Focusing rings

Collector Electrode Focusing Rings Drift Region Gating Grid... 

The main results of our experiments were as follows. In a transmitted beam we observed the usual, for the PFT, aberration self-focusing rings. The polarization of these rings was linear and orthogonal to the incident polarization, i.e., it corresponded to the e wave in the liquid crystal. 

Figure 2 shows the experimental dependences of the number of self-focusing rings on the laser radiation power W obtained for different angles of incidence Oq (the arrow on the abscissa corresponds to the specially measured threshold power for the PIT in a homeotropic cell with the same thickness and created in the same transverse cross section of the beam under normal incidence conditions). 

The generation of 3D finite element models suitable for analysis of cracks requires special attention in and around the erack region where focused rings of hex elements are used. In addition, the initial crack front may be straight, a simple elliptic section, or a general curve in space. 

Best Synthetic Methods is now 10 years old, is a family of 16 volumes and has been well received by the majority of chemists as a valuable aid in their synthetic endeavours, be they academic or commercial. The focus of the series so far has been on special methods, reagents or techniques. This volume is the first of a new sub-series with a focus on heterocycles and their synthesis. It is amazing the extent to which each heterocyclic type has its own specialized synthetic methodology. Whether the chemist is endeavouring to make a heterocycle by ring synthesis or wishes to introduce specific substituents, it is the intention that this new development will serve their needs in a practical, authoritative, fully illustrative and compact manner. Richard Sundberg is an authority on indole chemistry and it is a pleasure to have such a noted heterocyclist to initiate this venture. 

Although the number of ring atoms is the structural feature upon which we focus attention, we shall use the criteria of thermodynamics and kinetics to assess the feasibility of the reactions listed above. 

Given stringent requirements for effective sensitizers and the desire to use wavelengths further to the red for therapeutic appHcations, definition of newer sensitizers has been a principal area of research since about 1987. Expanded theoretical and experimental understanding of photophysics has been a key element in identifying new classes of potential sensitizers (93—98). Research has focused on cationic derivatives of Nile Blue (93), metaHo-phthalocyanines (94), naphthalocyanines (95), chlorin-type compounds (96), expanded ring porphyrinoids (97), as well as porphyrins other than hematoporphyrin and its derivatives (98). This work has also been reviewed (10,91). Instmmentation for photodynamic therapy has been reviewed (99). 

Estranes. Investigations into the total synthesis of steroids began in the 1930s shordy after the precise formula for cholesterol was estabUshed. The eadiest studies focused on equilenin (5) because of its relative stereochemical simplicity when compared to other steroid nuclei Initially, equilenin was synthesi2ed in 20 chemical steps with an overall yield of 2.7%. This synthesis helped to confirm the perhydro-l,2-cyclopentenophenanthrene ring system of... 

The anthraquiaone vat dyes can be classified into several groups on the basis of their chemical stmctures (/) benzanthrone dyes, (2) indanthrones, (3) anthrimides, (4) anthrimidocarbazoles, (5) acylaminoanthraquinones, (6) anthraquinoneazoles, (7) anthraquiaone acridones, (8) anthrapyrimidines, and 9) highly condensed ring systems. Most currendy (1993) available dyes have been known for many decades, and very few new dyes have been commercialized since the 1970s. Recendy, research and development efforts have focused on improved manufacturing of traditional vat dyes. 

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

Let us focus attention on the unfavorable ring closures. Why, for example, should formation of a five-membered ring by an endo-trig process be difficult The answer is provided by a consideration of the trajectory of approach of the nucleophile." If Z is an electron-attracting conjugating group of the type necessary to activate the double bond to nucleophilic attack, the reaction would involve the LUMO of the conjugated system, a 7t ... 

This section will focus primarily on a comparison of these ring systems with their heavier chalcogen analogues. The first selenium derivative benzo-l,2,5-selenadiazole was prepared more than 115 years ago by the condensation reaction of selenium dioxide with 1,2-diaminobenzene (Eq. 11.12) and other benzo derivatives may be prepared in a similar manner. The parent 1,2,5-selenadiazole has also been reported. This reagent has been employed to make the tellurium analogue via treatment with ethylmagnesium bromide followed by the addition of tellurium tetrachloride (Eq. 11.13). ... 

The biogenetie origin of jS-earboline systems with extended rings has been repeatedly diseussed. Almost invariably tryptophan or tryptamine is assumed to give rise to the major portion of the j8-earboline nueleus. Attention in these discussions was focused on the elaboration of the non-carboline portion of these molecules. Discussion of these... 

Nitrogen-containing fulvalenes have not been systematically studied by mass spectroscopy. Only isolated data for several examples of compounds have been reported. Most of the data consist of electron impact (El) mass spectra recorded for analytical purposes. Only a minor fraction dealt with the characterization of ion structures or focused on the effects of substituents, the ring size of fulvalenes, or the number and arrangement of nitrogen atoms and the fragmentation pathways. 

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