Unusual spatial structure

From the time of van t Hoff (1) and Le Bel (2) the tetrahedral arrangement of substituents around a tetravalent carbon atom has been part of the foundation of organic stereochemistry. However, in the last 30 years, thanks to the rapid development of synthetic methods of organic chemistry as well as of experimental and theoretical methods of structure elucidation, many exciting saturated hydrocarbons with very unusual spatial structure have been synthesized. Cubane (1) was obtained as early as 1964 (3), triprismane (2) (4), a tetra-f-butyl derivative of tetrahedrane (3) (5), small-ring propellanes with structures involving inverted carbon atoms, 4-6 ([3.2.1], [4.1.1] and... 

There are many globular proteins in a living cell, and they play a key role. We have already discussed this in Chapter 5. However, the theory of such systems is extremely hard a protein globule is perhaps one of the most complex objects in modern physics. What is striking and unusual is that proteins have a strictly defined spatial tertiary structure (see Sections 5.6.4 and 5.7). In a protein globule, not only averaged density, but the entire spatial structure of the whole chain is fixed. 

The origin of this unusual behaviour is partly clarified from Fig. 6.34(a) where the relevant curves 2 demonstrate the same kind of the non-monotonous behaviour as the critical exponents above. Since, according to its definition, equation (4.1.19), the reaction rate is a functional of the joint correlation function, this non-monotonicity of curve 2 arises due to the spatial re-arrangements in defect structure. It is confirmed by the correlation functions shown in Fig. 6.34(a). The distribution of BB pairs is quasi-stationary, XB(r,t) X°(r) = exp[(re/r)3], which describes their dynamic aggregation. (The only curve is plotted for XB in Fig. 6.35(a) for t = 102 (the dotted line) since for other time values XB changes not more than by 10 per cent.) This quasi-steady spatial particle distribution is formed quite rapidly already at t 10° it reaches the maximum value of XB(r, t) 103. The effect of the statistical aggregation practically is not observed here, probably, due to the diffusion separation of mobile B particles. 

The study of confined quantum systems has attracted increasing attention from several research groups in the world due to the unusual physical and chemical properties exhibited by such systems when subject to spatial limitation. Such novel properties, not present in conventional materials, have marked a new era for the synthesis of modern materials - structured at the nanoscale - and leading to what is now called nanotechnology. 

It appears that the incorporation of metal adatoms into adsorbate structures stabilizes the reaction intermediates, and therefore, can be expected to be a general phenomenon on catalytic metal surfaces, at least for metal particles large enough to be considered as metallic. The dynamic processes of incorporation, release, and mass transport of metal adatoms may occur on the time scale of surface reactions and affect the reactive behavior of the intermediates, that is to say, the reaction kinetics. Indeed, STM studies have shown that the kinetic oscillation in some surface reactions can be partially attributed to the spatial organization of reactive species on the surfaces and the structural change in such complex surfaces on the time scale of reaction [69]. The structural complexity of the active surfaces and the origin of unusual surface reaction kinetics are of interest, and may be connected. Recently, such a relationship was established in the autocatalytic decomposition of formate and acetate on the Ni(llO) surface [21]. 

A detailed description of the bonding in hydrates evidently requires a knowledge of the positions of the H atoms. In the earlier X-ray studies it was not possible to locate these atoms, and it was assumed that they were responsible for certain unusually short 0-0 or 0-X distances in the crystals. Later studies, particularly n.d. and n.m.r., have confirmed this and led to the precise location of the H atoms. It is now becoming possible to discuss not only the gross structures of the compounds, that is, the spatial arrangement of the heavier atoms, but also two aspects of the finer structure, namely, the positions of the H atoms in hydrogen bonds and the ordering of the protons. Reference to these topics will be made later. 

Superlattices, and other kinds of artificially structured materials in which composition varies periodically on a quantum-mechanically significant spatial scale, have excited much interest recently in the materials sciences. Superlattices often have unusual optical and electronic properties, and they may also display extraordinary chemical properties. There are interesting possibilities for synthesis of such materials by various electrodeposition methods. These are worthy of exploration. 

Pyridine A-oxide complexes have been prepared for Mg ", Ca ", Sr and Ba ", and arsine A-oxide complexes for Mg and Ca . An X-ray structure of Mg(C104)2-5Me3As0 shows the cation to be in a square pyramidal environment provided by the donor ligands the charge separation and unusual geometry are believed to be a consequence of the electronic and spatial needs of the ligand. ... 

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