Anisotropic moment densities, molecules

Absorption in the ultraviolet (k 100 - 400 nm) and the visible (A. 400 - 800 nm) is primarily the result of transitions in the electronic state of the molecule. In such a process, the transition dipole moment would be proportional the overlap in the densities of the charge distributions between the two electron orbitals involved in a transition. The periodic displacement of electrons from one state to another will cause the charge distribution to be anisotropic, with net negative and positive contributions in certain locations within the molecule. The result is the formation of a dipole moment. Very often, dye molecules that absorb in the visible are dispersed within a sample or attached to the molecules of a sample and are used to monitor its degree of alignment. However, since the relative orientation of such a dye molecule to the molecular axes of the constituent sample molecules is often unknown, the interpretation of these measurements can be difficult. 

One of the most important interactions of the electron spin of the paramagnetic molecule, which influences its line position, is that with the nearby nuclear spins. These are the nuclei of the molecule itself and nuclei in the local surrounding upto a distance of ca. 0.6 nm. There are two different contributions to this interaction an isotropic part (fcrmi-contact interaction), which arises from electron spin density of the unpaired electron at the nucleus, and an anisotropic part, which arises from through-space magnetic dipole-dipole interaction between the electron and nuclear magnetic moments. Whereas for intramolecular nuclei, the isotropic part can be very large, interactions with intermolecular nuclei further apart are mainly anisotropic. This offers the possibility to measure directly the distance to the nucleus by the... 

The purpose of this Chapter is to describe the dielectric properties of liquid crystals, and relate them to the relevant molecular properties. In order to do this, account must be taken of the orientational order of liquid crystal molecules, their number density and any interactions between molecules which influence molecular properties. Dielectric properties measure the response of a charge-free system to an applied electric field, and are a probe of molecular polarizability and dipole moment. Interactions between dipoles are of long range, and cannot be discounted in the molecular interpretation of the dielectric properties of condensed fluids, and so the theories for these properties are more complicated than for magnetic or optical properties. The dielectric behavior of liquid crystals reflects the collective response of mesogens as well as their molecular properties, and there is a coupling between the macroscopic polarization and the molecular response through the internal electric field. Consequently, the molecular description of the dielectric properties of liquid crystals phases requires the specification of the internal electric field in anisotropic media which is difficult. 

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