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Analytical Chemistry

Analytical Chemistry – Infrared (IR) Spectroscopy

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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8 replies on “Analytical Chemistry – Infrared (IR) Spectroscopy”

We did loads of this in organic chem in my degree. It was one of my favourite things. 3rd year we had to do a synthesis and an analysis and the latter included IR and NMR spectra (the two are complementary). IR has the advantage of being quite cheap, esp compared to NMR. IR is incredibly useful for identifying organic molecules in practice.

We did in my undergrad studies too – remember regularly queuing for the IR machine towards the end of organic lab days! They’re quite fun to figure out, coupled with NMR data too. Speaking of which, I should also make one at some point for NMR!

Wow! Thanks for such a brilliant and clear IR table.

By the way, I am quite curious about what the compound of the IR figure of example is.
Here is my guess: the peak at 3358 cm^-1 and the peak at 1670 cm^-1 signify the presence of carboxylic acid group.
The peaks around 2900 cm^-1 signify the presence of alkane structure, maybe some presence of aromatic ring?!
The peaks in fingerprint area is hard to tell. Maybe refers to the substitution type of aromatic ring?
The peaks around 1400 signify the stretch of alkanes or aromatics. The peaks around 1000 signify the presence of stretch mode of C-O bond, e.g. ether group, carboxylic acid group, etc.
The signals from 600 to 800 cm^-1 may tell the substitution pattern of aromatic ring.

To sum up, the compound could be an alkane with carboxylic acid group, perhaps aromatic ring with some substitution, perhaps some ether group, etc.

Welcome for any idea and comment! 🙂

You’re welcome!

The IR spectrum in the example is actually just the spectrum for ethanol. The peak for carbonyls is much more pronounced than that visible here – the peaks at just below 3000 are merely due to C-H bonds.

The stuff in the fingerprint region, because there are so many possible groups that could contribute, is pretty hard to actually interpret. IR will usually be used in combination with other spectroscopic methods, such as NMR, which are more useful for determining precise structure.

Thanks for reply!

Wow, it is ethane! However, I would like to argue this is an ethanol with impurities containing carbonyl or double bond moiety. The proof is provided as follows.

(1) Comparison with IR spectrum of liquid ethanol (provided in this article and other web site http://www.chemguide.co.uk/analysis/ir/interpret.html ) and gas ethanol (provided by NIST website http://webbook.nist.gov/cgi/cbook.cgi?ID=C64175&Type=IR-SPEC&Index=0#IR-SPEC ), it is found that the peak around 1700 cm^-1 is smaller in gas ethanol spectrum. This phenomena implies that concentrations of impurities are higher in liquid ethanol.

(2) I’ve checked how ethanol is produced in wikipedia ( http://en.wikipedia.org/wiki/Ethanol ). There are mainly two ways. One is to produce it from hydration of ethylene and the other is from fermentation of sugars with yeast. Either way, some reactant may be retained at last. That’s why there is a peak around 1700 cm^-1, which may be from unreacted double bond (ethylene) or unreacted carbonyl group in sugars.

To sum up, I’m quite surprised by the presence of signal around 1700 cm^-1 in ethanol IR spectrum, and it is rationalized as having some impurities in the sample.

Welcome for any comment and idea!

if the ethanol used was 200 proof, then the presence of aromatics might be due to benzene, which is commonly used to dehydrate the azeotropic mixture of ethanol/water (95/5%). The benzene (as well as methanol) renders the 200 proof ethanol non-consumable which in turn cuts the tax rate compared to consumable alcohol (at least in the US).
Hard to say, though, since benzene would be <1%

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