Dipole moments polarization contributions

In the second type of interaction contributing to van der Waals forces, a molecule with a permanent dipole moment polarizes a neighboring non-polar molecule. The two molecules then align with each other. To calculate the van der Waals interaction between the two molecules, let us first assume that the first molecule has a permanent dipole with a moment u and is separated from a polarizable molecule (dielectric constant ) by a distance r and oriented at some angle 0 to the axis of separation. The dipole is also oriented at some angle from the axis defining the separation between the two molecules. Overall, the picture would be very similar to Fig. 6 used for dipole-dipole interaction except that the interaction is induced as opposed to permanent. 

The polarity of a solid surface can be regarded as the strength of its average electrostatic field F. This field interacts with permanent or induced dipoles of adsorbed molecules, whereas its gradient interacts with permanent or induced quadrupoles. These interactions give rise to components of the adsorption energy such as EFfl (with permanent dipoles), Ef (polarization contribution) or PQ (with permanent quadrupoles). Therefore, the selection of the appropriate immersion systems, differing mainly in the molecular dipole moment n of the immersion liquid, can be expected to provide information on the value of EPfl since ... 

The linear and non-linear polarizabilities of a molecule in solution differ from those of the isolated molecule in the gas phase since the molecular properties are modified by solute-solvent interactions. Some of these interactions are present even in the absence of externally applied static or optical fields. For molecules with a non-zero dipole moment fj in the electronic ground state the dominant interaction is usually due to the reaction field contribution The molecular dipole moment polarizes the solvent environment and thus generates a polarization field which interacts with the solute. This field is given by (88) (Boettcher, 1973 Wortmann and Bishop, 1998). 

When an external electric field is applied to a dielectric material, the material gets a dipole moment (polarization of the dielectric). The phenomenon can be characterized by the dipole moment per unit volume, also known as the polarization density, that the material gains. For weak applied fields the response of the material is linear in the applied field but if the field is very strong, nonlinear contributions can play a role. At microscopic level, nonlinear contributions can be thought of as due to the interaction of the strong external field with the strong electric field inside atoms. 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

In contrast with water, methanol, ammonia, and other substances in Table 2.1, carbon dioxide, methane, ethane, and benzene have zero dipole moments. Because of the symmetrical structures of these molecules, the individual bond polarities and lone-pair contributions exactly cancel. 

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