Dipolar instabilities

Dipolar instabilities of impurities in solids were discovered in 1965 by Lombardo and Pohl in Li-doped KCl [49]. Since then, a large amount of off-centre and on-centre instabilities of monoatomic impurities in insulator and semiconductor materials have been reported. In many cases the impurity centres are not well characterized and the observed instabilities could be due to close defects. In this article we only consider centres with spontaneous instabilities driven by PJT mechanisms. An exhaustive review of all these centres is beyond the scope of this report and we have selected representative examples based on the authors interest. Some early reviews can be found in [93,152,153]. 

Ferroelectric domains are those macroscopic (greater than 20 A) regions in a solid having natural crystal polarity which is constant, but whose direction of polarity can be altered irreversibly by a practically imposable electric field. Implied in this definition is the existence within the material of a spontaneous electrical dipole moment hence the existence of piezoelectricity, and the reversing effects of applied fields hence the existence of hysteresis, and of the probability that at some temperature before complete crystal disruption the spontaneous polarization will be destroyed or directionally randomized on an atomic scale. At this temperature the dipolar instability between the decision of order or disorder will usually produce a high peak value of incremental permittivity. 

Recently, Denmark and coworkers have developed a new strategy for the construction of complex molecules using tandem [4+2]/[3+2]cycloaddition of nitroalkenes.149 In the review by Denmark, the definition of tandem reaction is described and tandem cascade cycloadditions, tandem consecutive cycloadditions, and tandem sequential cycloadditions are also defined. The use of nitroalkenes as heterodienes leads to the development of a general, high-yielding, and stereoselective method for the synthesis of cyclic nitronates (see Section 5.2). These dipoles undergo 1,3-dipolar cycloadditions. However, synthetic applications of this process are rare in contrast to the functionally equivalent cycloadditions of nitrile oxides. This is due to the lack of general methods for the preparation of nitronates and their instability. Thus, as illustrated in Scheme 8.29, the potential for a tandem process is formulated in the combination of [4+2] cycloaddition of a donor dienophile with [3+2]cycload-... 

The azido group possesses all the qualities required to make it an excellent starting group for synthesis. It can be easily introduced into aliphatic, aromatic and heterocyclic nuclei, even in the presence of various other functional groups. The dipolar character and the relative instability of the azido group enable it to react in numerous... 

The instability of the iso- compounds comes from the weak O-N or S-N bond. These bonds can be cleaved by reducing agents, which then usually reduce the remaining functional groups further. The hrst product from reduction of the N-O bond is an unstable imino-enol. The enol tautomerizes to the ketone and the imine maybe reduced further to the amine. We used this sort of chemistry on the products of 1,3-dipolar cycloadditions in Chapter 35 and isoxazoles are usually formed by such reactions. 

Two-fold cyclodehydration of 2-benzhydryl-l,3-indanediol 127 under optimized conditions affords only low yields of the parent compound 128 [65] (Scheme 26). However, this centrotriindane can be isolated completely from the reaction mixture owing to its extremely low solubility. A major portion of the starting material reacts to the singly cyclized diindene 130 bearing the double bond at the diquinane junction. This isomer appears to be the most stable one, it was also obtained by heating diindanols 37 and 41 in dipolar medium. The instability of 129, the isomer expected to be formed from both 127 and from 37 and 41, is reminiscent of the behaviour of the elusive triindenes 109 and 110 (Scheme 22). 

Acid fragment 3 was rapidly prepared by 1,3 dipolar cycloaddition of the requisite cinnamate 17 with tert-butyl azomethine ylide 16. However, the synthesis of the azomethine ylide 15 required rather harsh conditions in the alkylation step and suffered low yield in the methoxymethylation due to its inherent instability. This provided a racemic mixture of 3,4-tranY-disubstimted pyrrolidine... 

In the case of very strong dipolar couplings ( H- H) the experiment is performed using fast MAS rates (ca. 30 kHz) so that spectra with sufficient resolution can be recorded. The performance of this pulse sequence is very sensitive to the instabilities of MAS. The rotor synchronised 2D DQ NMR spectrum (the increment in A set to one rotor period) provides information on the spatial proximity of different protons based on the presence of peaks in the coz dimension. If the spins A (va) and B (vb) are close in space, the peaks at ((va + vb), va) and ((va + i b). i b) will appear in the 2Q spectrum (Figure 45). This provides semi-quantitative... 

In solution, benzyne and its derivatives are strained, short-lived intermediates which react with a broad array of nucleophiles, including carbanions, amines, alcohols and their salts, water, and even ethers. They also undergo pericyclic reactions such as Diels-Alder cycloadditions, 1,3-dipolar cycloadditions or ene reactions. The instability of benzyne and its derivatives is due to the strain caused by the deformation of the linear geometry of the formal sp-hybridized carbons of the triple bond to the angles close to 120° imposed by the ring geometry [2,3]. 

The normal hydrogen electrode (NHE) is the primary reference electrode and is used to define the accepted scale of standard potentials in aqueous media. It is also one of the most reproducible electrodes that are available. The hydrogen electrode has been successfully employed in dipolar aprotic solvents however, it is not frequently used. The aqueous saturated calomel electrode (SCE), connected to the electrolyte under study by a non-aqueous salt bridge, has become the reference electrode of choice for most investigators. Whether it is the SCE that is used, or any other suitable reference electrode for a given solvent, junction potentials will exist between the reference electrode and the electrolyte under study. These junction potentials will affect electrode potential measurements and will vary from one solvent/electrolyte system to another. In addition, the instability of the SCE in non-aqueous solvents has been noted. ... 

This chapter is concerned with experimental measurements of flexo-electric coefficients. After a brief introduction to flexoelectricity in nematic liquid crystaJs, some applications exploiting the flexoelectric effect and the influence of this effect on electrohydrodynamic instabilities are pointed out. Flexoelectricity axises in samples with a splay-bend distortion in the director field and as such its measurement is not as direct as for dielectric constants. The theoretical background needed to analyse electro-optic experiments and extract the flexocoefficients is outlined in Section 2.2. Various experimental techniques that have been developed are described in Section 2.3. These involve cells in which the alignment of the nematic director is homeotropic, or planar or hybrid. In the first case, the interdigitated electrode technique is particularly noteworthy, as it has been used to establish several features of flexoelectricity (1) the effect can arise purely from the quadrupolar nature of the medium, and (2) the dipolar contribution relaxes at a relatively low frequency. 

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