Crystalline hosts

Fig. 26. Clathrate receptor chemistry (a) a chiroselective crystalline host compound (clathrand) (b) a typical guest molecule to be included in the specified configuration and (c) the crystal stmcture of the respective clathrate (A and B denote host and C the guest species) (169).

A wide variety of guest molecules may be trapped by the Wemer-type crystalline host lattice, ranging, eg, from noble gases to condensed aromatic hydrocarbons. These clathrates may be formed from solution or by sorption. Kinetics of sorption—desorption have been studied (83). 

Solid-State Lasers. Sohd-state lasers (37) use glassy or crystalline host materials containing some active species. The term soHd-state as used in connection with lasers does not imply semiconductors rather it appHes to soHd materials containing impurity ions. The impurity ions are typically ions of the transition metals, such as chromium, or ions of the rare-earth series, such as neodymium (see Lanthanides). Most often, the soHd material is in the form of a cylindrical rod with the ends poHshed flat and parallel, but a variety of other forms have been used, including slabs and cylindrical rods with the ends cut at Brewster s angle. 

Insertion (intercalation) compounds. Insertion compounds are defined as products of a reversible reaction of suitable crystalline host materials with guest molecules (ions). Guests are introduced into the host lattice, whose structure is virtually intact except for a possible increase of some lattice constants. This reaction is called topotactic. A special case of topotactic insertion is reaction with host crystals possessing stacked layered structure. In this case, we speak about intercalation (from the Latin verb intercalare, used originally for inserting an extra month, mensis intercalarius, into the calendar). 

Fig. 1. Stereoscopic view of the two reacting molecules of Ham the crystalline host-guest complex 12a...

The organic inclusion compound 2,4,6-tris-(4-bromophenoxy)-l,3,5-triazine was used to create an ordered arrangement of endohedral fullerenes in a crystalline host matrix <06CPL327>. 

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

Solid-state lasers using substitutional neodymium (Nd3+ ions) as the active defects are widely available. Practical lasers contain about 1% Nd3+ dopant. The most common host materials are glass, yttrium aluminum garnet (YAG), Y3A15012, and calcium tungstate, CaW04. In the crystalline host structures, the defects responsible for amplification are NdY and Ndca-... 

The crystalline host rocks are covered by Palaeozoic sediments of the Western Interior Basin. These consist of Ordovician... 

Chapters 5 and 6 deal with the spectra of optically active centers. The term optically active center corresponds to a dopant ion and its environment (or to a color center), which produces absorption and/or emission bands that are different to those of the pure crystalline host. This is the case for a large variety of optical materials, such as phosphors, solid state lasers, and amplifiers. 

Asymmetric Induction in the Photochemistry of Crystalline Host-Guest Assemblies... 

The very nature of non-Kekule species as reactive intermediates suggests that studies of them under conditions far from those used in conventional investigations of the synthesis and reactions of stable molecules are indispensable. These requirements frequently are met by immobilizing the species in crystalline hosts or randomly oriented matrices, as is described in Chapter 17 by Bally in this book. Although some information available from crystal studies usually must be sacrificed in the random matrix technique, the latter is usually far more convenient, and most smdies of non-Kekule compounds in solids have used it. 

The zero-field values for TMM, D = 0.024 cm (2D = 513 G) and E < 0.001 cm , were compatible with the expected triplet ground state and with an effective interelectron distance of 4.8 A. Moreover, when the species was generated in a crystalline host, the lines were visibly further split into a... 

FAA FA FBC FC FEBEX FFFF FGD FP FSU FT FTIR FUETAP Flame atomic absorption Fly ash Fluidized bed combustion Filter cake Full-scale engineered barriers experiment (in crystalline host rock) Flow-field flow fractionation Flue gas desulphurization Fission products Former Soviet Union Fourier transforms Fourier transformed infrared spectroscopy Formed under elevated temperature and pressure... 

Semiconductor clusters have traditionally been prepared by the use of colloids, micelles, polymers, crystalline hosts, and glasses. The clusters prepared by these methods have poorly-defined surfaces and a broad size distribution, which is detrimental to the properties of the semiconductor materials. The synthesis of monodisperse clusters with very well-defined surfaces is still a challenge to synthetic chemists. However, some recent approaches used to overcome these problems are (i) synthesis of the clusters within a porous host lattice (such as a zeolite) acting as a template and (ii) controlled fusion of clusters. 

The luminescent material may be considered as a transformer of energy, i.e.. from ultraviolet photons to photons of lower energy from cathode rays to photons front electric fields to photons, etc. An inorganic luminescent material, or phosphor, usually consists of a crystalline host materia) to which is added a trace of an impurity (activator and eoactivaturl. 

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