Moments in Molecules

When two atoms with different electronegativities form a bond, the electron density in the bond is not equally distributed. For example, in the molecule hydrogen fluoride, HF, the electron density in the bond shifts away from the H atom toward the more electronegative fluorine atom. This results in a partial negative charge on F and a partial positive charge on 

The bond is said to be polar when such charge separation exists. The charge separation can be shown as 

The HF molecule is a linear diatomic molecule with one polar bond therefore, the molecule is polar and has a dipole moment. The dipole moment jx is the product of the charge Q and the distance between the charges, r  

Molecules with more than two atoms may or may not have permanent dipole moments. It depends on the geometry of the molecule. Carbon dioxide has two equal C=0 bond dipoles but because the molecule is linear the bond dipoles cancel and the molecule has no net dipole moment  

has two equal H—O bond dipoles. Water has a bent geometry and the vector sum of the bond dipoles does not cancel. The water molecule has a permanent net dipole moment. 

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Formula AsCls MW 181.28 pyramidal structure dipole moment in molecule in the gas phase 1.59 p/D Synonym arsenic (III) chloride arsenic chloride... 

Table AlO-9 Dipole Moments in Molecules Containing Heteroatoms. Hartree-Fock Models... 

Such a comparative study has been made by Byakov and his collaborators.29 255 They have shown that in the case of water the main contribution to the loss rate given by formula (6.3) comes from excitation of intramolecular vibrations rather than from dipole relaxation. This is all the more so in nonpolar media where the main channel of continuous losses is not the relaxation of constant dipole moments (which are zero) but the polarization losses due to the electron-inducing dipole moments in molecules. The possible exceptions are the media consisting of molecules with a high degree of symmetry, such as methane and neopentane, which have no active vibrations in the IR region. 

An additional interaction that is frequently seen in EPR spectra involves coupling of the electron and nuclear magnetic moments in molecules where there are nuclei with non-zero spins. These are the same isotopes as those used to generate NMR spectra and are summarised in Table 3.3. Spectra are split into 27+1 components, and the resulting peak separations are called hypefine splittings (which are... 

Dispersion interactions arise because there are rapid fluctuations in electronic charge distributions. Even a nonpolar molecule A has an instantaneous dipole moment /W< n8tant) that induces a dipole moment in molecule B that interacts with the instantaneous dipole to produce the instantaneous energy ... 

To sum up, the effect of an external field, i.e. the production of induced moments in molecules, whether the latter depend on Ppy P or Ppy can never give rise directly to a measurable contribution P<,. Theoretically an external field may indeed render anharmonic vibrations unsymmetrical and thus give rise to a measurable effect (see further below), but according to experiments by the molecular beam method this effect can only be made to account for a trifling part at most of the observed differences P — Pp or P — P oUd e dipole molecules Ca4 which were investigated. 

The absence of a dipole moment in molecules such as HgHala and BeHalg is in agreement with the proposed linear configuration of these molecules. The two equal moments of the bonds are directed in mutually opposite directions, with the result that the molecule has a zero moment, of bond moments is responsible for the zero moment of BHa molecules, which has a symmetrical planar configuration. 

Molecules in gas, liquid, solid and colloidal particles in a sol and biological macromole-cules in living systems interact with each other. Knowledge of these interactions is mandatory since they determine all of the static and also the dynamic properties of the system. First of all, we should discriminate between the chemical and physical interactions. Chemical interatomic forces form chemical bonds within a molecule. However, the inter-molecular forces between molecules are different from chemical interatomic forces because they are physical in nature. In the first part of this chapter (Section 2.1) we will examine the chemical bonding within a molecule and also the effects of geometry and dipole moments in molecules. We will consider the physical interactions between molecules in the rest of the chapter (Sections 2.2 to 2.9). 

In addition to orienting dipoles, electric fields induce dipole moments in molecules, since electrons and nuclei experience forces in opposite directions in the same electric field and since electrons, being less massive, move much more easily than nuclei in a field. The quantity that measures the ease with which the electron cloud in a certain molecule can be distorted is the molecular polarizability ccq. The magnitude of an induced dipole is given as... 

Molecule (b) will have a higlier dipole moment. In molecule (a), the t ans arrangement cancels the bond dipoles and tlie molecule is nonpolar. 

The practical example of a fuel container made of plastic emphasizes the important influence of dipole moments in molecules on the solubility behavior of plastics. The influence of ordered (crystalline) regions and unordered (amorphous) structures in the polymer is shown in Fig. 10. 

D18.1 Molecules with a permanent separation of electric charge have a permanent dipole moment. In molecules... 

Lazzeretti, P. Malagoli, M. Zanasi, R. Electronic current density (383) induced by magnetic fields and magnetic moments in molecules. 

The total induced dipole moment in molecule X (X = A, B, C,...) at position if due to the electric field caused by the other interacting molecules can be expressed in the form... 

Electronic magnetic dipole moments in molecules and atoms are measured in terms of Bohr magnetons in the same way that angular momentum is measured in terms of h. 

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