Electrostatic Potential Maps and Molecular Charge Distributions

Electrostatic Potential Maps and Molecular Charge Distributions 

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. 

The electrostatic potential map of benzene conveys the molecule s size as well as its charge distribution in a much more compact manner. The size and shape of the map are, of course, identical to that of the electron density surface, and indicate what part of the molecule is easily accessible to other molecules (the outside world ). The colors reveal 

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Electrostatic Potential Maps and Molecular Charge Distributions 29... 

Electron densities, bond densities, and spin densities, as well as particular molecular orbitals may be displayed as graphical surfaces. In addition, the value of the electrostatic potential or the absolute value of a particular molecular orbital may be mapped onto an electron density surface. These maps provide information about the environment around the accessible surface of a molecule. Electrostatic potential maps show overall charge distribution, while orbital maps reveal likely sites for electrophilic and/or nucleophilic attack. Surface displays may be combined with any type of model display. 

This chapter introduces a number of useful graphical models, including molecular orbitals, electron densities, spin densities, electrostatic potentials and local ionization potentials, and relates these models both to molecular size and shape and molecular charge distributions. The chapter also introduces and illustrates property maps which simultaneously depict molecular size and shape in addition to a molecular property. Properties include the electrostatic potential, the value of the LUMO, the local ionization potential and the spin density. 

As useful as molecular models are, they are limited in that they only show the location of the atoms and the space they occupy. Another important dimension to molecular structure is its electron distribution. We introduced electrostatic potential maps in Section 1.5 as a way of illustrating charge distribution and will continue to use them throughout the text. Figure 1.6(d) shows the electrostatic potential map of methane. Its overall shape is similar to the volume occupied by the space-filling model. The most electron-rich regions are closer to carbon and the most electron-poor ones are closer to the hydrogens. 

Problem 1.8 concerned the charge distribution in methane (CH4), chloromethane (CH3CI), and methyllithium (CH3Li). Inspect molecular models of each of these compounds, and compare them with respect to how charge is distributed among the various atoms (carbon, hydrogen, chlorine, and lithium). Compare their electrostatic potential maps. 

Although calculated electrostatic potential maps like these illustrate charge distribution and molecular shape well, it is important that you are able to draw the same conclusions based on what you would have predicted about the structures of BF3 and NH3 and their reaction product using orbital hybridization (Sections 1.13—1.15), VSEPR models (Section 1.17), consideration of formal charges (Section 1.5), and electronegativity (Sections 1.3A and 2.2). 

Organic Chemistry is supported by a well-developed illustration program. Besides tfaditional skeletal (line) structures and condensed formulas, there are numerous ball-and-stick molecular models and electrostatic potential maps to help students grasp the three-dimensional structure of molecules (including stereochemistry) and to better understand the distribution of electronic charge. 

Molecular models and electrostatic potential maps give students an appreciation of what molecules look like in three dimensions and show how charge is distributed within a molecule. 

Electrostatic potential maps have been added to Chapter 8 to show a more accurate distribution of charge in molecules. These maps are based on ab initio molecular modeling calculations and provide a convenient method for better student understanding of bond and molecular polarity. 

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