Analytical Chemistry - Infrared Spectroscopy

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I’ve been covering infrared spectroscopy recently with one of my A level classes, and realised that I haven’t really come across an aesthetically appealing reference chart for the frequencies of absorption – which seemed like as good an excuse as any to make one myself. So, here it is! Now, if you’re not a chemist, you may well be wondering what on earth IR spectroscopy is, so I’ve put together a brief explanation below.

In general, spectroscopy is the study of the interaction between light and matter. Infrared spectroscopy is a particular technique that can be used to help identify organic (carbon-based) compounds. Visible light is just a portion of the electromagnetic spectrum, and it’s the infrared section of the spectrum that’s utilised in this technique. It works by shining infrared light through the organic compound we want to identify; some of the frequencies are absorbed by the compound, and if we monitor the light that makes it through, the exact frequencies of the absorptions can be used to identify specific groups of atoms within the molecules.

That, then, is the simple explanation – but why do organic compounds absorb some of the frequencies in the first place? To explain that, we need to discuss chemical bonds in a little more detail. Chemical bonds aren’t rigid, immovable sticks; rather, they’re flexible, and are capable of both stretching and bending. In fact, they’re always in motion: the bonds vibrate, and they can absorb light of an energy comparable to this vibration. This absorption leads to it jumping to an ‘excited’ vibrational state.

We can spot these absorptions using a detector, which will record how much of the infrared light makes it through the compound. Some frequencies will pass through completely unabsorbed, whilst others will experience significant absorption as a result of the particular chemical bonds in the molecules. This leads to an outputted spectrum like the one below:

ethanol

The troughs in the spectrum are caused by the absorption of infrared frequencies by chemical bonds – often, these are characteristic of particular combinations of atoms, or functional groups. For example, in the spectrum above, the wide absorption on the left-hand side is caused by the presence of an O-H bond. The graphic shows several other characteristic frequencies of absorption, and the bonds that they are associated with. They allow chemists to identify features of chemical compounds, or, in combination with other spectroscopic methods, discern the precise structure of the compound.

This is just the briefest of overviews on IR spectroscopy; far more detail is offered by the links below.

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References & Further Reading