Background:
Summary of Methods Used
Simple Example:
Theory
The Vibrations
Research Topics:
Fatty Acid Methyl Esters (FAMEs)
School of Chemistry
The Queen's University of Belfast
A Simple Example: Part 2 Lets start at the beginning; As I explained earlier, we would predict that there would be 3n - 5 fundamental modes for a linear molecule. Since there are 3 atoms in CO2 then we would expect to find 4 distinct vibrations.
These files will take a short time to download, it shouldn't be more than about 30secs though so please be patient.
Animation of Vibration |
experimental/calculated frequency |
I.R/Raman Active? |
experimental 667 cm-1 calculated 640 cm-1 |
Raman inactive | |
experimental 667 cm-1 calculated 640 cm-1 |
Raman inactive | |
experimental 1351 cm-1 calculated 1372 cm-1 |
Raman active |
|
experimental 2349 cm-1 calculated 2436 cm-1 |
Raman inactive |
If you're not familiar with Chime then click and hold the left mouse button in the relevant window to manipulate the structures. Clicking the right mouse button will enable you to change the display type, stop and start the animation and various other things.
So, as predicted, there are four vibrational modes for this molecule, but we don't see four peaks in the spectrum In the symmetric stretching mode of CO2, the changes in the CO bond dipoles always cancel so there is no change in molecular dipole moment (it remains zero throughout this vibrational motion). This mode is, therefore, IR INACTIVE. However, for the antisymmetric stretch and the bends, a net oscillating dipole moment is created and so these modes are IR ACTIVE. This is a direct result of the symmetry of the O=C=O molecule. Additionally, the two bending motions shown above are expected to have the same frequency due to the cylindrical symmetry of this linear molecule. The result is that we expect to see only TWO I.R. absorption bands for CO2.
On the other hand, the Raman spectrum would show only one peak since neither of the bends nor the antisymmetric stretch result in a change of polarisability of the molecule.
Similar symmetry arguments can be applied to more complex molecules to predict the IR/Raman activity/inactivity of their vibrations and so, by comparison with observed spectra, it is possible to confirm or refute our assumptions about molecular structure.
Having seen how well the calculated spectrum compares with the experimental spectrum for this simplistic molecule we then need to ask the question - does the maths work as well for bigger molecules? The answer is that often it does, and more and more chemists are realising that they can use DFT methods to help them in their work.
This is where my work starts.
