Uniaxial reorientations

With neutron scattering, it has been found that the rotational motion of benzene in NaX corresponds to a uniaxial reorientation about the C-6 axis, with jumps of 60°. The mean time between successive jumps about the C-6 axis at 458 K was found to be 1.3 X 10 s. 

Fig. 3. - Close-packed (111) plane of Cao molecules depicting uniaxial reorientation about (111) directions (phase II, (90 -s- 260) K). The dimples correspond to the centres of electron-poor hexagonal/pen-tagonal faces the equatorial protrusions depict electron-rich 6 6 bonds. The constraints demanded by Po3 symmetry imply that the electron-rich and electron-poor regions have well-defined loci corresponding to constant latitudes relative to [111]. These electrostatic considerations are consistent with the assumption that the preferred easy reorientation direction is [111].

Below 8 K (Ar) and 15 K (COj) uniaxial reorientation process and distortion of the nuclear configuration from symmetry contribute to the temperature dependence. ... 

The ordering of the transverse molecular axes, which occurs in certain low-temperature smectic phases, has been studied by NMR and NQR methods [7.40]. These measurements show that the uniaxial reorientation of the molecular cores around their long axes are strongly biased. It is generally assumed that in nematic and smectic A phases, the uniaxial rotation (7-motion) is not biased. However, recent neutron quasielastic scattering experiments [7.41] in the nematic phase of MBBA seem to support the notion that the rigid benzylideneaniline core is restricted to a uniaxial rotational diffusion of finite angular excursion. Restricted libration within 7 =z 00/2 for internal motions in macromolecules has been considered by London and Avitabile [7.42], and Wittebort and Szabo [7.43]. 

The non-collective motions include the rotational and translational self-diffusion of molecules as in normal liquids. Molecular reorientations under the influence of a potential of mean torque set up by the neighbours have been described by the small step rotational diffusion model.118 124 The roto-translational diffusion of molecules in uniaxial smectic phases has also been theoretically treated.125,126 This theory has only been tested by a spin relaxation study of a solute in a smectic phase.127 Translational self-diffusion (TD)29 is an intermolecular relaxation mechanism, and is important when proton is used to probe spin relaxation in LC. TD also enters indirectly in the treatment of spin relaxation by DF. Theories for TD in isotropic liquids and cubic solids128 130 have been extended to LC in the nematic (N),131 smectic A (SmA),132 and smectic B (SmB)133 phases. In addition to the overall motion of the molecule, internal bond rotations within the flexible chain(s) of a meso-genic molecule can also cause spin relaxation. The conformational transitions in the side chain are usually much faster than the rotational diffusive motion of the molecular core. 

The spin-reorientation transition in Er2Fei4B compound can be attributed to the competing of the uniaxial Fe sub lattice and the planar rare-earth sublattice anisotropy, with the former being dominant at higher temperatures and the latter being dominant at lower ones. 

Abstract. - High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerene Cm in the temperature range 5 K to 320 K. Solid Cm adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the high-temperature face-centred-cubic phase (T > 260 K), the Cm molecules are completely orientation-ally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single (111) direction. In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists. 

In Eq. (2), the argument of the cosine function, which depends on 8Ntr and the reorientation angles (aR, fiR, yR), can be considered as the phase acquired within a time of N/2tr under the action of the chemical-shift difference tensor coA = (a>2 - principal values of the difference tensor are WA22 = 0, and the full-width anisotropy coA33 - ouAl, of A, i.e. the range of possible frequency differences, is 20 56... 

As revealed distinctively by experiments on the photoinduced regulation of in-plane alignment by silica plates whose surfaces were modified with azobenzenes,148-151 the azimuthal reorientation of liquid crystals was achieved by the uniaxial conformational change of command molecules localized at the topmost substrate surface. This situation was confirmed for polymer films by the results obtained with the following systems. The first consisted of the chemical modification of PVA thin films. Careful treatment of PVA films with azobenzene acid... 

Cross-linked liquid crystalline polymers with the optical axis being macroscopically and uniformly aligned are called liquid single crystalline elastomers (LSCE). Without an external field cross-linking of linear liquid crystalline polymers result in macroscopically non-ordered polydomain samples with an isotropic director orientation. The networks behave like crystal powder with respect to their optical properties. Applying a uniaxial strain to the polydomain network causes a reorientation process and the director of liquid crystalline elastomers becomes macroscopically aligned by the mechanical deformation. The samples become optically transparent (Figure 9.7). This process, however, does not lead to a permanent orientation of the director. 

An unusual feature of the heneicosanol measurements was a second scattering peak, which appeared to arise at high pressure at temperatures below 16°C. It was attributed to a weakly first-order phase transition analogous to the rotator Il-to-rotator I transition in lamellar crystalline n-alkanes with n = 23,25. In the rotator II phase, rapid reorientation of the chain around its axis leads to a pseudohexagonal structure. When the chains can no longer reorient, the symmetry of the structure is reduced to a uniaxially distorted hexagonal strueture. 

Fig.4. Atomic configurations of the unit cell for uniaxial compression along b direction, leading to H-abstraction. (a) Original cell at F = 281.7 A. (b) Molecular reorientation and methyl rotation at F/F = 0.79. (c) C-H bond stretch by 12% at V/Vo = 0.60. (d) Configuration at F/Fo = 0.51. In (c) and (d) the abstracted protons are shown in larger circles.

Dynamical properties of the commensurate and uniaxial incommensurate phases according to the model of Refs. 232, 340, and 342 could also be explored by the molecular dynamics technique used [203, 352]. It is found that in-plane and out-of-plane motions can be analyzed separately for ori-entationally ordered N2 on graphite [203]. The 40-ps simulations below the orientational ordering transition (see Ref. 342) show [203] that the amplitude of reorientation is small and the out-of-plane motion nearly harmonic in both phases, whereas the in-plane motion is more complex, because it is anhar-monic and collective. The out-of-plane motion in the disordered phases is still harmonic, but more strongly damped, and the in-plane dynamics cannot be analyzed any more in terms of a cumulant expansion. Thus, there is little qualitative difference between the reorientational motion observed in the commensurate and uniaxially compressed solids. Only the out-of-plane motion is slightly less damped in the uniaxial phase, and the fluctuations from the planar configuration are more pronounced. 

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic... 

The magnetisation mechanism with the highest coercive force is domain rotation (or coherent rotation), in which all the spins within the sample are collectively reoriented in the field direction. In uniaxial materials. 

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