Computational Activity to Visualize Stereoisomers in Molecules with an Axis of Chirality

Abstract

The topic of chirality is often a difficult concept to grasp for the typical undergraduate chemistry student. Often, the chirality of molecules is taught by using the concept of an asymmetric carbon center that generates molecules with nonsuperimposable mirror images. When students encounter a chiral molecule lacking a stereogenic center such as an asymmetric carbon, the confusion is amplified. In order to study this distinction, this computational study was designed to help students visualize molecules containing an axis of chirality but no stereogenic center. The molecules chosen for this activity are allenes, binaphthyls, and spiranes. Students first perform a visual inspection by drawing the possible enantiomers side-by-side in a 3D structure drawing program. They then compute vibrational circular dichroism (VCD) spectra for the enantiomers. Chiral molecules uniquely interact with circularly polarized light due to the dissymmetry in their molecular orbitals. Therefore, determining if a molecule is chiral may be achieved by computing its VCD spectrum: where chiral molecules produce a peaked spectrum and achiral molecules feature a zero-intensity flat-lined spectrum. For a given enantiomer, the VCD spectrum shows both positive and negative peaks. Just as enantiomers are mirror images, VCD spectra are mirror images of each other, meaning a positive peak from one enantiomer will be a negative peak from the other enantiomer.