ChemNews.Com VOL 6 NO 4

The basic approach followed when preparing material for lectures is generally something like the following, and student lab assignments will also include some or all of these steps:
• Sketch in the molecule and minimize the structure with a modeling packages like Chem3D Pro.
• Perform electronic structure calculations with an electronic structure program such as Gaussian 94.
• Examine the results of the Gaussian job, visualizing them as appropriate. Visualization options include:
• Examining optimized structures.
• Viewing and animating a series of structures as when following a reaction path computed by an IRC calculation.
• Plotting molecular orbitals and other volumetric data such as the electron density, electrostatic potential, spin density for radicals, and the gradient and the Laplacian of the electron density.
• Viewing isodensity surfaces and slices (cross sections) of these same properties.
• Painting the value of a second property on an isosurface of the electron density (for example, the value of the electrostatic potential plotted at each point on an isosurface of the electron density).

Items further down on the list will require increasingly more sophisticated (and more expensive) visualization software. However, students can benefit from the modeling and visualization that is possible from even one or two of the less expensive packages available.

Computational chemistry can be integrated into chemical education at all levels. For example, Professor Foresman has designed lecture examples and laboratory exercises for introductory chemistry, organic chemistry and physical chemistry courses.

In an organic chemistry course, the isomeric orientation on electrophilic substitution can be illustrated by examining the electron densities of the various transition structures that result during the nitration of nitrobenzene and chlorobenzene. Viewing slices through the electron density or the electrostatic potential mapped onto an isodensity surface clearly illustrates that the meta isomer is favored in the former while the para isomer is favored in the latter.

Similarly, in his physical chemistry course, Foresman uses graphical results such as optimized structures, illustrations of atomic orbitals and molecular orbitals, and plots of the potential energy surface for a reaction can be used to enhance both lecture and laboratory explorations of the central concepts. For example, the effects of dielectric media can be illustrated by studying the rotational barrier between the E and Z forms of n-methyl-2-nitrovinylamine (illustrated in the figure) in different solutions.

Such a study begins by locating the transition structure for the reaction and then estimating the barrier by comparing their relative energies. Then, students can visualize electron density and density difference in order to explain why the barrier changes in different solvents.

For further information about using computational chemistry in undergraduate education, you may contact Dr. Foresman via email at jbfeduc@gaussian.com.