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Infrared spectroscopy (4.2.4)
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Infrared (IR) spectroscopy is an analytical technique that enables chemists to identify vibrations from covalent bonds within molecules.
This is useful for working out the characteristic functional groups within organic compounds.
Vibrational movement of covalent bonds exists as two main types: bond stretching, and bond bending.
Each vibration is associated with a particular wavelength and frequency. When exposed to IR radiation the bonds absorb radiation at their characteristic wavelength and vibrate more vigorously.
Vibrational absorptions are seen where there are inverted peaks; in these regions there is a reduction in the radiation able to pass through the sample.
The deeper the peak the more radiation has been absorbed. These prominent peaks are most distinctive, and therefore most useful in characterisation.
The IR spectrum for propanal is shown below.
It has a prominent peak around 1700 cm-1 which is characteristic of the carbonyl functional group present in .
IR radiation is only absorbed by covalent bonds that have a permanent dipole, such as , and . The dipole does not need to be large but it must exist.
Vibrations from bonds without a permanent dipole, such as or , are invisible in IR spectrometry.
Non-polar molecules containing polar bonds, such as linear , have a combination of IR visible and invisible vibrating modes dependent on symmetry.
The most abundant atmospheric gases, and , show no change in dipole moment and do not interact with infrared radiation.
Some atmospheric gases do absorb IR radiation; these include , , and .
These gases are known as greenhouse gases.
When solar radiation enters the earth’s atmosphere, it is absorbed by the earth’s surface and oceans before being re-emitted at longer wavelengths, mainly as IR radiation.
The increased presence of atmospheric gases that interact with IR radiation cause a greater proportion of this radiation to be held within the atmosphere rather than entering space. This causes heating of the atmosphere, and scientists refer to this as global warming.
Human activity, most notably the extraction and combustion of fossil fuels, have led to much greater levels of atmospheric in recent history.
Increased reliance on renewable energy sources with reduced production such as nuclear, wind, wave, solar, and tidal power, alongside using carbon capture and storage methods should slow greenhouse gas production and global warming.
Different functional groups present in an organic compound can be identified from their IR stretching peaks.
A basic outline of the main groups and their characteristics absorption frequencies in IR is as shown below:
You will not be required to memorise this data.
The presence of the bond stretching is easy to locate, though of limited use in distinguishing one organic compound from another.
A strong, broad peak is characteristic of bond stretching in alcohols.
This notable peak is located just to the left of the landmark bond peak and is very popular in exam questions.
Be careful as the bond, although a weaker signal, is located in the same region as a bond.
The bond stretching, found in aldehydes, ketones, and other carbonyl-containing compounds is very easy to recognise as a strong narrow peak, almost in the middle of the spectrum around .
More details are required to fully identify the functional group.
The spectrum for the carboxylic acid group contains a stretch at and the very broad and strong peak of the group in carboxylic acids from .
Its breadth is due to hydrogen bonding between carboxylic acid groups causing variation in the vibration.
The carboxylic acid peak overlaps the bond peak, which can often be seen just below the broad carboxylic acid peak.
If there was a narrow peak and a peak this would indicate an alcohol and a carbonyl were present, not a carboxylic acid.
In amine’s, bond stretching appears as a characteristic peak in the IR spectrum, typically between .
This peak is often medium in intensity and can be either sharp or broad, depending on whether the amine is primary or secondary. It is significantly less intense than the of an alcohol, but is in the same region.
Most exam questions present an infrared spectrum and ask for the identity of the compound, or some structural features, deducible from the data.
Using the supplied data sheet spectroscopy table is essential here but a good understanding of the key peaks below will speed up your analysis.
IR spectra are most commonly assessed in combination with other techniques such as nuclear magnetic resonance (NMR) and mass spectrometry.
The absence of particular peaks from a spectrum can also help to confirm the identity of a functional group.
A commonly tested scenario is analysis of an alcohol oxidation. This involves distinguishing unreacted alcohol, aldehydes, ketones, and/or carboxylic acids.
Here we need to look for the absence, or presence, of the carboxylic acid’s broad peak. Likewise, the absence or presence of an alcohol peak. This will give a definitive answer on the presence of a carboxylic acid or an unreacted alcohol.
Aldehydes and ketones require additional data to identify. The IR spectra could be combined with NMR, mass spectrometry or qualitative tests such as using the Tollens reagent.
Infrared spectroscopy is used to identify and monitor levels of airborne pollutants and other volatile compounds that are not normally present at significant levels within the Earth’s atmosphere.
Molecules indicative of atmospheric pollution include unburnt hydrocarbon fragments with and bonds, combustion products such as and
Infrared has several specific uses in the fields of forensic investigation, including:
- Detection of accelerant residues from headspace sampling of fire-scene debris looking for evidence of arson.
- Matching components from vehicle paint to samples from road collisions.
- Analysis of breath samples for ethanol content from suspected drink drive cases.
Infrared analysis is a non-destructive technique so criminal evidence is not lost.