Before we get into infrared spectroscopy, it’s probably best to know what a spectrometer is. Well, it’s an instrument that is used to pass infrared radiation through a sample in order to measure how much of the radiation is absorbed.
The spectrometer has two beams which are split by rotating mirrors. One beam will go through the sample with the other passing through nothing. And the difference can then be measured to show the amount of radiation absorbed – so far so good!
When this is displayed on a graph it is known as the infrared spectrum. On the spectrum we use
cm-1 or wavenumbers as units, as this gives us values that are fairly sensible in terms of scale: numbers are in the 1000s rather than in the 1,000,000,000,000,000s etc.
We can also use infrared to detect impurities that can cause problems in more complicated syntheses.
Our infrared spectrum is of the whole sample, so we can see impurities if there are peaks that shouldn’t be there.
An example would be a C=O stretch when our sample is an alkane. This could come from carbon dioxide, which can be present if we breath on the sample while the machine is running a test – not good!
The covalent bonds present in the sample absorb only the frequencies that match their natural frequencies. Thus the frequencies that hit the detector are those that do not match any natural frequency in the molecule.
If we are looking at an infrared region below 1500 cm-1 this is known as the fingerprint region.
We can overlay the fingerprint region of a known spectrum with the one we have collected. This is a very accurate way of finding the molecule, and is even a way of finding whether we have for example butan-2-ol or butan-1-ol, ie specific enough to specify the position of the alcohol.