Elimination conformation

A is called anti-periplanar, and this type of elimination, in which H and X depart in opposite directions, is called anti elimination. Conformation B is syn-periplanar, and this type of elimination, with H and X leaving in the same direction, is called syn elimination. Many examples of both kinds have been discovered. In the absence of special effects (discussed below) anti elimination is usually greatly favored over syn elimination, probably because A is a staggered conformation (p. 139) and the molecule requires less energy to reach this transition state than it does to reach the eclipsed transition state B. A few of the many known examples of predominant or exclusive anti elimination follow. 

As discussed earlier, SAMs of alkanethiols on gold surfaces offer access to highly ordered, surface-confined molecular structures. The use of these assemblies as stable, well-defined spacers for studies of electron transfer between the gold electrode and redox eenters attached to the film surface [50, 73-83] or in solution [44, 46, 84] is depicted in Figure 5. The densely packed monolayer maintains a precise separation between a gold electrode surface and the pendant or dissolved redox center, and effectively eliminates conformational mobility that can complicate electron-transfer rate studies [73]. An advantage of using electrode-confined spacers is that... 

In the rigid 2,3-dihalogenonorbornane system, the only possible eliminating conformations in the substrate are planar syn and anti-clinal. Rate studies on the dibromo and dichloro substrates , the use of deuterium tracers "... 

To eliminate conformational differences, electrophoresis Is carried out In the so-called denaturing media. The first such experiment tried formaldehyde as the denaturing agent. Other reagents, which have been used include 8M urea and formamide. The latter is more frequently used since it has been found to allow unequivocal determination of moleculeir weights with considerable accuracy. In this technique formamide serves as the medium for polymerization of the acrylamide. The supporting electrolyte may be sodium chloride without any buffer or barbital may be used. The formamide method has been successfully used for molecular weight determination of many RNA species. 

Overlap with predefined site points. As we discussed earlier, we may have experimental or computational data suggesting that certain functional groups belong in certain places. We can check each docked conformation to make sure that a minimum number of site points has been hit, or to eliminate conformers which contain mismatches. 

The dimensionality of a data set is the number of variables that are used to describe eac object. For example, a conformation of a cyclohexane ring might be described in terms c the six torsion angles in the ring. However, it is often found that there are significai correlations between these variables. Under such circumstances, a cluster analysis is ofte facilitated by reducing the dimensionality of a data set to eliminate these correlation Principal components analysis (PCA) is a commonly used method for reducing the dimensior ality of a data set. 

One limitation of clique detection is that it needs to be run repeatedly with differei reference conformations and the run-time scales with the number of conformations pt molecule. The maximum likelihood method [Bamum et al. 1996] eliminates the need for reference conformation, effectively enabling every conformation of every molecule to a< as the reference. Despite this, the algorithm scales linearly with the number of conformatior per molecule, so enabling a larger number of conformations (up to a few hundred) to b handled. In addition, the method scores each of the possible pharmacophores based upo the extent to which it fits the set of input molecules and an estimate of its rarity. It is nc required that every molecule has to be able to match every feature for the pharmacophor to be considered. 

Equatorial halide is gauche to axial and equatorial hydrogens on adjacent carbon cannot undergo anti elimination in this conformation... 

The stereoselectivity of elimination of 5 bromononane on treatment with potassium ethox ide was described in Section 5 14 Draw Newman projections or make molecular models of 5 bromononane showing the conformations that lead to cis 4 nonene and trans 4 nonene respec tively Identify the proton that is lost in each case and suggest a mechanistic explanation for the observed stereoselectivity... 

Conformal coatings are protective coatings appHed to circuit board assembHes. They protect the interconnect conductors, solder joints, components, and the board itself they reduce permeabiHty to moisture, hostile chemical vapors, and solvents in the coating. Use of conformal coatings eliminates dendritic growth between conductors, conductor bridging from moisture condensation, and reduction in insulation resistance by water absorption. 

Anions of small heterocyclics are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Hiickel rule, and render it aromatic if flat geometry can be attained. Examples are found in Chapter 5.20. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 5.02.7 below). 

For example, c/s-4-t-butyleyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the tram isomer because only the cis isomer permits anti elimination from the favored chair conformation. ... 

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