Dipoles parallel orientation

When A = B, the expression at Eq. (8) represents the usual interaction energy of permanent dipoles. When A < B, parallel orientations of dipoles are favoured and when A > B, the interaction energy has a minimum for antiparallel dipoles. 

The geometrically optimized model of BD had a roughly linear conformation. This spontaneous ordering was unexpected given the general orientation of dipolar molecules. Azobenzenes that have permanent dipoles parallel to the molecular axis would intuitively be expected to tend to pair with their dipole oriented in the opposite direction. The linear geometry is probably due to the... 

There is also the normal dipole selection rule in operation, as illustrated in Figure 5.48, due to Liith (1981). Any dipole at a surface induces an image charge within the surface. If the dipole orientation is normal to the surface, the effect is enhanced by the image dipole. If, however, the orientation is parallel to the surface, the effect is annihilated by the image dipole. This orientation selection rule thus strongly favours normally oriented dipoles. 

In a microscope, standard polarized epi-illumination cannot distinguish order from disorder in the polar direction (defined as the optical axis) because epi-illumination is polarized transverse to the optical axis and observation is along the optical axis at 180°. However, microscope TIR illumination can be partially polarized in the optical axis direction (the z-direction of Section 7.2) and can thereby detect order in the polar angle direction. Timbs and Thompson(102) used this feature to confirm that the popular lipid probe 3,3 -dioctadecylindocarbocyanine (dil) resides in a supported lipid monolayer with its dipoles parallel to the membrane surface, but labeled antibodies bound to the membrane exhibit totally random orientations. 

Without the external field, the Stockmayer fluid near the wall exhibits symmetric density oscillations that die out as they reach the middle of the film. Near the surface, the fluid dipoles are oriented parallel to the walls. Upon turning on the electric field, the density profile of the Stockmayer fluid exhibits pronounced oscillations throughout the film. The amplitude of these oscillations increases with increasing field strength until a saturation point is reached at which all the fluid dipoles are oriented parallel to the field (perpendicular to the walls). The density profile remains symmetric. The dipole-dipole correlation function and its transverse [] and longitudinal [] com-... 

Near one metal electrode the potential for a dipole oriented normal to the interface tends to add with that of its image in the metal electrode. The potentials of the dipole and its image tend to cancel for a dipole oriented parallel to the interface. The addition or cancellation of the dipole potential with its image makes it more probable for an inelastic transition to occur for a dipole oriented normal to the interface. Near the center of a tunneling junction, however, the reverse is true for two reasons. 1) The cancellation of potentials for the dipole oriented parallel is least important at the center. 2) The inelastic transition matrix element involves an integration over the barrier volume of the interaction potential times some nearly spatially homogeneous electronic wavefunction terms. If the dipole is oriented normal to the interfaces and located in the center of the barrier, the potential is an odd function in z ( if z defines the normal to the interfaces ), and integrates to... 

The IETS intensities for the methyl group vibrations of this species are shown in Fig. 9. The theoretical predictions of Kirtley and Hall (34) using KSH, and taking methyl group dipole derivatives from infrared measurements of ethane, assuming the C-S bond normal vs parallel to the interface, are also shown in Fig. 9. Note that for an orientation with the C-S bond normal, the symmetric C-H modes ( 2 and 9 ), which have net dipoles parallel to the C-S bond, are favored over the anti-symmetric modes ( 4,7, and 11), which have net dipole moments perpendicular to the C-S bond, but that for the C-S bond parallel to the surface the situation is reversed. The better, although by no means perfect, agreement between theory and experiment for the C-S bond normal tends to support the proposed orientation of Hall and Hansma. 

Parallel oriented dipoles lower the value of Tc as does a decrease in the heat of adsorption caused by a heterogeneous surface. 

In Secs. VIII,2 and 3 we saw that two phenomena may cause a decrease of the heat of physical adsorption of gases on metals or on ionic adsorbents first the mutual repulsion of parallel oriented dipoles and, second, a heterogeneous character of the surface. 

The radical path is most consistent with the existing experimental data. A correlation of half-wave potentials with Taft polar (a ) and steric (Es) constants 478)indicates that a parallel orientation of the C-X dipole with regard to the electrode suiface is most favourable for an electron transfer to the antibonding o orbital of the C-X bond. Thereby a radical anion 162) is formed 471 which rapidly dissociates to halide ion and radical. The more facile reduction of exo-2-... 

Fluorophores with dipoles perpendicular to excitation light will not absorb. Fluorophores with dipoles parallel to excitation light will absorb the most. Thus, polarized excitation will induce photoselection in the fluorophore absorption. The electric vector of excitation light is oriented parallel to or in the same direction as the z-axis. Emitted light will be measured with a polarizer. When emission is parallel to the excitation, the measured intensity is called I. When the emission is perpendicular to the excitation light, the measured intensity is called 11 - Light polarization and anisotropy are obtained according to Equations (11.1) and (11.2) ... 

The distance that separates the two molecules goes from 10 to 60-100 A. Below 10 A, electron transfer may occur between the two molecules, inducing an energy transfer from donor to acceptor, k2 values hold from 0 to 4. For aligned and parallel transition dipoles (maximal energy transfer) k2 is 4, and if the dipoles are oriented perpendicular to each other (very weak energy transfer), k2 is 0. When k2 is not known, its value is considered to be equal to 2/3. This value corresponds to a random relative orientation of the dipoles. 

Further insight into the charge transfer features was recently obtained44 through single-crystal polarized optical studies of plastocyanin. In these experiments, absorption spectra were obtained (Fig. 14) with polarized light which had its electric dipole direction oriented parallel and perpendicular to the crystal a axis (Fig. 15). These spectra were... 

On metal surfaces, two additional selection mles apply. The first is that only vibrations perpendicular to the surface are HREELS active. This mle follows from two phenomena unique at metal surfaces " (i) Electromagnetic waves polarized perpendicularly to the plane of incidence (parallel to the plane of the surface) undergo a 180° phase shift upon reflection. That is, at the metal surface, the out-of-phase electric-field vectors of the incident and reflected waves cancel each other as a result, no field exists that can couple with dipoles that oscillate parallel to the surface, (ii) The dynamic dipole moment generated by an oscillator that vibrates in the surface-parallel direction is cancelled by that of its image dipole (Figure 1) hence, there the net dynamic dipole moment is zero. On the other hand, if the real dipole is oriented perpendicularly to the surface, its dynamic dipole moment is reinforced by that of its image dipole. This selection mle is the same as that for infrared reflection-absorption spectroscopy (1RAS).°... 

Both processes (86) and (87) apparently involve formation or destruction of the zwitterion dipole moment Therefore, application of an electric field must di lace the respective chemical equilibrium to some extent depending on the an e 0 between the directions of the zwitterion dipole and the field E. For 0 < 90° an increase of the number of zwitterions is favoured whereas for 0 > 90° this number will be decreased. Therefore, the whole system tends to develop preferential orientation of dipoles parallel to the field. Proton transfer of the kind involved here has generally been proved to be practically difiiision controlled so that the reaction rates could be extremely high. The chemical mechanism of dielectric polarization may thus be fast enough in comparison with the rotational difiusion of the zwitterion, especially if the latter is a macromolecule (e.g . a protein). 

Because the orientations and spatial locations of the two chromophores may vary over the ensemble of molecules, and because they can also change during the time the donor is in the excited state, the measured effect of is usually an average over the appropriate spatial/temporal distributions. Whenever Eo and the acceptor dipole moment pA have parallel orientations, the rate of FRET is maximum for that placement in space for the acceptor relative to the donor. For pA oriented parallel to Ed, the possible maximum values of are between 1 and 4 that is, the actual maximum value depends on the value of 0 ) (see below). The minimum value of is zero for every position of the acceptor relative to the donor whenever Ed and the acceptor dipole moment pA are oriented perpendicular to each other. 

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