Polarisation effects

In the case of symmetrical molecules such as carbon tetrachloride, benzene, polyethylene and polyisobutylene the only polarisation effect is electronic and such materials have low dielectric constants. Since electronic polarisation may be assumed to be instantaneous, the influence of frequency and temperature will be very small. Furthermore, since the charge displacement is able to remain in phase with the alternating field there are negligible power losses. 

There is an important practical distinction between electronic and dipole polarisation whereas the former involves only movement of electrons the latter entails movement of part of or even the whole of the molecule. Molecular movements take a finite time and complete orientation as induced by an alternating current may or may not be possible depending on the frequency of the change of direction of the electric field. Thus at zero frequency the dielectric constant will be at a maximum and this will remain approximately constant until the dipole orientation time is of the same order as the reciprocal of the frequency. Dipole movement will now be limited and the dipole polarisation effect and the dielectric constant will be reduced. As the frequency further increases, the dipole polarisation effect will tend to zero and the dielectric constant will tend to be dependent only on the electronic polarisation Figure 6.3). Where there are two dipole species differing in ease of orientation there will be two points of inflection in the dielectric constant-frequency curve. 

When dipoles are directly attached to the chain their movement will obviously depend on the ability of chain segments to move. Thus the dipole polarisation effect will be much less below the glass transition temperature, than above it Figure 6.4). For this reason unplasticised PVC, poly(ethylene terephthalate) and the bis-phenol A polycarbonates are better high-frequency insulators at room temperature, which is below the glass temperature of each of these polymers, than would be expected in polymers of similar polarity but with the polar groups in the side chains. 

Because of a small dipole polarisation effect the dielectric constant is somewhat higher than that for PTFE and the polyolefins but lower than those of polar polymers such as the phenolic resins. The dielectric constant is almost... 

An interesting point concerns polarisation effects in the Raman spectra, which are commonly observed in low-dimensional materials. Since CNTs are onedimensional (ID) materials, the use of light polarised parallel or perpendicular to the tube axis will give information about the low dimensionality of the CNTs. The availability of purified samples of aligned CNTs would allow us to obtain the symmetry of a mode directly from the measured Raman intensity by changing the experimental geometry, such as the polarisation of the light and the sample orientation, as discussed in this chapter. 

From Fig. 10.40 it will be seen that contact between the electrolyte (soil or water) and the copper-rod electrode is by porous plug. The crystals of CUSO4 maintain the copper ion activity at a constant value should the halfcell become polarised during measurements. The temperature coefficient of such a cell is extremely low, being of the order of 1 x 10" V/°C and can thus be ignored for all practical purposes. To avoid errors due to polarisation effects, it is necessary to restrict the current density on the copper rod to a... 

Finally, there are groups of liquid crystals where, at the current time, force fields are not particularly useful. These include most metal-containing liquid crystals. Some attempts have been made to generalise traditional force fields to allow them to cover more of the periodic table [40, 43]. However, many of these attempts are simple extensions of the force fields used for simple organic systems, and do not attempt to take into account the additional strong polarisation effects that occur in many metal-containing liquid crystals, and which strongly influence both molecular structure and intermolecular interactions. 

Co valency and polarisation effects compHcate calculations to be made on these problems. The polarisation effects that influence the crystal structures may be estimated more readily, as they depend on size and charge of ions and are easy detectable. Thus the increase in Me—F-distances, which always occurs if MeFe-octahedra are linked together, is a consequence of polarisation and contrapolarisation that cancel out in the shared fluoride ions. An instructive example of two sorts of Me—F-distances which may be explained by polarisation effects is that of the pentafluorides (page 27). The differences are not so extreme in other cases however. 

Polarisation effects should be relatively more important when the activation barrier is small or unimportant (i.e. the reactants diffuse together to form the encounter pair, as discussed in ref. 87b) than when the activation barrier is large (i.e. the reactants diffuse together and then have to obtain considerable energy to cross from the reactant to product potential energy surface). Clearly, much more work in this area is required and it is especially appropriate to consider these ideas when discussing proton transfer reactions. 

The introduction of 7i-bonding interactions between the metal and the ligand results in a metal-to-ligand or ligand-to-metal transfer of electron density. This occurs in accord with the electroneutrality principle, and in many cases, it opposes the polarisation effects of the metal ion. Furthermore, the effect will be expressed in orbitals possessing rather specific symmetry properties which may play important roles in the reactivity of the ligands. 

In these reactions the polarisation effect of the metal can activate an alkyl group towards nucleophilic attack. The incoming nucleophile is most commonly a halide or pseudohalide, and iodide or thiocyanate are particularly active in these processes. Numerous examples of such reactions are known, which are accelerated by co-ordination of the metal to oxygen. An example is shown in Fig. 4-46 in which the metal stabilises a phos-phonate leaving group. 

Let us start by considering the reaction of the copper(n) complex 6.49 with formaldehyde. Initially we might expect the diimine 6.50 to be formed, but this ignores the nature of the intermediates. As we saw earlier, the reaction of an amine with an aldehyde initially produces an aminol. Consider the addition of the second molecule of formaldehyde to 6.49. The product will be 6.51, which contains an imine and an aminol (Fig. 6-43). The imine is co-ordinated to a metal ion, and the polarisation effect is likely to increase the electrophilic character of the carbon. The hydroxy group of the aminol is nucleophilic and it is correctly oriented for an intramolecular attack upon the co-ordinated imine. The result is the formation of the copper(n) macrocyclic complex 6.52. 

Tennyson, J. and Danby, G. (1987). The ab initio inclusion of polarisation effects in low-energy positron-molecule collisions using the R-matrix method. In Atomic Physics with Positrons, eds. J.W. Humberston and E.A.G. Armour (Plenum Press) pp. 111-121. 

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