Dimensionally extended network

Networks with Si-Si linkages do not oeeur in Nature due to the sensitivity of the Si-Si bond towards air and moisture. Nevertheless, there are several manmade materials, whieh have already gained teehnologieal importance or look set to become important in the near future. In the following 14 chapters, Si-Si bond containing species of different size and dimensionality are presented, from low-molecular weight units to three-dimensional extended networks. 

The simultaneous formation of the polyimide and the three dimensional epoxy network occurs because of the polyamic acid containing reactive groups that can initially open the oxirane groups of the DGEBA and then subsequently intramolecularly cyclize to become polyimide as the epoxy network extends. These reactions are summarized in Fig. 4. 

Tetrathiafulvalene (TTF) derivatives containing fused 1,2,5-thiadiazole rings have been prepared in the interest of synthetic organic conductors. The extended -conjugation and unique two-dimensional crystal network of the derivative (13) may be expected to confer useful conducting... 

The two-dimensional structures are extended networks formed by the linking of the metal-oxygen polyhedra and the phosphate tetrahedra. These are sheet structures and often resemble those of naturally occurring clay minerals. The sheets are usually anionic and the protonated (cationic) amine molecules, located between the two sheets, render the framework neutral. The two-dimensional structures are intermediates between the one-dimensional chains and the three-dimensional structures, and the literature on phosphate networks contains descriptions of several layered materials, owing to the wide compositional diversity exhibited by them [22-24]. The layered materials are of interest because they act as precursors for the three-dimensional structures. 

Ammonia and aU other amines, NR3, are bases. A solntion of ammonia in water is mildly alkaline (pHb = 4.75). Ammonia dissolves readily in water (1.176m at 273 K, 0.702 m at 298 K in IL of water) under evolntion of heat (AH = —37.1 kJmol ). Hydrogen bonds are formed between water and ammonia. Two hydrogen-bonded ammonium hydrates have been isolated, NH3 H2O (m.p. 194.15 K) and 2NH3 H2O (m.p. 194.32K). In the solid state, both compounds have a three-dimensional extended hydrogen bond network. 

Vitreous P2O5 and phosphate-rich glasses are difficult to prepare, owing to the volatility of P2O5, and are very moisture sensitive. Vibrational and MASNMR spectra show that they contain three-dimensional phosphate networks comparable to those in the extended forms of P2O5 and ultraphosphates. 

Recent particle tracking simulations in soil network models indicate that solute dispersion is more sensitive to the water retention curve than to the particular combination of pore-size distribution and topology that determine its shape (Vogel, 2000). Numerical particle tracking techniques have also been used to simulate solute dispersion in fractured media. Examples for two-dimensional randomly intersecting fracture networks include the models developed by Hull et al. (1987), Smith and Schwartz (1984), Robinson and Gale (1990), and Clemo and Smith (1997). Recently Nordqvist et al. (1996) and Margolin et al. (1998) have extended this approach to three-dimensional fracture networks. 

Since tetraethynylethenes represent a repeat unit in more than one two-dimensional carbon network including 45 and 46 (Fig. 13-1) [1] the preparation of a specific network cannot be accomplished by simple oxidative polymerization of 20, but rather requires a more characteristic macrocydic precursor as starting material. Macrocyclic precursors to extended carbon sheets are perethynylated dehydroannulenes [56] and expanded radialenes, novel carbon-rich materials with interesting and unusual structures and functions. 

In addition to these two major families of magnets, other types of cations, with different geometries, shapes, and charges also stabilize different extended bimetallic oxalate complexes of controlled and increased dimensionality, such as dimers, trimers, tetramers, 1-D chains, and 3-D extended networks from templating ... 

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