Small Caged Molecules

Small Caged Molecules Used to Control Protein Activity... 

Clathrate hydrates Solid cages of water that form around small gas molecules such as methane, hydrogen, or carbon dioxide under conditions of high pressure and low temperature such as found on the deep sea floor and within the sediments. 

In this chapter, the chemistry and the structures of Zintl ions and cage molecules are summarized and put in the context of intermetalloid clusters. The emphasis is put on the observation that irrespective of the starting material (Zintl ions or small low-valent organometallic compounds) the same types of intermetalloid clusters are formed. 

The specifications for the small help gas molecules that fill the small cages of sH hydrate are less rigorous and all guests that adequately fill the small cages in si and sll hydrate are thought to be suitable sH help gas molecules, for example, Ar, O2, Kr, CO, Xe, CH4, H2S, and then to a lesser extent SO2, CO2, CH3CI, and acetylene. CH4, Xe, H2S, H2 have been confirmed to be small sH help gas molecules by structural tools, such as NMR spectroscopy, x-ray, and neutron diffraction. 

Davidson (1971) indicated that occupancy of the smaller cages is incomplete for molecules less than 5.0 A in diameter, and that larger cages in each structure seem to be almost completely occupied. Via the use of NMR, Ripmeester and Davidson (1981) have determined that for 129Xe, the occupancy of the small cages of structure I is 0.74 times that of the large cages. 

A number of studies have indicated that a labile cage-like (512) cluster of 20 water molecules around a hydrate guest former may not form preferentially at the initial stages of nucleation. MD simulations of xenon hydrate formation from a xenon-ice system showed that there is no preferential formation of cavities with 20 water molecules, which would be similar to the small hydrate cage. Rather, the statistical mean cage size distribution was found to be between 24 and 27 water molecules. Tse et al. (2002) suggest that this supports the experimental observation that sll SF6 hydrate formation does not require occupation of small cages. 

Small amounts of phosphorous can be used to vulcanize polymeric sulfur. Larger amounts yield the P4S4 bird-cage molecules mentioned in Section 6.2 on Other Phosphorous-Containing Polymers in Chapter 6, together with phosphorous-sulfur copolymers. The copolymers are hydrolytically unstable.56... 

Clathrate hydrates are inclusion compounds formed by the enclosure of a small guest molecule within a hydrogen bonded cage of solid-state water. Clathrate hydrates are co-crystals and are thus distinct from ice, which is made of pure water, and hence can have different physical properties to ice such as a different melting point. The classic example of a clathrate hydrate is the burning snowball of methane clathrate hydrate. The combustion of the methane in the clathrate is self-sustaining, Figure 7.1. Many... 

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