Spherical-like cages

All hydrate structures have repetitive crystal units, as shown in Figure 1.5, composed of asymmetric, spherical-like cages of hydrogen-bonded water molecules. Each cage typically contains at most one guest molecule, held within the cage by dispersion forces. The hydrate crystalline structures and mechanical properties are discussed in Chapter 2. Throughout this book the common name... 

Finally, when talking about amplifying nanoparticles, the applications are of course not only limited to spherical particles as mainly employed today and discussed here, but aspherical objects like cages, rods or wires offer a wealth of additional tuning modes and will presumably receive pronounced attention in the future [142]. 

The final section of the volume contains three complementary review articles on carbon nanoparticles. The first by Y. Saito reviews the state of knowledge about carbon cages encapsulating metal and carbide phases. The structure of onion-like graphite particles, the spherical analog of the cylindrical carbon nanotubes, is reviewed by D. Ugarte, the dominant researcher in this area. The volume concludes with a review of metal-coated fullerenes by T. P. Martin and co-workers, who pioneered studies on this topic. 

By the last two assumptions the theory, strictly speaking, is only applicable to the monatomic gases A, Kr, Xe, to a somewhat lesser extent to the almost spherical molecules CH4, CF4, SFe, and perhaps to nonpolar diatomic molecules. The rotation of even slightly nonspherical molecules like Q2 and N2 will not be free in the entire cavity when such a molecule comes close to the wall of its cage it will have to orient itself parallel to this wall. Furthermore, some of the cavities are somewhat oblate (cf. Section I.B), and thus the rotation of relatively large, oblong molecules may be seriously... 

Crossley (89) described the self-assembly of a spherical cage-like structure made up of two zinc porphyrin dimers bound to a tetramine ligand. The 2 1 complex is stable at 10-6m concentrations, but addition of excess of ligand causes dissociation of the capsule with formation of a 1 1 complex. 

Fullerenes are cage-like carbon structures which derive from a graphene sheet where a few six-membered rings are replaced by five-membered ones which forces the layer into a bent shape. Placing the pentagons at suitable positions, a spherical structure of 60 carbon atoms is obtained - the buckminsterfullerene. Fullerenes with... 

Compared to small two-dimensional molecules, for example the planar benzene, the structures of these three-dimensional systems are aesthetically appealing. The beauty and the unprecedented spherical architecture of these molecular cages immediately attracted the attention of many scientists. Indeed, Buckminsterfullerene CgQ rapidly became one of the most intensively investigated molecules. For synthetic chemists the challenge arose to synthesize exohedrally modified derivatives, in which the properties of fullerenes can be combined with those of other classes of materials. The following initial questions concerned the derivatiza-tion of fullerenes What kind of reactivity do the fullerenes have Do they behave like a three-dimensional superbenzene What are the structures of exohedral fullerene derivatives and how stable are they ... 

Figure 3. GGA and LDA equilibrium structures of cage-Uke (Au ) and amorphous-like (Au ) type for several Au clusters with magic numbers fulfilling the spherical aromaticity rule n=2(L- -l), where L is an integer number (see text).

The effect is like that of a Faraday cage that confines the electron within a spherical surface. It is therefore no longer appropriate to formulate the wave function with inhnite boundary conditions, assuming tp(r0) —> 0, for r0 < oo, instead. 

The FAU framework structure is composed of sodalite cages, linked by 6-6 secondary building units (SBUs) like the carbon atom in the structure of diamond. The formed cube has an axis, a 24.3 A, and the framework produced by the union of these cubes contains windows, which lead to an approximately spherical cavity with a radius, R 6.9 A, known as the supercage or (1-cagc [108,109], These supercages have tetrahedral symmetry, and are opened through four 12-member ring (MR) windows, each with a diameter of d 1A A [108,109],... 

In order to collect SERS spectra, nanoscaled structures are usually either fixed onto large surfaces or free in suspension in the form of nanoparticles. Suitable particles are available in a variety of forms simple spherical particles, rods, [13] rice-like shapes, [14] sea urchins [15], or even cages [16]. Their size can vary from 1 to 500 nm, and this factor has an important impact on their surface plasmon resonance properties. 

In small crystals at least, the C o molecules seem to assemble themselves into a somewhat ordered array as if they are effectively spherical, which is entirely consistent with the hypothesis that they are shaped like soccer balls. The excess between the nearest-neighbour distance (10.02 A) and the diameter calculated for the carbon cage itself (7.1 A) must represent the effective van der Waals diameter set by the repulsion of the V electron clouds extending outward from each carbon atom. Because the van der Waals diameter of carbon is usually considered to be 3.3-3.4 A the packing seems a little tighter than one might expect for soccer-ball-shaped C o molecules. The reason for this has not yet been determined. 

The most successful example is the control of window size of caged mesoporous silica materials synthesized with block copolymer as templates. The pore-entrance diameter of SBA-16 increases as a function of synthesis or hydrothermal-treatment temperature. This is likely to be related to the known phenomenon of the decrease in hydrophilicity of PEO blocks as the temperature increases. In the mesostructure of the F127-silica composite, the cores of the (spherical) micelles are constituted by PPO blocks, whereas the micelle corona, which consists of PEO blocks, interacts with the silica framework. At lower temperature, PEO blocks are expected to favorably interact with hydrophilic silica species and thus to have a tendency to be intimately mixed with the silica framework. When the F127-silica composite is subjected to the treatment at higher temperatures,... 

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