Molecular distortion induced electric field

Polarizability (of a molecule) — There are numerous different mechanisms that contribute to the total polarizability of a molecule. The three most important of these are termed electron polarizability, molecular-distortion polarizability, and orientation polarizability. All these parameters are measured as statistical averages over large numbers of molecules present in the bulk phase. (1) -> Electron polarizability a is a measure of the ease with which electrons tend to be displaced from their zero-field positions by the applied -> electric field. Thus, the electron polarizability of a molecule is defined as the ratio of induced dipole moment pincj (coulomb meters) to the inducing electric field E (volts per meter) ... 

Now let us examine the molecular origin of Molecular polarity may be the result of either a permanent dipole moment p or an induced dipole moment ind here the latter arises from the distortion of the charge distribution in a molecule due to an electric field. We saw in Chap. 8 that each of these types of polarity are sources of intermolecular attraction. In the present discussion we assume that no permanent dipoles are present and note that the induced dipole moment is proportional to the net field strength at the molecule ... 

Similar to the situation for quadrupole-induced relaxation, the quadrupole splitting in liquid crystals is zero if the molecular symmetry is tetrahedral or higher. Electric field gradients are zero for such symmetries so there can be no quadrupolar interaction. However, one expects to see small splittings from tetrahedral or octahedral derivatives because of structural distortions. These predominately arise from specific interactions with extraneous materials such as lipophilic headgroups in surfactant systems, as seen, for instance, in both cationic and anionic octahedral cobalt(III) species [23], Much larger splittings will be expected from other structure... 

The fundamentals of the Raman effect can be understood by consideration of a classical model, in which an incident beam of radiation (i.e., laser beam, for all practical purposes, in flame diagnostics) passes through an ensemble of molecules. The resultant laser beam electric field distorts the electronic cloud distribution of each molecule, causing oscillating dipoles these induced dipoles are related to the incident laser beam electric field by the molecular polarizability. The dipoles, in turn, produce a secondary radiating field at essentially the same frequency as that for the incident beam. This radiation is termed Rayleigh scattering. 

In the last few years, several workers have analyzed charge density distribution in molecular crystals with non-linear optical (NLO) properties [71-74]. The NLO response can, in principle, be explained by an anharmonic distortion of the electron density distribution due to the electric field of an applied optical pulse. The polarization P induced in a molecule is... 

Molecular polarisability is the result of two mechanisms (a) distortional polarisation and (b) orientation polarisation. Distortional polarisation is the result of the change of electric charge distribution in a molecule due to an applied electric field, thereby inducing an electric dipole. This distortional polarisation is coined ad. Permanent dipoles are also present in the absence of an electric field. At the application of an electric field they will orient more or less in the direction of the electric field, resulting in orientation polarisation. However, the permanent dipoles will not completely align with the electric field due to thermal agitation. It appears that the contribution of molecular polarisability from rotation is approximately equal to p2/(3kT). Accordingly, the total molecular polarisability is... 

Molecular quadrupole moments may be obtained directly by observing the optical birefringence induced in a gas by an electric field-gradient Exx = —Eyy <35—37 The birefringence nx -ny, like the dielectric polarization in Debye s treatment, is comprised of a temperature-independent contribution due to distortion of the molecule and an orientation term proportional to T l 35 ... 

In addition to orienting dipoles, electric fields induce dipole moments in molecules, since electrons and nuclei experience forces in opposite directions in the same electric field and since electrons, being less massive, move much more easily than nuclei in a field. The quantity that measures the ease with which the electron cloud in a certain molecule can be distorted is the molecular polarizability ccq. The magnitude of an induced dipole is given as... 

The van der Waals radius determines the shortest distance over which intermolecu-iar forces operate it is aiways larger than the covalent radius. Intermolecular forces are much weaker than bonding (intramolecular) forces. Ion-dipole forces occur between ions and poiar molecules. Dipole-dipole forces occur between oppositely charged poles on polar molecules. Hydrogen bonding, a special type of dipole-dipole force, occurs when H bonded to N, O, or F is attracted to the lone pair of N, O, or F in another molecule. Electron clouds can be distorted (polarized) in an electric field. Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion forces. 

London (dispersion) forces are generally small, but the exact size or extent depends on the polarizability of the atom. This is a measure of the ease with which the electron density of an atom or molecule can be distorted by an electric field. The larger the molecule or atom, the greater the volume occupied by the electrons and the greater the polarization and the larger the size of the temporary or induced dipole (Figure 4-71). Hence, London (dispersion) forces increase with relative atomic or relative molecular mass. 

Apart from these particular additional processes their are at least two mechanisms that always contribute in the absorption spectra. The "collision-induced contribution arises from the rotational modulation of transient dipoles during collisions. The charge distribution of one molecule is distorted by the electric field from the charge distribution of its collision partner. The resulting induced dipole moment is varying within the time scale of collisions, that is much faster than the molecular motions themselves so that it contributes higher frequency absorption to the permanent moment s rotational absorption spectrum. 

The formation of the virtual state relies on the distortion of the electron wavefunction by the incident radiation, rather than on the strength of a molecular dipole moment. Therefore, the likelihood of this process depends on the polarizability a of the molecule—the ability of the electric field of the radiation to momentarily deform the electron cloud around the molecule and induce a dipole moment. For a spherical electron distribution, this induced dipole moment has a magnitude given in Eq. 3.36 ... 

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