The vibration generator 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.
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.
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:
v H2S
Ř A1
stretch (IR active)
Ř A1
bend (IR active)
Ř B1
stretch (IR active)
v CF2Cl2
Ř A1 stretch (IR
active)
Ř A1 stretch (IR
active)
Ř A1 bend (IR
active)
Ř A1 bend (IR
active)
Ř A2
bend (IR inactive)
Ř B1 stretch (IR
active)
Ř B1 bend (IR
active)
Ř B2 stretch (IR
active)
Ř B2 bend (IR
active)