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
Some examples of vibrations of FAMEs
For the visualization of the vibrations shown here I've used the medium sized C10H20O2 FAME and I've selected only 7 of the 90 normal modes.
These vibrations are found in different areas of the spectrum and so they show different types of movement. I'm highlighting only three areas here;
 the area from around 1000 cm-1 - 1300 cm-1 |
| the area from around 1400 cm-1 - 1500 cm-1 |
| the carbonyl stretching area at approx. 1750 cm-1 |
First things first though - here are the I.R. and the Raman spectra for this molecule.
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| Experimental I.R. | Calculated I.R. |
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| Experimental Raman | Calculated Raman |
If you look at these spectra carefully you will be able to see that the bands in the experimental spectra are well modelled by the calculated spectra. As usual it is neccesary to live with intensities that are not so good, but the positions of the bands are predicted with much greater accuracy. So, lets look at some of these vibrations. If the molecules appear to be quite small then please click "refresh" on your browser toolbar. If that doesn't work then press and hold the 'shift' key and press the left mouse button to zoom in on the display.
Section 1; the area from around 1000 cm-1 - 1300 cm-1. I've picked out three vibrations here. All the explanations are given below. Don't forget that you can move these molecules around so that you can view the vibration from various angles.
Animation of Vibration |
This is the "walking" vibration. It is found at a calculated (scaled 0.967) position of 1004 cm-1 and although it is both I.R. and Raman active, is appears as only a very, very small peak on the spectrum. Nevertheless, it is nice to look at and clearly shows how the whole molecule is involved in some of these vibrations. Offically this is called a CH2 twisting vibration. |
This is the in-phase "rowing" vibration found at 1217 cm-1 (scaled as above). This vibration leads to a very strong I.R. band but only a very weak Raman band. Once again, the whole molecule is involved in this vibration and it is easy to see that there will be similar vibrations where the movements are out-of-phase with each other. I show only one example here (below). |
This is also a "rowing" vibration but this time it is an out-of-phase example. I hope you can see that if these were real rowers in a boat they wouldn't be going anywhere! This vibration is found at 1259 cm-1 (scaled as above). Again, this vibration leads to a very strong I.R. band but only a very weak Raman band. |
Section 2; the area from around 1300 cm-1 - 1500 cm-1. Again, I've selected only three vibrations here.
Animation of Vibration |
This is the 'umbrella' vibration (because it looks like an umbrella opening and closing) of the methyl group on the alkyl side of the FAME. As you can see, the atoms close to this group also move a little, but the carbonyl group and the methyl group on the ester side barely move at all. So this is a more localised vibration than those above. It occurs at 1393 cm-1 and is both I.R. and Raman active. |
This is the umbrella vibration at the other end of the molecule. Again, you can see that the further along the alkyl chain you look, the less the atoms move so that by the time you look at the 6th carbon in the chain there is no movement at all. This vibration is found at slighly higher wavenumber (1444 cm-1) and is a much stronger peak in both I.R. and Raman spectra. |
In this case the vibration (occurring at 1457 cm-1) is a scissor type motion and involves most of the CH2 units in the molecule as well as both methyl groups.Although it is both I.R. and Raman active is appears as a very weak peak in I.R. spectrum but a very strong peak in the Raman spectrum. |
Section 3; the area at ca. 1735 cm-1. Only one vibration is found here.
Animation of Vibration |
This is the carbonyl C=O stretch found at ca.1735 cm-1 This shows the true power of Group Frequency Tables since the appearance of a vibrational band in this position informs you that a C=O bond is present. Again, the vibrational motion is fairly well localised on the C=O bond itself and movement of other atoms falls off rapidly as the distance from the carbonyl is increased. |
