6.2 Chemical Bonds Vibrate
Elizabeth Johnson
Learning Objectives
- Describe the characteristics of vibrational movements of molecules and minerals.
- Determine the number of vibrational normal modes of a molecule.
Prior Knowledge and Skills
3.5 The Dual Nature of Electromagnetic Energy
3.7 Electromagnetic Energy: Units Conversion
Key Terms
- Vibrational normal mode
- Degenerate modes
Guided Inquiry
Watch the following animations of molecular vibrations:
How to use: click on the “i” button next to each vibration to bring up a video.
H2O: https://viva.pressbooks.pub/analyticalmethodsingeosciences/chapter/h2o-vibrations/
CO2: https://viva.pressbooks.pub/analyticalmethodsingeosciences/chapter/co2-vibrations/
SiO44- : https://viva.pressbooks.pub/analyticalmethodsingeosciences/chapter/sio44-vibrations/
These are examples of vibrational modes, or the vibrations that describe all possible internal motions of the molecule.
6.2.1. What is the difference in motion between a symmetric stretch and an asymmetric stretch?
6.2.2. Based upon the animations, do the different vibrational modes look like they have the same frequency (energy) or different frequencies?
6.2.3. Look at the symmetric bend for the CO2 molecule. In this figure, is the carbon atom moving in the y direction (vertically), or in the z direction (into and out of the page)?
6.2.4. If the CO2 molecule did a symmetric bend in the other direction, would you expect for it to have the same energy, or a different energy, from the symmetric bend in the animation?
The animations do not show degenerate modes, or multiple modes that have the same vibrational energy. We can calculate the total number of vibrational modes for a molecule:
Equation 6.2.1 Linear molecule: 3n-5
Equation 6.2.2 Non-linear molecule: 3n-6
6.2.5. How many vibrational modes does H2O have?
6.2.6. How many vibrational modes does CO2 have?
6.2.7. How many vibrational modes does a silicon tetrahedron (SiO44-) have?
Other Types of Vibrational Modes
Vibrational modes are not limited to stretching and bending. Below, six types of molecular vibrational modes are shown.
| Symmetric Stretch | Asymmetric Stretch | Scissoring or bending |
| Twisting | Wagging | Rocking |
Figure 6.2.1 Six vibrational mode motions (Public Domain; Tiago Becerra Paolini).
6.2.8. What is the difference between “wagging” and “twisting” in terms of atomic motion?
Mineral Structures and Vibrations
What do vibrational modes look like in a solid such as a crystal structure?
Each link below is from the website “Animating the CRYSTAL vibrational frequencies” http://www.crystal.unito.it/animations-of-vibrational-modes.php.
Forsterite: http://www.crystal.unito.it/vibs/forsterite/
Calcite: http://www.crystal.unito.it/vibs/calcite/
Beryl: http://www.crystal.unito.it/vibs/beryl/
How to use: Choose the forsterite model. The middle column is a list of vibrational modes. Scroll down to 979.2 cm-1 (B1G is a symmetry designation) and click on that number. The mineral should start mov