Cage of molecules

The isolated molecule C28 point symmetry group is Td. The 42 covalent bonds of three different types take part in forming of the cage of molecule ... 

Note 1 In polymerizations, the best-known result of the cage effect is the reduced efficiency of initiation of a radical polymerization due to a fast reaction between the two radicals, formed by the homolytic dissociation of initiator, within a transient cage of molecules. The reaction occurs before the radieals are able to diffuse apart. 

Experimental work In kinetics of polymerization In liquid crystalline media Is sketchy at best. Hopes have been formulated for the possibility of regulating stereo-placements and Inducement of topotactlc effects by free-radical polymerization of liquid crystalline monomers In bulk or In liquid crystalline solvents, due to the high degree of orientational order (35, 52). Such effects have yet to be established. Most of the reported data appear to be due to factors other than molecular orientation and are only Indirectly related to the liquid crystalline order of the medium. For example, factors such as Incomplete miscibility of monomer and solvent, phase separation of the polymer, enhancement of viscosity of the medium, and caging of molecules of initiator (53, 54, 55, 56) can be invoked to explain the observed kinetic effects. 

Figure 16.4. The cages of molecules in a rhombic 3-D structure are bridged along a [111] space diagonal. Thick lines indicate (3 -I- 3)-type bonds between C6-C3 andC3-C6 atoms on the adjacent cages of molecules.

The equipartition principle is a classic result which implies continuous energy states. Internal vibrations and to a lesser extent molecular rotations can only be understood in terms of quantized energy states. For the present discussion, this complication can be overlooked, since the sort of vibration a molecule experiences in a cage of other molecules is a sufficiently loose one (compared to internal vibrations) to be adequately approximated by the classic result. 

Since methane is almost always a byproduct of organic decay, it is not surprising that vast potential reserves of methane have been found trapped in ocean floor sediments. Methane forms continually by tiny bacteria breaking down the remains of sea life. In the early 197Qs it was discovered that this methane can dissolve under the enormous pressure and cold temperatures found at the ocean bottom. It becomes locked in a cage of water molecules to form a methane hydrate (methane weakly combined chemically with water). This "stored" methane is a resource often extending hundreds of meters down from the sea floor. 

Molecular as well as ionic substances can form hydrates, but of an entirely different nature. In these crystals, sometimes referred to as clathrates, a molecule (such as CFI4, CHCI3) is quite literally trapped in an ice-like cage of water molecules. Perhaps the best-known molecular hydrate is that of chlorine, which has the approximate composition Cl2- 7.3H20. This compound was discovered by the great... 

In the 1930s when high-pressure natural gas (95% methane) pipelines were being built in the United States, it was found that the lines often became plugged in cold weather by a white, waxy solid that contained both water and methane (CFIJ molecules. Twenty years later. Walter Claussen at the University of Illinois deduced Ihe structure of that solid, a hydrate of methane. Notice (Figure B) that CH4 molecules are trapped within a three-dimensional cage of H20 molecules. 

Sulfurous acid is an equilibrium mixture of two molecules (12a and 12b) in the former, it resembles phosphorous acid, with one of the H atoms attached directly to the S atom. These molecules are also in equilibrium with molecules of S02, each of which is surrounded by a cage of water molecules. The evidence for this equilibrium is that crystals of composition S02-aH20, with x about 7, are obtained when the solution is cooled. Such substances, in which a molecule occupies a cage formed by other molecules, are called clathrates. Methane, carbon dioxide, and the noble gases also form clathrates with water. 

Owing to restrictions on the motion of molecules in the liquid state, a pair of radicals formed in solution may execute many oscillations in their respective cages consisting of surrounding molecules before they diffuse apart. Thus, recombination with restoration of the original molecule during the interval of their existence as immediate neighbors in the liquid may well occur. The rate of decomposition would then depend not only on the rate of fission of the initiator into radicals but also on the rate at which they subsequently diffuse apart. It is, of course, also possible that the pair of benzoate radicals may combine to yield decomposition products, e.g., phenyl benzoate and carbon dioxide, instead of the reactant. 

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

The broad, featureless nature of the Si NMR signal is likely due to one of three phenomena that are well-known to cause such effects in NMR spectroscopy a multiplicity of chemical and physical environments about the silicon atoms restricted segmental motion in polycyclic or cage-like molecules or ne broadening due to ySi magnetic interactions with the numerous N quadrupoles. 

Fig. 4. Inclusion cage of Dianin s compound (5). The matrix is constructed via a cyclic H-bonded hexagonal system of host molecules (on top and on bottom of the macrocage O atoms as bold dots, H-bonds as dotted lines) bulky parts of the host molecules interlock (equatorial of the cage). The cage can be filled with molecules of fitting size (e.g. one molecule of chloroform) (Adapted from Ref. 16)...

The Hamiltonian in Eq. (104) may describe both the process of tunnel inversion or isomerization of a molecule and the inertia effects arising from the symmetric vibrations of the reaction complex AH- B in the cage of the solvent or solid matrix (Fig. 9). In the latter case, the coordinate and the frequency of the symmetric vibration correspond to R and w0. 

It has been estimated (4) that in most common solvents at room temperature two reactant molecules within a cage of solvent molecules will collide from 10 to a 1000 times before they separate. The number of collisions per encounter will reflect variations in solvent viscosity, molecular separation distances, and the strength of the pertinent intermolecular forces. High viscosities, high liquid densities, and low temperatures favor many collisions per encounter. 

Water intrusion-extrusion isotherms performed at room temperature on hydrophobic pure silica chabazite show that the water-Si-CHA system displays a real spring behavior. However, Pressure/Volume differences are observed between the first and the second cycle indicating that some water molecules interact with the inorganic framework after the first intrusion. 29Si and especially H solid state NMR and powder X-ray diffraction demonstrated the creation of new defect sites upon the intrusion-extrusion of water and the existence of two kinds of water molecules trapped in the super-cage of the Si-CHA a first layer of water strongly hydrogen bonded with the silanols of the framework and a subsequent layer of liquid-like physisorbed water molecules in interaction with the first water layer. 

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

---