Polar Molecules The Influence of Molecular Geometry

In Chapter 7 we saw that the unequal sharing of electrons between two atoms with different electronegativities, A(EN) 0, results in polar bond. For heteronuclear diatomic molecules such as HF, this bond polarity results in polar molecule.T] en the entire molecule acts as a dipole, and we would find that the molecule has a measurable dipole Ttwment, that is, it is greater than zero. 

In this section we will discuss the ideas of cancellation of dipoles in general terms, using general atomic symbols A and B. Then we will apply these ideas to specific molecular geometries and molecular polarities in Parts B of Sections 8-5 through 8-12. 

Let us consider a heteronuclear triatomic molecule with the formula AB2 (A is the central atom). Such a molecule must have one of the following two molecular geometries  

In the case of the angular arrangement, the two equal dipoles do not cemcel, but add to give a dipole moment greater than zero. The angular molecular arrangement represents a polar molecule. 

The arrow head indicates the area of partial negative charge and the tail (without a cross) represents the region of partial posrtive charge. 

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A Preview of the Chapter 8-2 Valence Shell Electron Pair Repulsion (VSEPR) Theory 8-3 Polar Molecules The Influence of Molecular Geometry 8-4 Valence Bond (VB) Theory... 

The origin of the effect here represented by x0) can be derived from modelistic considerations. Solvent molecules are mobile entities and their contribution to the dielectric response is a combination of different effects in particular the orientation of the molecule under the influence of the field, changes in its internal geometry and its vibrational response, and electronic polarization. With static fields of moderate intensity all the cited effects contribute to give a linear response, summarized by the constant value e of the permittivity. This molecular description of the dielectric response of a liquid is... 

MD simulations of electrolytes for lithium batteries retain the atomistic representation of the electrolyte molecules but do not treat electrons explicitly. Instead the influence of electrons on intermolecular interactions is subsumed into the description of the interatomic interactions that constitute the atomistic potential or force field. The interatomic potential used in MD simulations is made up of dispersion/ repulsion terms. Coulomb interactions described by partial atomic charges, and in some cases, dipole polarizability described by atom-based polarizabilities. The importance of explicit inclusion of polarization effects is considered below. In the most accurate force fields, interatomic potentials are informed by high-level QC calculations. Specifically, QC calculations provide molecular geometries, conformational energetic, binding energies, electrostatic potential distributions, and dipole polarizabilities that can be used to parameterize atomic force fields. 

Molecular geometry is tremendously important in understanding the physical and chemical behavior of a substance. Molecular polarity, for example, is one of the most important consequences of molecular geometry, because molecular polarity influences physical, chemical, and biological properties. Recall from Section 8.4 that a bond between two atoms of different electronegativities is polar and that a diatomic molecule containing a polar bond is a polar molecule. Whether a molecule made up of three or more atoms is polar depends not only on the polarity of the individual bonds, but also on its molecular geometry. 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

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