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The alert successfully saved. The alert did not successfully save. Please try again later. Citation Only. A very high absorption of this sort tells you important things about the bonds in the compound.
A graph is produced showing how the percentage transmittance varies with the frequency of the infra-red radiation. Notice that an unusual measure of frequency is used on the horizontal axis. Wavenumber is defined like this:. Similarly, don't worry about the change of scale half-way across the horizontal axis. You will find infra-red spectra where the scale is consistent all the way across, infra-red spectra where the scale changes at around cm -1 , and very occasionally where the scale changes again at around cm As you will see when we look at how to interpret infra-red spectra, this does not cause any problems - you simply need to be careful reading the horizontal scale.
Each frequency of light including infra-red has a certain energy. If a particular frequency is being absorbed as it passes through the compound being investigated, it must mean that its energy is being transferred to the compound. In covalent bonds, atoms aren't joined by rigid links - the two atoms are held together because both nuclei are attracted to the same pair of electrons.
The two nuclei can vibrate backwards and forwards - towards and away from each other - around an average position. The diagram shows the stretching that happens in a carbon-oxygen single bond. There will, of course, be other atoms attached to both the carbon and the oxygen. For example, it could be the carbon-oxygen bond in methanol, CH 3 OH. The energy involved in this vibration depends on things like the length of the bond and the mass of the atoms at either end.
That means that each different bond will vibrate in a different way, involving different amounts of energy. Bonds are vibrating all the time, but if you shine exactly the right amount of energy on a bond, you can kick it into a higher state of vibration. The amount of energy it needs to do this will vary from bond to bond, and so each different bond will absorb a different frequency and hence energy of infra-red radiation. As well as stretching, bonds can also bend.
The diagram shows the bending of the bonds in a water molecule. The effect of this, of course, is that the bond angle between the two hydrogen-oxygen bonds fluctuates slightly around its average value. Imagine a lab model of a water molecule where the atoms are joined together with springs.
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