Dipole antiparallel

Granicher (1957), which considered only the majority carriers in each region. It is somewhat more difficult to picture the polarization processes in terms of molecular orientations. In the regions where orientational defects provide the relaxation mechanism, their motion in the applied field simply reorients molecules or, if the effective charge associated with the defects is included, then this contributes a polarization in the same direction. In regions where ion states provide the relaxation mechanism, however, motion of these states in the direction of the field tends to orient molecular dipoles antiparallel to the field. The polarizations pro-... 

Parallel bond dipoles Antiparallel bond dipoles more repulsive less repulsive... 

The nematic and smectic mesophases are normally dipolar inasmuch as molecules with terminal dipole moments will organize with the dipoles antiparallel. In some liquid crystals, of which the cyanobiphenyls (Figure 4) are the archetypal example [1], such antiparallel interactions are crucial, because in these materials, it is the antiparallel... 

A study of the effect of substitution patterns in oxadiazoles and isoxazoles and their effect on the UV spectra in the lO -lO M concentration range was performed. Hypso-chromic effects and deviations from Beer s law were observed and were believed to be associated with antiparallel, sandwich-type self-association via dipole-dipole interactions. Beer s law is followed when the molecular dipole moments are small or when self-association is sterically hindered. 

Figure 5.3. The classical picture the energies of dipoles varies continuously from parallel alignment with the applied magnetic field (-m-B) to antiparallel (+m-B). On the right is shown the distribution of molecules that results and the lower mean energy of the ensemble relative to the field-free environment.

The frustration effects are implicit in many physical systems, as different as spin glass magnets, adsorbed monomolecular films and liquid crystals [32, 54, 55], In the case of polar mesogens the dipolar frustrations may be modelled by a spin system on a triangular lattice (Fig, 5), The corresponding Hamiltonian consists of a two particle dipolar potential that has competing parallel dipole and antiparallel dipole interactions [321, The system is analyzed in terms of dimers and trimers of dipoles. When the dipolar forces between two of them cancel, the third dipole experiences no overall interaction. It is free to permeate out of the layer, thus frustrating smectic order. 

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 parallel or antiparallel orientations of dipoles to each other may form certain angles 9 or 9k with the Ox axis of the lattice, which is drawn along the chains with the smallest intersite distances46 (ai a2 < aj - a2 < ai + a2 ). The energies of these ground states can also be conveniently calculated in the chain representation with somewhat more awkward46 expressions than Eqs. (2.2.10) and (2.2.11). 

According to Eq. (2.23), the magnitude of the ion-dipole interaction is maximized in collinear arrangements ( cos 0 = 1), and is attractive or repulsive according to whether the oppositely charged pole of the dipole is aligned toward or away from the ion. Similarly, the dipole-dipole attraction (2.25) is maximized when the dipoles Pi, and are parallel (attraction) or antiparallel (repulsion). 

It may be noted that simple MOPAC AMI calculations suggest that the dipole moment of NOBOW is oriented antiparallel to the molecular arrow. As indicated in Figure 8.25, this means that for an up field, the molecular arrows are pointing down. Given the definition of the sign of P in FLCs, this also means that domains of the ShiCaPa phase with positive chirality have negative ferroelectric polarization, and vice versa. 

Antiparallel dipole ordering to produce an antiferroelectric crystal is also commonly encountered. Other ways of ordering electric dipoles are not so well characterized, but parallels with the situation in magnetic materials occur. 

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