Selectivity cavities

Figure 14 External potassium coordination sites (upper) showing the four principal ligand contacts (Gly backbone carbonyls) to a surface-bound K+ ion and the fully solvated external ion that are held in the vicinity of the pore by electrostatics. Stereoviews (lower) showing the positions of potassium ions (green) and potassium-bound waters (red) in the central cavity, selectivity fdter, and two extracellular ions (reproduced with permission from Zhou et al ).

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M5 and M6E are typically encapsulated structures and similarly to M4E species are accessible only when E is a first row element. Larger main group elements are not appropriate for these cavities. Selected examples of clusters with stoichiometries M4E, M5E, and M6E are shown in Table 3.2. The most important family of clusters with interstitial or encapsulated atoms are by far the carbides. 

Crown ethers are heterocycles consisting of oxyethylene units, and their size depends on the number of these repeating units. They are known to capture cations into their cavities selectively. Specific cation recognitions of the crown ethers led to the development of various applications such as metal cation sensors and phase transfer cata-lysts.2 Since the hydrophilicities of the crown ethers increase through complexation with cations, introducing crown ether units into thermosensitive polymers should control their solubilities and LCST behaviors in the presence of cations as stimuli. 

Fig. 1. Arranging binding groups around cavities selected examples.

A new dimension to acid-base systems has been developed with the use of zeolites. As illustrated in Fig. XVIII-21, the alumino-silicate faujasite has an open structure of interconnected cavities. By exchanging for alkali metal (or NH4 and then driving off ammonia), acid zeolites can be obtained whose acidity is comparable to that of sulfuric acid and having excellent catalytic properties (see Section XVIII-9D). Using spectral shifts, zeolites can be put on a relative acidity scale [195]. An important added feature is that the size of the channels and cavities, which can be controlled, gives selectivity in that only... 

Most effective differentiation of the receptor between substrates will occur when multiple interactions are involved in the recognition process. The more binding regions (contact area) present, the stronger and more selective will be the recognition (17). This is the case for receptor molecules that contain intramolecular cavities, clefts or pockets into which the substrate may fit (Fig. 1). 

In a word, all these receptors are more or less able to discriminate against cations that are either smaller or larger than thek cavity (44). However, in a strict sense, discrimination of metal-ion spheres does not concern with molecular recognition but selection of the carbon ball C q certainly does. In fact, the fuUerene C q has been included into the cavity of octa-/ f2 butylcalix[8]arene (Fig. 8c) shutting out C q and making a very convenient and efficient C q purification possible without any expensive apparatus (45). 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Collectors Fitting into Fattice Cavities. Lattice site fitting of collectors at sohd walls has been invoked as a means of explaining the selective behavior of amines (cationic coUectors) as reagents in the flotation-separation of soluble salt minerals such as KCl and NaCl (22). 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

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