Orientation influence, interacting molecules

The Influence of the Number and Relative Orientations of the Interacting Molecules... 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

The problem of influence of the electric field intensity on the permittivity of solvents has been discussed in many papers. The high permittivity of water results from the intermolecular forces and is a cumulative property. The electric field intensity is the lowest at the potential of zero charge (pzc), thus allowing water molecules to adsorb in clusters. When the electrode is polarized, the associated molecules, linked with hydrogen bonds, can dissociate due to a change in the energy of their interaction with the electrode. Moreover, the orientation of water molecules may also change when the potential is switched from one side of the pzc to the otha. 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

A dipole-dipole interaction, or Keesom force, is analogous to the interaction between two magnets. For non-hydrogen bonding molecules with fixed dipoles, these interactions are likely to influence the orientation of the molecules in the crystal. This is because, unlike the Debye force which is always attractive, the interaction between two dipoles is only attractive if the dipoles are properly oriented with respect to one another, as is the case with magnets. 

Whenever two phases come in contact with one another, an interfacial region forms within which physical and chemical characteristics of each phase are disturbed relative to interior (bulk) regions of each phase. At the air-water interface, for example, the directional orientation of water molecules is more pionounced than in bulk solution, in order to compensate for the lack of hydiogen-bonding partners on the gas-phase side of the interface. As a consequence, the dielectric constant and other solvent characteristics that influence liemical reactions are perturbed to some degree. Solute molecules added to mi water or solvent-water systems may reside predominately in one phase or the other, or may concentrate in the interfacial region. Whether or not solute molecules are surface-active depends on the relative energies of possible dilute solute, solute-solvent, and solvent-solvent interactions (Tanford, 1980). 

C-NMR spectroscopy is also an appropriate method to study solvent effects. A charge-transfer interaction, for example, induces a down-field shift of the C-signal by changing the electronic environment between C50 and solvent Yet an easy prediction of signal positions is not possible because other effects like the size, shape, and orientation of solvent molecules also influence the chemical shift. 

Molecular polarisability is discussed in detail in chapter 9. Orientational polarisation involves the rotation, under the influence of an electric field, of molecules, or parts of molecules, that have permanent electric dipoles. This effect clearly leads to a possible mechanism for orienting molecules and, in particular, for reorienting the domains of liquid crystals or LCPs that have permanent dipoles. It is less obvious that deformational polarisation can lead to orientation. If, however, the polarisability of a molecule is anisotropic, as is usually the case, the polarisability and hence the induced electric dipole will differ for different orientations of the molecule in the field. The interaction energy with the field is therefore reduced if the molecule rotates so that its direction of maximum polarisability coincides with the field direction. Electric-field orientation is largely of importance in device applications. 

As mentioned in introduction, the competition of molecular motion and deformation applied by the solvent evaporation determines the molecular orientation in a solution-cast film. In other words, CTA chains orient in the film plane by applied uniaxial compression deformation due to the solvent evaporation. At the same time, TCP molecules orient accompanied with the CTA chains. Therefore, the orientation relaxation of TCP molecules will be affected strongly by the existence of a solvent, although the relaxation time is reduced for both CTA and TCP. The experimental results indicate that the nematic interaction, i.e., orientation of TCP molecules, occurs only at the final stage of evaporation. Therefore, a solvent that retards the evaporation rate at the final stage to obtain smooth surface will have a strong influence on the orientation of additives. 

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