 kcal/mol) is significantly more exothermic than in the constant pressure case (—3.4 1.3 kcal/mol). The latter number agrees very well with the experimental value of —3.3 kcal/mol, also obtained at constant pressure. The calculated enthalpies of solvation were decomposed into solute-water and water-water (solvent reorganization) terms. The solute-water contribution is comparable and favorable (—5.4 kcal/mol) in both the constant volume and constant pressure calculations. The solvent reorganization term, in contrast, shows a large ensemble dependence. In the constant-pressure case, the solvent reorganization term has a value of 2.0 1.3 kcal/mol. The overall favorable enthalpy of hydration of krypton at constant pressure therefore results from the solute-water attractions rather than from a... 

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