INFRA-RED SPECTRUM

WHAT IS AN INFRA-RED SPECTRUM? This page describes what an infra-red spectrum is and how it arises from bond vibrations within organic molecules. The background to infra-red spectroscopy How an infra-red spectrum is produced You probably know that visible light is made up of a continuous range of different electromagnetic frequencies - each frequency can be seen as a different colour. Infra-red radiation also consists of a continuous range of frequencies - it so happens that our eyes can't detect them. If you shine a range of infra-red frequencies one at a time through a sample of an organic compound, you find that some frequencies get absorbed by the compound. A detector on the other side of the compound would show that some frequencies pass through the compound with almost no loss, but other frequencies are strongly absorbed. How much of a particular frequency gets through the compound is measured as percentage transmittance. A percentage transmittance of 100 would mean that all of that frequency passed straight through the compound without any being absorbed. In practice, that never happens - there is always some small loss, giving a transmittance of perhaps 95% as the best you can achieve. 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 infra-red spectrum looks like A graph is produced showing how the percentage transmittance varies with the frequency of the infra-red radiation.
	Note:  The infra-red spectra on this page have been produced from graphs taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan. It is possible that small errors may have been introduced during the process of converting them for use on this site, but these won't affect the argument in any way.
Notice that an unusual measure of frequency is used on the horizontal axis. Wavenumber is defined like this: Don't worry about this - just accept it! Similarly, don't worry about the change of scale half-way across the horizontal axis. You will find infra-red spectra where the scale is consistent all the way across, infra-red spectra where the scale changes at around 2000 cm-1, and very occasionally where the scale changes again at around 1000 cm-1. As you will see when we look at how to interpret infra-red spectra, this doesn't cause any problems - you simply need to be careful reading the horizontal scale. What causes some frequencies to be absorbed? Each frequency of light (including infra-red) 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 infra-red 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 - towards and away from each other - around an average position. The diagram shows the stretching that happens in a carbon-oxygen single bond. There will, of course, be other atoms attached to both the carbon and the oxygen. For example, it could be the carbon-oxygen bond in methanol, CH3OH. 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 amount 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 hence energy) of infra-red radiation. Bond bending As well as stretching, bonds can also bend. The diagram shows the bending of the bonds in a water molecule. The effect of this, of course, is that the bond angle between the two hydrogen-oxygen bonds fluctuates slightly around its average value. Imagine a lab model of a water molecule where the atoms are joined together with springs. These bending vibrations are what you would see if you shook the model gently. Again, bonds will be vibrating like this all the time and, again, 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 infra-red radiation in order to make this jump from one state to a higher one. Tying all this together Look again at the infra-red spectrum of propan-1-ol, CH3CH2CH2OH: In the diagram, three sample absorptions are picked out to show you the bond vibrations which produced them. Notice that bond stretching and bending produce different troughs in the spectrum.
	THE FINGERPRINT REGION OF AN INFRA-RED SPECTRUM This page explains what the fingerprint region of an infra-red spectrum is, and how it can be used to identify an organic molecule. Note:  It would be helpful if you first read the introductory page on infra-red spectra if you haven't already done so. What is the fingerprint region This is a typical infra-red spectrum: Note:  The infra-red spectra on this page have been produced from graphs taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan. It is possible that small errors may have been introduced during the process of converting them for use on this site, but these won't affect the argument in any way. Each trough is caused because energy is being absorbed from that particular frequency of infra-red 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 side of the spectrum is used to identify the presence of an oxygen-hydrogen bond in an -OH group. Note:  Using troughs in this way to identify particular bonds is covered on a separate page. INTERPRETING AN INFRA-RED SPECTRUM This page explains how to use an infra-red spectrum to identify the presence of a few simple bonds in organic compounds. Note:  This page follows directly on from the introductory page on infra-red spectra. If you haven't already done so, you should read that page before you go on. The infra-red spectrum for a simple carboxylic acid Ethanoic acid Ethanoic acid has the structure: You will see that it contains the following bonds: carbon-oxygen double, C=O carbon-oxygen single, C-O oxygen-hydrogen, O-H carbon-hydrogen, C-H carbon-carbon single, C-C The carbon-carbon bond has absorptions which occur over a wide range of wavenumbers in the fingerprint region - that makes it very difficult to pick out on an infra-red spectrum. The carbon-oxygen single bond also has an absorbtion in the fingerprint region, varying between 1000 and 1300 cm-1 depending on the molecule it is in. You have to be very wary about picking out a particular trough as being due to a C-O bond. Note:  If you aren't sure what the fingerprint region is, follow this link before you go on. The other bonds in ethanoic acid have easily recognised absorptions outside the fingerprint region. The C-H bond (where the hydrogen is attached to a carbon which is singly-bonded to everything else) absorbs somewhere in the range from 2853 - 2962 cm-1. Because that bond is present in most organic compounds, that's not terribly useful! What it means is that you can ignore a trough just under 3000 cm-1, because that is probably just due to C-H bonds. The carbon-oxygen double bond, C=O, is one of the really useful absorptions, found in the range 1680 - 1750 cm-1. Its position varies slightly depending on what sort of compound it is in. The other really useful bond is the O-H bond. This absorbs differently depending on its environment. It is easily recognised in an acid because it produces a very broad trough in the range 2500 - 3300 cm-1. Note:  You will never have to remember where these absorptions occur. You will always be given this data in an A'level exam. The infra-red spectrum for ethanoic acid looks like this: The possible absorption due to the C-O single bond is queried because it lies in the fingerprint region. You couldn't be sure that this trough wasn't caused by something else. Note:  The infra-red spectra on this page have been produced from graphs taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan. It is possible that small errors may have been introduced during the process of converting them for use on this site, but these won't affect the argument in any way. The infra-red spectrum for an alcohol Ethanol The O-H bond in an alcohol absorbs at a higher wavenumber than it does in an acid - somewhere between 3230 - 3550 cm-1. In fact this absorption would be at a higher number still if the alcohol isn't hydrogen bonded - for example, in the gas state. All the infra-red spectra on this page are from liquids - so that possibility will never apply. Notice the absorption due to the C-H bonds just under 3000 cm-1, and also the troughs between 1000 and 1100 cm-1 - one of which will be due to the C-O bond. The infra-red spectrum for an ester Ethyl ethanoate This time the O-H absorption is missing completely. Don't confuse it with the C-H trough fractionally less than 3000 cm-1. The presence of the C=O double bond is seen at about 1740 cm-1. The C-O single bond is the absorption at about 1240 cm-1. Whether or not you could pick that out would depend on the detail given by the table of data which you get in your exam, because C-O single bonds vary anywhere between 1000 and 1300 cm-1 depending on what sort of compound they are in. Some tables of data fine it down, so that they will tell you that an absorption from 1230 - 1250 is the C-O bond in an ethanoate. The infra-red spectrum for a ketone Propanone You will find that this is very similar to the infra-red spectrum for ethyl ethanoate, an ester. Again, there is no trough due to the O-H bond, and again there is a marked absorption at about 1700 cm-1 due to the C=O. Confusingly, there are also absorptions which look as if they might be due to C-O single bonds - which, of course, aren't present in propanone. This reinforces the care you have to take in trying to identify any absorptions in the fingerprint region. Aldehydes will have similar infra-red spectra to ketones. The infra-red spectrum for a hydroxy-acid 2-hydroxypropanoic acid (lactic acid) This is interesting because it contains two different sorts of O-H bond - the one in the acid and the simple "alcohol" type in the chain attached to the -COOH group. The O-H bond in the acid group absorbs between 2500 and 3300, the one in the chain between 3230 and 3550 cm-1. Taken together, that gives this immense trough covering the whole range from 2500 to 3550 cm-1. Lost in that trough as well will be absorptions due to the C-H bonds. Notice also the presence of the strong C=O absorption at about 1730 cm-1. The infra-red spectrum for a primary amine 1-aminobutane Primary amines contain the -NH2 group, and so have N-H bonds. These absorb somewhere between 3100 and 3500 cm-1. That double trough (typical of primary amines) can be seen clearly on the spectrum to the left of the C-H absorptions.