Symmetric molecular systems dipole moments

In a symmetrical octahedral system such as SFg, each polar S—F bond has a counterpart pointing in the opposite direction. The bond polarities cancel in pairs, leaving this molecule without a dipole moment. Example examines molecular variations on octahedral geometry. 

An electronic transition takes place through the interaction of the electric held of incident electromagnetic radiation with a component of the dipole moment of the absorbing atomic or molecular system. Such transitions usually involving light in the visible region of the spectrum can occur only between states that differ in parity that is, one state must have a symmetric (g) wave function and the other an antisymmetric (n) wave function. 

Linear Dipolar Electric Polarization. Even atoms and microsystems with symmetrically distributed charges are known to exhibit polarization under the influence of an external electric field or the internal (molecular) electric field of a neighbour. Such a system is said to become endowed with an induced electric dipole (or higher multipoles). In many a case it sufiSces to assume that the induced dipole moment p E) is proportional to the field strength inducing it ... 

The center of diffusion is a unique point, because when the molecular coordinate system is centered at CD, the coupling tensor is symmetric. Moreover, for molecules with elements of symmetry (e.g., spheres, cylinders, ellipsoids), Dj. CD vanishes, and it is clear that for such bodies the dipole moment referred to CD is the quantity to be compared to the experimental dipole moment. Similar situations can be expected for globular proteins because their shape is close to a sphere or an ellipsoid. Brownian dynamics simulations of electrooptical relaxation experiment indicated that even for a large molecule such as tRNA, the difference is negligible between transient dichroism and derived dipole moments with and without inclusion of the coupling tensor when the coordinate system is centered at CD. Therefore it can be expected that calculated dipole moments of a molecule relative to its center of diffusion should closely correspond to those derived from orientational behavior. Some possibility for a bias of experimental dipole moments exists, however, since none of the orientation mechanisms considered in Eqs. [96] and [97] includes a possible contribution due to the translation-rotation coupling diffusion tensor. 

Figure 29 Uniaxially oriented system. The distribution of molecular axis is symmetric around the reference axis. A dipole-transition moment is aligned symmetrically around the molecular axis at an angle (. ...

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