Electric polarization, dipole

High electric polarity (dipole moment) of substituent groups in the organics... 

Electric polarization, dipole moments and other related physical quantities, such as multipole moments and polarizabilities, constitute another group of both local and molecular descriptors, which can be defined either in terms of classical physics or quantum mechanics. They encode information about the charge distribution in molecules [Bbttcher et al, 1973]. They are particularly important in modelling solvation properties of compounds which depend on solute/solvent interactions and in fact are frequently used to represent the -> dipolarity/polarizability term in - linear solvation energy relationships. Moreover, they can be used to model the polar interactions which contribute to the determination of the -> lipophilicity of compounds. 

This energy depends on the interaction with the surroimding matter which results from electrical polarization, dipole orientation, van der Waals forees, and ehemical bond formation. Ligands strongly bound to a central ion or atom whieh remain connected with the redox species independent of its state of charge can be treated as a unit already existing in vacuum. For an illustration of the concepts, a polar liquid will be considered as the interacting medium. 

A ferroelectric crystal is one that has an electric dipole moment even in the absence of an external electric held. This arises because the centre of positive charge in the crystal does not coincide with the centre of negative charge. The phenomenon was discovered in 1920 by J. Valasek in Rochelle salt, which is the H-bonded hydrated d-tartrate NaKC4H406.4H 0. In such compounds the dielectric constant can rise to enormous values of lO or more due to presence of a stable permanent electric polarization. Before considering the effect further, it will be helpful to recall various dehnitions and SI units ... 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

Attractive or repulsive forces between molecular entities or groups within the same molecular entity (i.e., both intermolecular and intramolecular) not due to bond formation or to electrostatic interactions of ions or ionic groups with one another or with neutral molecules. The origin of van der Waals forces is in electric polarization of uncharged atoms, groups, or molecules and includes dipole-dipole interactions, dipole-induced dipole interactions, and London forces (induced dipole-induced dipole interactions). 

Polypeptides are electrically polar, carrying permanent dipoles at the planar CO-NH groups of the backbone chain and generally at some atomic groups of the side-chains. Because of the vector nature of dipoles, we must speak of the mean-square dipole moment, averaged over all possible conformations of the backbone chain and all accessible orientations of the side-chains when the dipolar nature of a polypeptide in solution is considered. The of a polypeptide thus may depend on what conformation the molecule assumes in a given solvent. 

See lext. XD = X-ray diffraction 1R = infrared spectrum R = Raman spectrum UV = ultraviolet spectrum H-NMR = ]HNMR spectrum C-NMR = 13CNMR spectrum F-NMR =, 9FNMR spectrum MS = mass spectrum PES — photoelectron spectrum E - electric polarization and dielectric loss measurements D = dipole moment measurements TDPAC = time differential perturbed angular correlation measurements GC = gas chromatography TA = thermal analysis M = molecular weight A = electrical conductance. c Isolated as the THF adduct M(dik)Cl3-C4HgO. 

Electrical polarization work The transfer of a quantity of dipole moment (extensive) through a difference in electric field strength (intensive). 

In a ferroelectric material, the dipole moments of molecules remain aligned in the absence of an external field. This alignment gives the material a permanent electric polarization. 

Most important is, however, the fact that Pr strongly depends on VF2 content. This is because the copolymers adopt the all-trans highly polar conformation (see Sect 3) and with increasing number of VF2 units the resulting dipole moment within each crystal increases. In fact, it has been shown that the electric polarization in these copolymers increases with the fraction of ferroelectric crystals in the material... 

The expectation value of the property A at the space-time point (r, t) depends in general on the perturbing force F at all earlier times t — t and at all other points r in the system. This dependence springs from the fact that it takes the system a certain time to respond to the perturbation that is, there can be a time lag between the imposition of the perturbation and the response of the system. The spatial dependence arises from the fact that if a force is applied at one point of the system it will induce certain properties at this point which will perturb other parts of the system. For example, when a molecule is excited by a weak field its dipole moment may change, thereby changing the electrical polarization at other points in the system. Another simple example of these nonlocal changes is that of a neutron which when introduced into a system produces a density fluctuation. This density fluctuation propagates to other points in the medium in the form of sound waves. 

Intermolecular forces, known collectively as van der Waals forces, are the attractions responsible for holding particles together in the liquid and solid phases. There are several kinds of intermolecular forces, all of which arise from electrical attractions Dipole-dipole forces occur between two polar molecules. London dispersion forces are characteristic of all molecules and result from the presence of temporary dipole moments caused by momentarily unsymmetrical electron distributions. A hydrogen bond is the attraction between a positively polarized hydrogen atom bonded to O, N, or F and a lone pair of electrons on an O, N, or F atom of another molecule. In addition, ion-dipole forces occur between an ion and a polar molecule. 

Movement of an electron from the ground electronic state of a molecule to an excited state creates a momentary dipole, called an electric transition dipole. Thus, associated with each electric transition is a polarization (electric transition dipole moment) that has both direction and intensity which vary according to the nature of the chromophore and the particular excitation. When two or more chromophores lie sufficiently close together, their electric transition dipoles may interact through dipole-dipole (or exciton) coupling. Exciton coupling arises from the interaction of two (or more) chromophores through... 

A simple electrostatic model facilitates understanding the physical meaning of the Curie behavior of [Mn(taa)] in the HS phase. Electric polarization P produced by a number of reorienting molecular dipoles /r under a local field Eloc obeys a simple Curie law,... 

Drude first proposed that the rotatory power of a dissymmetric substance could be understood if its absorption of light involved the motion of a charged particle along a helical path within the molecule [8]. This type of motion would result in the simultaneous production of an electric dipole from the translatory motion and a magnetic dipole from the rotatory motion. The model requires that the electric and dipole moments have at least some components which are collinear with each other, or else stereospecific interaction with circularly polarized light would not be possible. 

Figure 27). The associated electric transition dipole strengths are 2.7 xlO36 and 88 xlO36 (cgs units), respectively.[1] In composite systems, where two anthracenes are fused to bicyclo[2.2.2]octane (Figure 27), the intense long axis-polarized transition dipoles are... 

Material Media and Their Reaction to External Fields. In a material medium, a charge distribution can induce some charge separations, or dipoles, which help to minimize the total energy. Similarly, an external magnetic field will induce some magnetic dipoles in the medium to counteract this field. To handle these effects, an electric polarization (or electrical dipole moment per unit volume) P and a magnetization (or magnetic dipole moment per unit volume) M are defined. If the medium is linear and isotropic, these two new vectors P and M are proportional to E and to H, respectively ... 

Electric Dipole Moments. In many cases the (di)electric polarization P is proportional to the electric field strength E. The relation between the electric displacement D and the electric field strength E is given by... 

Solids can also be subdivided by their electrical polarization properties. The preponderant fraction of solids (crystalline or amorphous) are dielectric They have no net electrical polarization. If the individual components (molecules or clusters of ions) do have a net electric dipole moment, and these add nonlinearly, then one has electrets. There are also nanoferroelectrics. 

Refs. [i] Bottcher CJF (1973) Theory of electric polarization Dielectrics in static fields, vol. 1. (1978) Dielectrics in time-dependent fields, vol. 2. Elsevier, Amsterdam [ii] Tide DR (2003) Dipole Moments. In Tide DR (ed) CRC handbook of chemistry and physics, 84til edn. CRC Press, Boca Raton, pp 9-42 - 9-51 [Hi] Miller TM (2003) Atomic and molecular polarizabilities. In Tide DR (ed) CRC handbook of chemistry and physics, 84th edn. CRC Press, Boca Raton, pp 10-163 -10-177 [ivJFred-erikse HPR (2003) Polarizabilities of atoms and ions in solids. In LideDR (ed) CRC handbook of chemistry and physics, 84til edn. CRC Press, Boca Raton, pp 12-17 -12-18... 

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