The Crystalline Solid State

Solid-state chemistry uses the same principles for bonding as those for molecules. The differences from molecular bonding come from the magnitude of the molecules in the solid state. In many cases, a macroscopic crystal can reasonably be described as a single molecule, with molecular orbitals extending throughout. This description leads to significant differences in the molecular orbitals and behavior of solids compared with those of small molecules. There are two major classifications of solid materials crystals and amorphous materials. Our attention in this chapter is on crystalline solids composed of atoms or ions. 

We will first describe the common structures of crystals and then give the molecular orbital explanation of their bonding. Finally, we will describe some of the thermodynamic and electronic properties of these materials and their uses. 

Crystalline solids have atoms, ions, or molecules packed in regular geometric arrays, with the structural unit called the unit cell. Some of the common crystal geometries are described in this section. In addition, we will consider the role of the relative sizes of the components in determining the structure. Use of a model kit, such as the one available from ICE, makes the study of these structures much easier. 

The crystal structures of metals are simple. Those of some minerals can be very complex, but usually have simpler structures that can be recognized within the more complex structure. The unit cell is a structural component that, when repeated in all 

Institute for Chemical Education, Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, WI 53706. Sources for other model kits are given in A. B. Ellis, M. I. Geselbracht, B. J. Johnson, G. C. Lisensky, and W. R. Robinson, Teaching General Chemistry A Materials Science Companion, American Chemical Society, Washington, DC, 1993. 

Atoms at the comers of rectangular unit cells are shared equally by eight unit cells and contribute to each ( of the atom is counted as part of each cell). The total for a single unit cell is 8 X = 1 atom for all of the comers. 

Atoms at the comers of nonrectangular unit cells also contribute one atom total to the unit cell small fractions on one comer are matched by larger fractions on another. 

Atoms on edges of unit cells are shared by four unit cells—two in one layer, two in the adjacent layer— and contribute to each. 

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Shortly after, Doetschman and Hutchison reported the first example of a reactive carbene in the crystalline solid state, by preparing diphenylcarbene from diphenyldi-azomethane in mixed crystals with 1,1-diphenylethylene 84 (Scheme 7.23). When the mixed crystals were irradiated, carbene 85 was detected by electron paramagnetic resonance (EPR) and the disappearance of the signal was monitored to determine its kinetic behavior. Two reactions were shown to take place under topochemical... 

These particular molecules were chosen since they exemplify the interesting and unique properties - of an entire class of 17-donor species. These properties include the formation of stable, colored radical cations and the known ability to interact strongly in the crystalline solid state. - ... 

In the crystalline solid state, there is little vibrational or translational freedom, and hence diffusion into a crystalline lattice is slow and difficult. As the temperature of a solid is raised by the input of heat, vibrational and translational motion increases. At a particular temperature - termed the melting point - this motion overcomes the attractive forces holding the lattice together and the liquid state is produced. The liquid state, on cooling, returns to the solid state as crystallization occurs and heat is released by the formation of strong attractive forces. 

This opening chapter has introduced many of the principles and ideas that lie behind a discussion of the crystalline solid state. We have discussed in detail the structure of a number of important ionic crystal structures and shown how they can be linked to a simple view of ions as hard spheres that pack together as closely as possible, but can also be viewed as the linking of octahedra or tetrahedra in various ways. Taking these ideas further, we have investigated the size of these ions in terms of their radii, and... 

Reactions that occur in the crystalline solid state offer an opportunity to document the structure of the reactant and its environment to a level of detail that is not... 

In our laboratories we have shown that the crystalline solid state may exert a very high degree of control on the selectivity of several highly reactive intermediates. The reaction in Scheme 27 illustrates the transformation of a diazo compound... 

Photochemical reactions in the solid state may also go to limited conversion when the product does not compete for incident light but may act as an efficient energy quencher. This is likely to be of importance in triplet state reactions when the product may act as an energy trap. Although this is a factor that may also limit the conversion of photochemical reactions in solution, because of efficient energy delocalization by dipole-dipole and exchange mechanisms it is likely to be more important in the crystalline solid state. 

Visible Absorption Bands of Charge-Transfer Salts in the Crystalline Solid State and... 

Patel, G. N., Miller, G. G. Copolymerization of diacetylenes in the crystalline solid state — a method for recording latent finger prints. Preprint 1976... 

In contrast to these lyotropic mesogenic materials, which require the presence of a solvent in order to produce a mesophase, thermotropic mesogens are those compounds which exhibit a mesophase in the melt state at temperatures above the crystalline solid state and before the formation of an isotropic melt Mesophase-forming polymers may possess either one of the two basic structures shown in Fig. 1 the polymer may either contain the mesogenic group (the part of the polymer molecule which is responsible for liquid crystallinity) directly in the main chain, or the mesogen may be present as a pendant group in the side chain. 

Similar reaction products are observed with gamma-irradiation in solutionj but in the crystalline solid state, the mechanism appears to be different and no glucose is formed. 

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