An Intuitive Approach to Estimating Electron Distributions in Molecules

 

One of the fundamental questions of chemistry is how electrons are distributed in molecules. Many questions concerning chemical reactivity can be answered if you know which regions of a molecule are "electron-rich" and which are "electron-poor". For example, if the first step in a particular chemical reaction involves the addition of a hydrogen ion to a molecule, you can begin to predict where in the molecule the hydrogen ion might attach itself by knowing where the electron density is highest, since the positive hydrogen ion will be attracted to the more electron-rich regions of the molecule. While the terms electron density, electron-poor and electron-rich can be defined more precisely, let us begin with a more descriptive and intuitive approach to the question of electron distribution.

You already know how to draw the Lewis structure(s) of various molecules and ions, so you have some idea of where in the molecule there might be a greater number of electrons, and hence more electron density. Regions of multiple bonds will, in general, have more electron density than regions of single bonds. In addition, you know about bond polarity and the unequal distribution of electron density in a bond involving two different atoms. This latter concept is most often discussed using the relative electronegativities of the two atoms.

The electronegativity scale that you are accustomed to seeing was developed by Pauling using arguments based on bond energies. A more instructive electronegativity scale is that of Mulliken and Jaffe. In this scale the electronegativity is defined as the average of the ionization energy and electron affinity of an atom. Since electron affinities (which are really the zeroth ionization energy of an atom) are generally much smaller than ionization energies, you can say that electronegativity is roughly proportional to ionization energy. Although the numbers you calculate using the Mulliken-Jaffe approach are different than the usual Pauling numbers, the trends are very similar. Thinking of electronegativity as varying as ionization energy makes the concept more real. The atoms from which it is hardest to remove an electron (the highest ionization energy) have the highest eletronegativity, consistent with our anthropomorphic view of electronegativity as a measure of how tightly an atom holds onto electrons.

The molecular viewer Chime is capable of generating pictures of electrostatic potential surfaces of molecules that are approximations based on the two ideas described above - the nature of the bonds in the molecule and the relative electronegativities of the various atoms in the molecule. Before you begin more sophisticated calculations of electron distributions you should play with this feature of Chime for a number fo molecules and ions and begin to develop the ability to make some predictions about electron distributions in simple molecules.

Here is a brief tutorial that demonstrates how to use Chime to generate electrostatic potential surfaces. If you are unfamiliar with using Chime, here is a tutorial on the basic features of the program. If your personal computer does not have Chime installed you may download it (free) here.

Once you have worked through the tutorial(s) and feel comfortable using Chime, look at the following molecules and see how the results of using Chime to generate an electrostatic potential surface compare with predictions that you make about where the regions of highest and lowest electron density (also often referred to loosely (and not strictly correctly) as the most negative and most positive regions of the molecule) are. You will first need to draw some Lewis structures.

ammonia
acetic acid
acetate ion
aniline
1,1-dichloroethane
prozac - here you should use the wireframe structure and VSEPR ideas to help you determine where the multiple bonds, if any, are
when pyridine is protonated (an H⁺ is added to the molecule) to produce the pyridinium ion, where is the hydrogen ion attached

There are many additional molecules that you can examine in our on-line collection.

Once you have your electrostatic potential surface on the screen there are several additional ways that you can examine it in Chime. Perhaps the most useful is to make the surface partially transparent. This is done in Chime by right-clicking then going Select --> Display List --> Toggle Transparency. At this point you can also change the display mode for the molecule by, for example, right-clickng and the Display --> Ball & Stick.