Bridging elements

Another model assumes that gel zones are formed by hydrated lead dioxide (PbO(OH)2) and act as bridging elements between the crystallite particles. Electrons can move along the polymer chains of this gel and so cause electronic conductivity between the crystalline zones 137],... 

Particularly useful is the introduction of a sulphur atom as bridging element. One of the most classical examples, in which the sulphur atom introduces the necessary "rigidity" in order to ensure a complete acyclic diastereoselection of polyolefins, is the synthesis of the Cig-juvenile hormone [1] (Scheme 9.1). 

Regioselective control elements blocking and activating groups. Bridging elements... 

Metal atoms have also been used as bridging elements to fix the molecular... 

Fig. 5.40 Circuits for conductivity measurements with two-electrode cell (a) and four-electrode cell (b). In (a), S AC voltage source D detector I, II, III bridge elements. In (b), S constant-current source POT potentiometer Rs variable resistor C and C electrodes for current flow P and P electrodes for voltage measurement.

The corrole synthesis was established by Johnson s group.239 Their first attempt to cyclize tetrapyrrole with various bridging elements did not lead to corroies but to their 10-hetero analogues (Scheme 68). 

Tetraacetylenes such as 115 and 116 contain the 1,5-hexadiyne group as a bridging element. Since the base-catalyzed isomerization of this unit to hexa-l,3-dien-5-yne (6) constitutes the basic reaction of Sondheimer s annulene chemistry [75], it appeared attractive to attempt to apply this classic reaction of planar aromatic chemistry to a layered precursor and create three-dimensional relatives of Sondheimer s dehydroannulenes. Indeed, both 115 and 116 could be isomerized to their fully conjugated isomers 129 and 130, respectively, by treatment with potassium tert-butoxide in tert-butanol, the original Sondheimer conditions (Scheme 28). From the X-ray structure obtained for 130, it was concluded that both hydro-... 

Bridging with tri-functional reagents in 1,3,5-position at the wide, or at the narrow rim led to a more or less rigid skeleton. In such a series of 1,3,5-bridged calix[6]arenes 84 chirality is caused by the CTV moiety used as bridging element.157,158 The calix[6]cryptands 85a and 85b on the other hand are inherently chiral due to the 1,2,4-tri-substitution pattern on the narrow rim.159... 

Structures and substitution patterns of bridge elements in ID tt systems conjugation efficiency. The electronic requirements for optimum molecular second-order polarizabilities devised on the basis of the two-state model (see pp. 143 and 168) and the technical requirements to translate molecular properties into stable bulk materials partially coincide with the requirements for dyes in classical domains of application, e.g. in textile dyeing and colour... 

Scheme 7 Selection of donor and acceptor groups, and bridge elements.

The bridging element in all the compounds described up to now is carbon. Analogs in which the bridging element is a heteroatom are also known. 

The use of lead azide in one-ampere, no-fire detonator designs is shown in Figure 5. In these designs, the high-temperature stability of lead azide is needed if the detonators are not to fire with a 1-A current flow. To keep the azide below its initiation temperature, heat conduction paths are provided from a thin-film, resistive-bridge element (Figure 5b) through the electrical connections and the... 

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