Molecular shapes and electronic

Jurs-PNSA-2 (characterization of positively charged surface area as a function of molecular shape and electronic considerations)... 

In the above, qualitative description of the expected trends of interrelations of changes in molecular shape and electronic state, an important element was missing a quantitative description of molecular shape and a numerical measure of shape changes. A precise, quantitative molecular shape description is also needed in the study of most other problems of chemistry, as well as in various related subjects, such as biochemistry, pharmacology, medicinal chemistry, and drug design. 

The connection between a molecule s electron density surface, an electrostatic potential surface, and the molecule s electrostatic potential map can be illustrated for benzene. The electron density surface defines molecular shape and size. It performs the same function as a conventional space-filling model by indicating how close two benzenes can get in a liquid or crystalline state. 

What Are the Key Ideas The central ideas of this chapter are, first, that electrostatic repulsions between electron pairs determine molecular shapes and, second, that chemical bonds can be discussed in terms of two quantum mechanical theories that describe the distribution of electrons in molecules. 

Self-Test 3.8A Describe (a) the electron arrangement, (b) the molecular shape, and (c) the hybridization of the central chlorine atom in chlorine trifluoride. 

A low electron density surface roughly shows the outline of a molecule s electron cloud. This surface gives information about molecular shape and volume, and usually looks the same as a van der Waals or space-filling model of the molecule. 

The correlation between mobility and sphericity has given rise to different speculations relating molecular shape and physical properties that could influence electron transport. However, it should be stressed that the liquid structure is important as well (Stephens, 1986). For example, although the electron mobility in liquid NP is several orders of magnitude larger than that in liquid... 

It is clear that the combination of different architectures and the precise localization of functionalities within a single macromolecule provide unique opportunities for the control of molecular shape as well as molecular, optical, and electronic properties. A significant hurdle that still remains today is the relatively demanding multistep process used to prepare dendrons and hybrids. This, in turn, translates into limited availability but, as high added-value applications emerge, it is clear that current, as well as yet-to-be-developed, syntheses will be used to prepare specialty materials that benefit from the unique properties derived from the combination of dendritic and linear architectures. 

Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements... 

One direct approach to the separation of chiral compounds is called molecular imprint polymers (MIPs) that involves the formation of a three-dimensional cavity with the shape and electronic features that are complementary to the imprinted or target molecule. 

As molecular orbital theory evolved over the years, several variations of Langmuir s definition of isosterism were expressed by others these are discussed in more recent publications [68, 69]. Burger s definition [68] of isosterism encompasses the aspects of the previous definitions and states that isosteres are chemical substances, atoms, or substituents that possess near equal or similar molecular shape and volume, approximately the same distribution of electrons, and which exhibit similar physicochemical properties. A few examples of isosteric atoms and substituents are provided in Figure 4.6. Many more examples are available in the literature [69-72], including metal isosteres [73]. 

Thus pure MM (MM by itself) can t calculate UV spectra, the shapes and energies of molecular orbitals, and electron distribution and derivative properties of this, like atomic charges, dipole moments, and more arcane molecular features like bond paths (associated with atoms-in-molecules theory, AIM [1]). 

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