Dipole vector reorientation

Rose and Benjamin studied the water dipole and the water H-H vector reorientation dynamics at the water/Pt( 100) interface and the results are reproduced in Fig. 4. As in the case of the translational diffusion, the effect of the surface is to significantly slow down the adsorbed water layer. We note that the effect is very short range, and that the rotational motion of water molecules in the second layer is already very close to the one in bulk water. 

Dipolar ions like CN and OH can be incorporated into solids like NaCl and KCl. Several small dopant ions like Cu and Li ions get stabilized in off-centre positions (slightly away from the lattice positions) in host lattices like KCl, giving rise to dipoles. These dipoles, which are present in the field of the crystal potential, are both polarizable and orientable in an external field, hence the name paraelectric impurities. Molecular ions like SJ, SeJ, Nf and O J can also be incorporated into alkali halides. Their optical spectra and relaxation behaviour are of diagnostic value in studying the host lattices. These impurities are characterized by an electric dipole vector and an elastic dipole tensor. The dipole moments and the orientation direction of a variety of paraelectric impurities have been studied in recent years. The reorientation movements may be classical or involve quantum-mechanical tunnelling. 

An important signature of the dynamics of water molecules is the reorientation of its dipole vector that can be probed by dielectric and NMR measurements. We have calculated the single molecule dipole-dipole time correlation function (TCF), defined as,... 

Geometrical scheme of dipole s reorientations is given in Fig. 5. Because of perfect reflections of a dipole-moment vector p from conical walls, a surface covered by this vector consists of identical sectors with a central angle 2a. The sectors are inclined at the angle n/2 — 0) to the cone axis C, where 0 is the angle between this axis and the direction n of the angular-velocity vector of... 

It is of interest to examine the nature of the motions of a sidechain as large as a tryptophan residue in the protein interior. Reorientation of the transition dipole vector of tryptophan is due mainly to rotation about the C —C(3 and C,3—Cr bonds, which correspond to the x1 and x2 dihedral angles of the side-chain, although larger-scale collective motions of the backbone are also involved. In the protein environment, the fluctuations of x1 and x2 are expected to be anticorrelated so that large variations in the two angles result in a... 

The reorientational dynamics of water molecules has been analyzed by calculating the relaxation times of the molecular dipole vector relative to the laboratory fixed... 

In addition to the direct analogs of the nonpolarizable-particle expressions considered above, Wertheim has also derived a family of expressions involving as well as e, where is the high-frequency dielectric constant that corresponds to e computed in a static system in which molecular reorientation is suppressed. In the model under consideration, the relevant aspect of such reorientation is reorientation of the permanent dipole vector, so the computation of effectively reduces to the computation of e in the absence of the permanent dipole moment the expressions involving then follow trivially from the expressions above by subtraction. For example, corresponding to (4.21), there is the expression... 

A survey of the data available on the motion of polymers has enabled certain characteristic types of behaviour to be identified. A polymer being a long chain-like structure may be expected in the molecular weight. The effects of increase in the chain length of a polymer have been illustrated by dielectric relaxation (6/, At low molecular weight the relaxation which reflects the rate of reorientation of a dipole vector within the polymer will be approximately proportional to the chain length (7),... 

Given the specific, internuclear dipole-dipole contribution terms, p,y, or the cross-relaxation terms, determined by the methods just described, internuclear distances, r , can be calculated according to Eq. 30, assuming isotropic motion in the extreme narrowing region. The values for T<.(y) can be readily estimated from carbon-13 or deuterium spin-lattice relaxation-times. For most organic molecules in solution, carbon-13 / , values conveniently provide the motional information necessary, and, hence, the type of relaxation model to be used, for a pertinent description of molecular reorientations. A prerequisite to this treatment is the assumption that interproton vectors and C- H vectors are characterized by the same rotational correlation-time. For rotational isotropic motion, internuclear distances can be compared according to... 

Since P must remain normal to z and n, the polarization vector forms a helix, where P is everywhere normal to the helix axis. While locally a macroscopic dipole is present, globally this polarization averages to zero due to the presence of the SmC helix. Such a structure is sometimes termed a helical antiferroelectric. But, even with a helix of infinite pitch (i.e., no helix), which can happen in the SmC phase, bulk samples of SmC material still are not ferroelectric. A ferroelectric material must possess at least two degenerate states, or orientations of the polarization, which exist in distinct free-energy wells, and which can be interconverted by application of an electric field. In the case of a bulk SmC material with infinite pitch, all orientations of the director on the tilt cone are degenerate. In this case the polarization would simply line up parallel to an applied field oriented along any axis in the smectic layer plane, with no wells or barriers (and no hysteresis) associated with the reorientation of the polarization. While interesting, such behavior is not that of a true ferroelectric. 

Figure 9. Proposed allowed equilibrium conformational states for poly (a-olefin sulfones) in solution. Note that the sulfone dipoles cancel and that during the transitions ttt g tg g tg there is no net reorientation of these dipoles (dielectrically inactive motions), but there is a reorientation of backbone C-H vectors (C-13 NMR active motions).

Photoreorientation of azobenzene chromophores by irradiation with polarized light is a very important photoinduced structural change. For azobenzene moieties, there is a widely accepted mechanism for the photo-reorientation The azobenzene moieties that are parallel with their long axis (and therefore with their transition dipole) to the electric field vector... 

The relationship between the structure of a polymer chain and it dynamics has long been a focus for work in polymer science. It is on the local level that the dynamics of a polymer chain are most directly linked to the monomer structure. The techniques of time-resolved optical spectroscopy provide a uniquely detailed picture of local segmental motions. This is accomplished through the direct observation of the time dependence of the orientation autocorrelation function of a bond in the polymer chain. Optical techniques include fluorescence anisotropy decay experiments (J ) and transient absorption measurements(7 ). A common feature of these methods is the use of polymer chains with chromophore labels attached. The transition dipole of the attached chromophore defines the vector whose reorientation is observed in the experiment. A common labeling scheme is to bond the chromophore into the polymer chain such that the transition dipole is rigidly affixed either para 1 lei (1-7) or perpendicular(8,9) to the chain backbone. 

The first process prevails at relatively low frequencies. The electric component E of radiation orients dipole moments p along the field direction, while chaotic molecular motions hinder this orientation p and E are the vectors, and the field E is assumed to vary harmonically with time t. Due to inertia of reorienting molecules the time dependence of the polarization lags behind the time dependence E(f), so that heating of the medium occurs (the heating effect is not considered in this work). The dielectric spectrum obeys the Debye relaxation, for which the absorption monotonically increases with frequency. 

Distances in solution at physiological temperatures can at least be estimated under conditions where the reorientation rate of the spin-spin-vector is reduced by other mechanisms, e.g., embedding the proteins in membranes or upon addition of viscosity agents [79]. In this case the dipole-dipole interaction is partly averaged out, making accurate distance measurement difficult. Quantitative... 

The applied electric field interacts with the instantaneous dipole moments that may arise from uneven charge distribution due to the fluctuating shape of the individual microemulsion droplets. The ensuing reorientation of an instantaneous dipole by the field may involve the rotation of a droplet as a whole or a peristaltic type of rotation in which the instantaneous dipole moment vector rotates in (and the associated structural distortion propagates as a wave on the surface of) the droplet, which itself is either stationary or may also rotate but not necessarily in a correlated fashion. A peristaltic rotation of the instantaneous dipole may be the only mode of rotation on the observed time scale for droplets locked in clusters. If any of these descriptions applies, the time constant of the associated rise of the birefringence (t/) may be referred to as rotational relaxation time. 

In the previous sections the expressions for the admittance of materials were developed on the assumption that they had no dc conductivity. The real part of the admittance arose from the dissipative process of dipole reorientation. Energy was absorbed by the system when the orientation of dipoles was changed with respect to the electric field vector. 

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