Electrical dissymmetry

A molecule with more than one polar bond in which all the effects of the polar bonds do not cancel out has a dipole moment—an electrical dissymmetry that causes an inter-molecular attraction between this molecule and other similar ones. This attraction is called a dipolar attraction, and it lowers the ability of the substance to exist in the gas phase. However, if the polar bonds in a molecule are oriented so that their effects are canceled out, as in carbon dioxide, then a molecule with no dipole results (Section 13.5). 

There are other sources of electrical dissymmetry in a bond, some... 

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

Compared to the colligative methods light scattering can yield information on a possible dissymmetry of the aggregates. For anisotropic particles the direction of the electric field associated with the incident light may not coincide with the shift of the electron cloud. The intensity of light scattered at (usually) 90° from anisotropic aggregates is increased over the value predicted on the basis of isotropy by the Cabannes factor. 

One kind of relaxation time has already been discussed in Section 4.6.8, namely, the time of adjustment to dissymmetry of the ionic atmosphere around an ion when an applied electric field is switched on. Its understanding is basic to our picture of ionic... 

The relaxation effect is a bit more difficult to explain. It has to do with the fact that when an ion moves in a given direction, inertia causes the ionic cloud around it to become egg shaped, and this dissymmetric ionic cloud has more counter charge toward its rear than toward its front. The dissymmetry of charges acts to counteract the effect of the directional electric field applied through the solution, and so this also slows the ion down. Both these effects combine to explain why mobility falls with increasing concentration, for the two effects increase in strength with the square root of the concentration. 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

When a central ion moves in an electric field the ionic cloud surrounding the ion is permanently formed. This requires a certain time called the relaxation time. Therefore, as illustrated in Fig. 6-1, the charge density around the central ion is no longer symmetrical, but is lower in front of the central ion than behind it. This dissymmetry in charge distribution leads to an electrostatic deceleration of the central ion which reduces the ion mobility. 

Inside a molecule, a dissymmetry of the electronic cloud repartition exists between the different atoms. When an electric field is applied, it can modify the electron repartition and consequently the equilibrium location of the atoms in the molecule. This effect is called atomic polarisation. 

Even without an electric field, permanent dipole moments exist as a consequence of the dissymmetry of the charge distribution over a molecule. When an electric field is applied, those dipoles orient themselves. It is the dipolar polarisation described by Debye for dipoles in solution. 

In the context discussed above, characterizing a chromophore means an assignment of the absorption band to its corresponding quantum mechanical states. The electric transition moment direction of the absorption band belonging to the chromophore that can be experimentally determined from the degree of anisotropy R (eqn [26]) is an important information for an unequivocal assignment. Furthermore, the dissymmetry factor g (eqn [30]) rewritten by eqns [14] and [17] into... 

The fourth, the so-called inherent dissymmetric chromophore, does not possess local symmetry. Therefore, the transitions belonging to these chromophores are magnetically and electrically allowed. Inherent dissymmetric chromophores are often found with so-called form chiral molecules for which atropisomers like binaphthols are typical representatives. Further examples are chromophores that come into being by exciton coupHng. In both cases the dissymmetry factor g is in the order of 10 to 10 and the CD is easily measurable in spite of the fact that the absorption coefficients of these compounds are often very high (e between 10" and 10 ). 

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