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Infrared (IR) Spectroscopy; - Coggle Diagram
Infrared (IR) Spectroscopy;
How an IR spectrum is produced;
Shine a range of IR frequencies through a sample of an organic compound, some frequencies get absorbed by the compound.
A detector on the other side of the compound detects which frequencies pass through the compound and which were absorbed.
How much of a particular frequency gets through the compound is measured as a
percentage transmittance.
A percentage transmittance of 100 would mean that all of the frequency passed straight through the compound without being absorbed.
This never happens, 95 is the highest you are realistically going to get.
A transmittance of only 5% would mean that nearly all of that particular frequency is absorbed by the compound. A very high absorption of this sort tells you important things about the bonds in the compound.
What an IR spectrum looks like;
A graph is produced showing how the percentage transmittance varies with the frequency of the IR radiation.
x axis - wave number (cm^-1).
wave number = 1/wavelength in cm = cm^-1.
y axis - transmittance (%).
What causes some frequencies to be absorbed;
Each frequency of light (including IR) 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.
Energies in IR radiation correspond to the energies involved in bond vibrations.
Bond stretching;
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 around an average position.
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 about 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 therefore energy) of IR radiation.
Bond bending;
Bonds can also bend. Due to bending atoms often fluctuate slightly around their average value.
The bonds will always be vibrating like this and if you shine exactly the right amount of energy on the bond, you can kick it into a higher state of vibration. Since the energies involved with the bending will be different for each kind of bond, each different bond will absorb a different frequency of IR radiation in order to make this jump from one state to a higher one.
The fingerprint region;
Each trough is caused because energy is being absorbed from that particular frequency of IR radiation to excite bonds in the molecule to a higher state of vibration - either stretching or bending.
Some of the troughs are easily used to identify particular bonds in a molecule. For example, the big trough at the left hand of the spectrum is used to identify the presence on an oxygen-hydrogen bond in an -OH group.
The region to the right-hand side of the diagram (from about 1500 to 500 cm^-1) usually contains a very complicated series of absorptions. These are mainly due to all manner of bending vibrations within the molecule. This is called the
fingerprint region.
It is much more difficult to pick out individual bonds in this region than it is in the "cleaner" region at higher wave numbers. The importance of the fingerprint region is that each different compound produces a different pattern of troughs in this part of the spectrum.
Using the fingerprint region;
The pattern in the fingerprint region is completely different and could therefore be used to identify the compound.
So - to positively identify an unknown compound, use its IR spectrum to identify what sort of compound it is by looking for specific bong adsorptions.
You can then compare the fingerprint region of its IR spectrum with known spectra measured under exactly the same conditions to found out which alcohol (or whatever) you had.
