We can spot these absorptions using a detector, which will record how much of the infrared light makes it through the compound. This absorption leads to it jumping to an ‘excited’ vibrational state. In fact, they’re always in motion: the bonds vibrate, and they can absorb light of an energy comparable to this vibration. Chemical bonds aren’t rigid, immovable sticks rather, they’re flexible, and are capable of both stretching and bending. 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. 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. 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. Infrared spectroscopy is a particular technique that can be used to help identify organic (carbon-based) compounds. In general, spectroscopy is the study of the interaction between light and matter. 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. 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.
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