Alternating induced polarities

Prevost and Kirrmann cite Ingold s experiments providing evidence of the inadequacy of Lapworth and Robinson s theory of alternate induced polarity in their 1931 memoir (1320). Their reference is to Ingold,... 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

As a result of these substituent-induced polarizations, the complementary conjugative interactions at each ring site become somewhat imbalanced (so that, e.g., the donor-acceptor interaction from C3—C4 to C5—C(, is 23.1 kcal mol-1, but that in the opposite direction is only 16.4 kcal mol-1). From the polarization pattern in (3.133) one can recognize that excess pi density is accumulated at the ortho (C2, C6) and para (C4) positions, and thus that the reactivity of these sites should increase with respect to electrophilic attack. This is in accord with the well-known o, /(-directing effect of amino substitution in electrophilic aromatic substitution reactions. Although the localized NBO analysis has been carried out for the specific Kckule structure of aniline shown in Fig. 3.40, it is easy to verify that exactly the same physical conclusions are drawn if one starts from the alternative Kekule structure. 

Material response is typically studied using either direct (constant) applied voltage (DC) or alternating applied voltage (AC). The AC response as a function of frequency is characteristic of a material. In the future, such electric spectra may be used as a product identification tool, much like IR spectroscopy. Factors such as current strength, duration of measurement, specimen shape, temperature, and applied pressure affect the electric responses of materials. The response may be delayed because of a number of factors including the interaction between polymer chains, the presence within the chain of specific molecular groupings, and effects related to interactions in the specific atoms themselves. A number of properties, such as relaxation time, power loss, dissipation factor, and power factor are measures of this lag. The movement of dipoles (related to the dipole polarization (P) within a polymer can be divided into two types an orientation polarization (P ) and a dislocation or induced polarization. 

The order in Problem 7.7(c) may occur because the valence electrons of a larger atom could be more available for bonding with the C, being further away from the nucleus and less firmly held. Alternatively, the greater ease of distortion of the valence shell (induced polarity) makes easier the approach of the larger atom to the C atom. This property is called polarizability. The larger, more polarizable species (e.g. I, Br, S, and P) exhibit enhanced nucleophilicity they are called soft bases. The smaller, more weakly polarizable bases (e.g. N, O, and F) have diminished nucleophilicity they are called hard bases. 

Figure 2.9. Optical anisotropy induced by the light in a film of the compound Alizarin Yellow GG-PEI kinetics of the parallel (dashed black line) and orthogonal (solid gray line) polarization components of the transmitted probe beam, (a) Induction and relaxation, solid black line presents two-exponential decay fit to the data, and (b) switching by alternation of polarization of the irradiating beam. Source Kulikovska et al., 2007. Reproduced with permission from American Chemical Society.

In the field-off state the macroscopic polarization of the antiferroelectric phase is zero. With increasing field, the induced polarization, at first, increases linearly with field and then, at a certain threshold, the antiferroelectric (AF) structure with alternating molecular tilt transforms in the ferroelectric one (F) with a uniform tilt, see Fig. 13.24a. Correspondingly, the macroscopic polarization jumps from a low value to the level of the local polarization Po [34]. The process is quite similar to that observed in crystalline antiferroelectrics. With a certain precaution we can speak about a field-induced AF-F non-equilibrium phase transition . The magnitude of the switched polarization in some antiferroelectric materials can be quite... 

An alternative possibility to increase the sensitivity of solid-state NMR for the study of catalytic processes is the application of spin-polarized para-hydrogen. The most prominent example of this technique is the so-called Para Hydrogen Induced Polarization or short PHIP experiment, originally invented for liquid-state NMR studies of homogeneous catalysis. PHIP is not restricted to homogeneous hydration reactions and liquid state NMR, but is also applicable to studies of heterogeneous hydration... 

Scheme III consists of two layer pairs with opposing fiber orientations and relative polarizations. To induce warping deformation and consequently beam twist, the relative polarization around the cross-section alternates from quadrant to quadrant. Similar results could be achieved just by using lengthwise oriented fibers with such an alternating relative polarization. But without adjustable fiber orientations, it would not be possible to adapt to given stiffness properties, especially with regard to torsion.

An interesting alternative method for formulating f/(jt) was proposed in 1929 by de Boer and Zwikker [80], who suggested that the adsorption of nonpolar molecules be explained by assuming that the polar adsorbent surface induces dipoles in the first adsorbed layer and that these in turn induce dipoles in the next layer, and so on. As shown in Section VI-8, this approach leads to... 

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