This paper illustrates how valence shell electron pair repulsion (VSEPR) theory and molecular modeling can be used in a complimentary fashion in the undergraduate curriculum to predict the three-dimensional structure of molecules. Students use the familiar VSEPR model to sketch the three-dimensional structures of molecules predicted by a comparison of the relative magnitudes of electrostatic repulsion between electron pair domains on the central atom. Despite the simplicity and elegance of the VSEPR model, however, students often have difficulty visualizing the three-dimensional shapes of molecules and learning the more subtle features of the model, such as the bond length and bond angle deviations from ideal geometry that accompany the presence of lone pair or multiple bond domains or that result from differences in the electronegativity of the bonded atoms, partial charges and molecular dipole moments, and site preferences in the trigonal bipyramidal electron geometry. Students therefore also employ a molecular modeling approach to predict the three-dimensional shapes of molecules by performing both molecular mechanics and semiempirical quantum mechanical calculations to minimize the structures of these molecules. A major strength of the molecular modeling approach is that it allows students to quantitatively explore the more subtle implications of VSEPR theory using an alternative model. Student responses on end-of-the-semester evaluations ranked this exercise highly among the seven projects performed during the semester, commenting specifically on how the software allowed them to visualize the three-dimensional aspects of molecules more effectively than simple ball-and-stick models and on how the molecular modeling helped them to explore the more detailed structural features predicted by VSEPR theory.
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Pfennig et al. (1999) studied this question.