Bend distortion

Defect-stabilised mesophase created when a smectic A mesophase is subjected to a twist or bend distortion. 

Note 1 The twist and bend distortions can be stabilised by an array of screw or edge dislocations. 

The results are consistent with the predictions of the ligand field theory and with the observed photochemical reactions. Bond bending distortions are observed in metal-nitrosyl compounds. The photochemical reactions of CotCCO NO and the photohydrogenation catalyzed by RhCPPhjJ NO provide indirect support for the bending. 

Excited state Raman spectroscopy of Fe(CN)5N02-provides direct support for the bending distortion. 

Excited State Bending Distortions of the MNP Group and Their Photochemical and Spectroscopic Consequences... 

FIGURE 6.22 Generation of a conical intersection (CI) by crossing of the twin states along the bending distortion mode, (a) in H3 and (b) in SN2 systems. Symmetry labels refer to the mirror plan, m. 

The three elastic constants are the Frank elastic constants, called after Frank, who introduced them already in 1958. They originate from the deformation of the director field as shown in Fig. 15.52. A continuous small deformation of an oriented material can be distinguished into three basis distortions splay, twist and bend distortions They are required to describe the resistance offered by the nematic phase to orientational distortions. As an example, values for Miesowicz viscosities and Frank elastic constants are presented in Table 15.10. It should be mentioned that those material constants are not known for many LCs and LCPs. Nevertheless, they have to be substituted in specific rheological constitutive equations in order to describe the rheological peculiarities of LCPs. Accordingly, the viscosity and the dynamic moduli will be functions of the Miesowicz viscosities and/or the Frank elastic constants. Several theories have been presented that are more or less able to explain the rheological peculiarities. Well-known are the Leslie-Ericksen theory and the Larson-Doi theory. It is far beyond the scope of this book to go into detail of these theories. The reader is referred to, e.g. Aciemo and Collyer (General References, 1996). 

Fig. 19. Changes in frontier orbital energies upon bending distortions of acetylene and ethylene (4-31G)...

A simple rule for the occurrence of trans-h A distorted structures 2 at homopolar double bonds was derived from an elementary molecular orbital model treating a-jt mixing and a valence bond treatment . The relation between the singlet-triplet separation (A st) of the constituent ER2 and a +jt bond energy Ea+ was used as a criterion for determining the expected structure of R2E=ER2. The trans-b ai geometry 2 occurs when l/AEa+ < A sT < l/2Ea+n- The first part of the inequality determines the irawi-bending distortion of the double bond, while the second part determines the existence of a direct E=E link. 

There is a considerable transfer of 7t-charge to the sulfur in the planar form, and the NH2 resists out-of-plane bending distortions that would decrease the interaction in the dipolar canonical structure. Thus, the traditional picture of amide resonance is more appropriate for thioamides than for amides. 

A great deal of effort has gone into determining the factors that influence the geometries of diatomic and related polyatomic molecules when coordinated to transition metal centers. A bending distortion changes the coordination mode from linear (19) to bent (20) and, further, through a kinked conformation (21) to side-on rj -coordination (22). The process culminates in oxidative addition in which the X—Y bond is broken (23). 

As before, we shall begin by considering a planar sample in which the director is confined to the xy plane. In such a case, a wedge disclination involves only splay and bend distortions and we need to take into account only the splay-bend anisotropy (kjj + fejj). 

Fig. 3.13.3. A hybrid aligned cell for the determination of the anisotropy of the flexoelectric coefficients. In this geometry, the director has a splay-bend distortion which gives rise to a flexoelectric polarization P. On applying an electric field E, the director is twisted by an angle (j> cc — which can be measured optically.

In other words, both twist and bend distortions are absent, leaving only the splay term in the Oseen-Frank free energy expression (3.3.7). It is seen from fig. 5.3.1, that by merely bending or corrugating the layers a splay deformation can be readily achieved without affecting the layer thickness. 

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