Included in the CoolMolecules site is a vibration generator that can model molecular motion and show the symmetry of each molecule's normal modes. By analyzing a molecule's vibrations, it is possible to predict the number of absorption bands that will appear in that molecule's infrared spectrum.

Suggested Activity #1

Investigate the effects of atomic mass on vibration using diatomic molecules. Since vibration involves no net translation of the molecule's center of mass, the less massive atom will move farther from its equilibrium position than will the more massive atom. For example, consider the following molecules and note the relative sizes of the arrows on each atom.

Suggested Activity #2

Predict the number of peaks in the infrared spectrum of a molecule by observing how many vibration modes change the molecule’s dipole moment. If a vibration mode changes the dipole moment, that mode will be responsible for a peak in the infrared spectrum. For advanced classes, character tables can be used to verify predictions about infrared activity. Infrared active vibrations have the same symmetry as the Cartesian coordinates x, y, and z. Some good molecules to consider are those structures that were determined using electron diffraction.