Dipole moments permanent

The study of electrons in other polar fluids is quite extensive but theoretical studies have been limited to model calculations. Extensive reviews of these are found in Kestner (1976, 1987) and Feng and Kevan (1980). The general features are remarkably explained by steric restrictions and the magnitude dipole moments, permanent and induced. 

Fig. 17.11 Plot of the computed transition dipole moment, permanent dipole moment, permanent...

For reasons of symmetry the X2 molecule already treated does not exhibit any permanent dipole moment. Permanent dipole moments are always the result of the charge being distributed asymmetrically in the bonded state, and thus occur in diatomic molecules formed from different atoms. According to... 

Blanton S A ef a/1997 Dielectric dispersion measurements of CdSe nanocrystals colloids observations of a permanent dipole moment Phys. Rev. Lett. 79 865... 

Wall M C, Stewart B A and Mullin A S 1998 State resolved oollisional relaxation of highly vibrationally exoited pyridine ( ii = 38,000 om ) and CO2 influenoe of a permanent dipole moment J. Chem. Rhys. 108 6185-96... 

Our discussion of elecfronic effects has concentrated so far on permanent features of the cliarge distribution. Electrostatic interactions also arise from changes in the charge distribution of a molecule or atom caused by an external field, a process called polarisation. The primary effect of the external electric field (which in our case will be caused by neighbouring molecules) is to induce a dipole in the molecule. The magnitude of the induced dipole moment ginj is proportional to the electric field E, with the constant of proportionahty being the polarisability a ... 

The permanent dipole moment of an isolated molecule depends on the magnitude of the charge and on the distance separating the positive and negative charges. It is defined as... 

As argued above, this result is found to work best for substances in which both the 1,1 and 2,2 forces are either London or dipole-dipole. Even the case of one molecule with a permanent dipole moment interacting with a molecule which has only polarizability and no permanent dipole moment-such species interact by permanent dipole-induced dipole attraction-is not satisfactorily approximated by Eq. (8.46). In this context the like dissolves like rule means like with respect to the origin of intermolecular forces. 

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 ... 

If molecules have permanent dipole moments and can orient themselves with respect to the field, then Eq. (10.17) must be modified by inclusion of a term associated with p. 

Figure 4.18 Molecules (a)-(g) have a permanent dipole moment but molecules (h) and (i) do not...

A molecule has a permanent dipole moment if any of the translational symmetry species of the point group to which the molecule belongs is totally symmetric. 

Although symmetry properties can tell us whether a molecule has a permanent dipole moment, they cannot tell us anything about the magnitude of a non-zero dipole moment. This can be determined most accurately from the microwave or millimetre wave spectrum of the molecule concerned (see Section 5.2.3). 

A molecule has a permanent dipole moment if any of the symmetry species of the translations and/or T( and/or 1/ is totally symmetric. Using the appropriate character table apply this principle to each of these molecules and indicate the direction of any non-zero dipole moment. 

The other four molecules, (c)-(f), do not have permanent dipole moments, as inspection of the relevant character tables in Appendix A will confirm. 

The molecule must have a permanent dipole moment (pi 0). 

This is the same as Equation (5.14) for a diatomic or linear polyatomic molecule and, again, the transitions show an equal spacing of 2B. The requirement that the molecule must have a permanent dipole moment applies to symmetric rotors also. 

Although these molecules form much the largest group we shall take up the smallest space in considering their rotational spectra. The reason for this is that there are no closed formulae for their rotational term values. Instead, these term values can be determined accurately only by a matrix diagonalization for each value of J, which remains a good quantum number. The selection mle A/ = 0, 1 applies and the molecule must have a permanent dipole moment. 

We tend to think of a spherical rotor molecule, such as methane (see Figure 4.12a), as having no permanent dipole moment and, therefore, no infrared, millimetre wave or microwave... 

In Chapter 4, on molecular symmetry, 1 have added two new sections. One of these concerns the relationship between symmetry and chirality, which is of great importance in synthetic organic chemistry. The other relates to the connection between the symmetry of a molecule and whether it has a permanent dipole moment. 

The first of the two specific iateraction terms is due to the attractive iateraction between the permanent dipole moment ]l of a. molecule and... 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

Applications. Molecules couple to an electromagnetic field through their electric dipoles, so only those having a permanent dipole moment exhibit significant rotational spectra. For such species, microwave spectroscopy yields highly precise moments of inertia and details of centrifugal... 

This particularly valuable technique for studying molecules which possess permanent dipole moments in the vapour phase has been reviewed on many occasions. For its application to heterocyclic compounds the excellent account by Sheridan <74PMH(6)53) should be consulted. 

If a surface is polar, its resulting electric field will induce a dipole moment in a molecule with no permanent dipole and, through this polarization, increase the extent of adsorption. Similarly, a molecule with a permanent dipole moment will polarize an otherwise nonpolar surface, thereby increasing the attraction. 

For a polar surface and molecules with permanent dipole moments, attraction is strong, as for water adsorption on a hydrophilic adsorbent. Similarly, for a polar surface, a molecule with a permanent quadrupole moment vidll be attracted more strongly than a similar molecule with a weaker moment for example, nitrogen is adsorbed more strongly than oxygen on zeolites (Sherman and Yon, gen. refs.). 

This dispersion interaction must be added to the dipole-dipole interactions between molecules, such as HCl, NH3 and H2O which have a permanent dipole, fi. The magnitude of die dipole moment depends on tire differences in electronegativity of the atoms in the molecule. Here again, the energy of interaction varies as (orientation effect). 

From a quantum mechanical perspective, an atom or molecule would be considered to have no permanent dipole moment if the probability of finding electrons is symmetric about the nucleus. For example the probability of finding the electron in the ground state of hydrogen is constant with respect to its solid... 

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