Dipole Moments and Optical Activity

The existence of a dipole moment tells us something about the molecular geometry or symmetry, and vice versa. For example, the existence of a dipole moment for H20 and NH3 implies that the former cannot be linear and the latter cannot be trigonal planar. Indeed, it is not difficult to show that molecules that possess dipole moments belong only to point groups C , Cs, or Cm. 

For a molecule to be optically active, it and its mirror image cannot be superimposed on each other i.e., they are dissymmetric (see Section 6.1.3). In the language of symmetry operations, this means that an optically active molecule cannot have any Sn element. In particular, since Si is a and S2 is i, any molecule that possesses a symmetry plane or a center of inversion cannot be optically active. 

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Symmetry also plays an important part in the determination of the structure of molecules. Here, a great deal of the evidence comes from the measurement of crystal structures, infra-red spectra, ultra-violet spectra, dipole moments, and optical activities. All of these are properties which depend on molecular symmetry. In connection with the spectroscopic evidence, it is interesting to note that in the preface to his famous book on group theory, Wigner writes ... 

Symmetry is a fundamental concept of paramount importance in art, mathematics, and all areas of natural science. In the context of chemistry, once we know the symmetry characteristics (i.e., point group) of a molecule, it is often possible for us to draw qualitative inferences about its electronic structure, its vibrational spectra, as well as other properties such as dipole moment and optical activity. 

Oxirane (1) and methyloxirane (3) are miscible with water, ethyloxirane is very soluble in water, while compounds such as cyclopentene oxide and higher oxiranes are essentially insoluble (B-73MI50501) (for a discussion of the solubilities of heterocycles, see (63PMH(l)l77)). Other physical properties of heterocycles, such as dipole moments and electrochemical properties, are discussed in various chapters of pmh. The optical activity of chiral oxiranes has been investigated by ab initio molecular orbital methods (8UA1023). 

However, it is necessary to determine precisely the orientation of the dipole moments, and to ensure that the observed CD is not the resultant from a mixture of con formers. Moreover, even with a correct choice of the dipoles, there are often shortcomings. For instance, studies of l,l -spirobisindanes have shown substantial discrepancies between the theoretical and experimental results for several products of known configuration 27). It has been shown that the signs of the bands stand in direct relation to the configuration of the spiro-atoms, which means that the optical activity is really governed by the coupling mechanism including other types of interaction. 

At the end of this chapter we will show that knowledge of symmetry can lead us directly to predict whether a molecule can have a dipole moment and whether it can exhibit optical activity. 

Fig-9. EO coefficient data, as a function of chromophore number density, for CLD-type chromophores with (solid circles) and without (open triangles) isophorone protection of the polyene bridge. The maximum achievable electro-optic activity is smaller for the naked polyene bridge structure and the maximum of the curve is shifted to lower number density. The dipole moment and molecular first hyperpolarizability values are comparable (The unprotected polyene bridge variation may exhibit slightly higher values of p(3)... 

The interaction of light with a molecule can be described by the influence of its eleetric field on the electron density. The eleetric field E acting on the molecule (or material) induces a distortion of the electron density, with (induced) dipole moment / ind (or polarization P on the maeroscopic level). For weak electric fields the magnitude of this induced moment (polarization) can be expected to be linear with the amplitude of the electric field. This linear proportionality suffiees to explain the linear optical properties, e.g., refraction and optical activity. Yet, when the electric... 

Thus the polarizability tensor of the molecule is written as a sum of local atomic polarizabilities, each modified through dipolar interactions with the electric dipole moments on all the other atoms induced by the electric vector of the incident light wave. Similarly for the local atomic polarizabilities appearing in the origin-dependent parts of the optical activity tensors. But unlike the bond polarizability development, no allowance can be made for intrinsic local optical activity tensors Gj ap and Aj since these now pertain to spherical atoms. We refer to the original articles for the explicit Raman intensity and optical activity expressions generated by the atom dipole interaction theory. 

A variation on HPLC-CDS is based on magneto-optical rotation. In a longitudinal electric field, almost all molecules show optical activity [134]. The electrical field orients the molecules through their permanent dipole moments and/ or through any anisotropic electrical polarizability. Kawazumi et al. [135] used this principle to build a detector for HPLC. Although the device was complicated, and prone to respond to minute changes in the refractive index or temperature of the mobile phase, it showed promise as a universal detector. This universality would only apply to detectability. Compounds with strong dipole moments and those that are easily polarizable would have more intense responses than compounds that do not. 

L. D. Barron, in Molecular Light Scattering and Optical Activity, Cambridge University Press, Cambridge, U.K., 1982, pp. 317-321. P. L. Polavarapu, Mol. Phys., 49, 645 (1983). A Comparison of Bond Moment and Charge Flow Models for Vibrational Circular Dichroism Intensities. J. R. Escribano, T. B. Freedman, and 1- A. Nafie, /. Phys. Chem., 91, 46 (1987). A Bond-Origin-Independent Formulation of the Bond Dipole Model of Vibrational Circular Dichroism. 

The vibrational signatures are drastically modified in the nanosized particles compared to the corresponding bulk structures. These modifications are revealed by a widening and position shifts of the vibrational bands. Furthermore, the structural reconstruction at the surface of nanograins and the electron-phonon interactions play a key role on the optical and vibrational properties. In this context, spectroscopic methods such as Raman and IR are suitable tools to probe the surface reconstruction and size effects through the features of the bands related to active vibration modes. The fundamental IR absorption or Raman spectra are based on rules related to dipole moments and polarizabilities of molecules. Thus, for IR technique, when clusters or molecular complexes are considered, the intensity of their characteristic bands may be calculated according to the following formulation ... 

It seems rational to conclude that polyamide C6-DMPIP in solution behaves approximately as a stiff chain, and this result is confirmed by dipole moment data on this polymer. The experimental value of the mean dipole moment per residue, (ju / ) % was in fact found [12] to be independent of the molecular weight and it is essentially constant both for racemic and optically active C6-iWP/P polymers [12] (2.21 D, Table II). 

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