3.4.11 Structure Determination - Infrared spectroscopy
Students should:
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Infrared double beam spectrometer
A spectrometer is an instrument used for passing IR radiation through a sample and measuring how much of the radiation is absorbed by the sample.
The IR spectrometer consists of a variable wavelength IR source, whose beam is split by rotating mirrors into two parallel beams. One beam passes through the sample, which is usually a liquid, or a mixture of a hydrocarbon 'solvent' ('Nujol') and a finely powdered solid, smeared between two discs of potassium chloride. The second, reference beam just passes through some empty potassium chloride discs.
Both beams then pass to the analyser, which, by the difference in intensity between the two beams, produces an output of frequency against radiation absorbed.
When this is displayed as a graph, it is known as an IR spectrum.
Infrared spectra
An infrared spectrum shows a series of abosorptions drawn on a graph type background. The y-axis is the transmittance and the x-axis is the frequency. Traditionally the frequency is shown as a 'wavenumber'. This is the reciprocal of the wavelength in centimetres. This convertion gives a frequency scale with conveniently sized numbers. The frequency is directly proportional to the energy of the electromagnetic radiation.
IR spectra usually range from 4000 to 400 cm-1.
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The energy of infrared radiation The high energy end of the scale 4000cm-1 = wavelength 0.00025 cm = 2500 nm The low energy end of the scale 400cm-1 = wavelength 0.0025 cm = 25000 nm |
Using IR spectra
Infra-red radiation has the correct frequency to interact with the bonds in molecules, when they are polar. These can absorb the radiation at specific wavelengths to change their vibrational states. As with all energy values at atomic and molecular level, these vibrations are quantised, i.e. energy absorption is not continuous, but occurs at frequencies corresponding to the difference in energy between two quantum states.
Vibration modes
Each bond type has a different absorption frequency and a scan over a range of frequencies shows absorptions corresponding to the bonds present in the molecule. In reality, the patterns produced in the spectra by IR absorptions are complex and only a few bond types can be identified, such as carbonyl groups C=O, and hydroxyl (alcohol) groups O-H. Example IR data
Bond strength
The bond strength of an individual bond depends on the electron density between the two nuclei being held together. Groups or atoms that withdraw electrons (electronegative) tend to reduce the electron density of neighbouring groups. Consequently, a bond stretch may absorb at high frequency in one molecule and at a lower frequency in a different molecule.
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Example: Compare the stretching frequencies of the carbonyl group in carboxylic acids and aldehydes. R-COOH, Carbonyl stretch = 1700 - 1725 cm-1 RCHO, Carbonyl stretch = 1710 - 1740 cm-1 |
Any molecular influence that increases the strength of a bond by increasing the electron density within the bond also increases the energy required to change vibrational states. More energy equates to higher frequency.
For example, a C=N double bond is about twice as strong as a C-N single bond,
and the CN triple bond is also stronger than the double bond. The infrared stretching
frequencies of these groups vary in the same order, ranging from 1100 cm-1 for
C - N, to 1660 cm-1 for C = N, to 2220 cm-1 for C 
N
Identification of bonds
Different bonds give rise to different stretching and bending vibrational frequencies and so a simple comparison of the absorptions recorded can be compared with known frequencies. This does not give definite identification of the molecule being analysed, but it does give clues as to the possible component parts of the molecule.
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Example: An IR spectrum shows a strong absorption at a frequency of between 1680 and 1750 cm-1. What information does this provide? This frequency is indicative of a carbonyl group, C=O. The reason for the range is because the strength of the C=O bond depends also on the atoms to which it is attached. |
The fingerprint region
Use is made of the complexity of the spectrum as no two compounds have exactly the same series of absorptions. This means that a complex region of the spectrum, known as the fingerprint region, can be used to compare an unknown substance with a database of known substances. If the unknown compound has a spectrum identical to a known spectrum, then a positive identification has been made. The fingerprint region appears at the right hand side of the diagram below between wavenumbers 500cm-1 - 1500cm-1.
The lower 'x' axis has units of cm-1. This is known as the wavenumber, it is the reciprocal of the wavelength in centimetres. This unit of measurement is traditional for IR spectra and is used for its convenient numbers. In the diagram a strong absorption can be seen at about 3000 cm-1 corresponding to the bond between the benzene ring and a hydrogen atom. IR data
Analytical procedure
Determine which functional groups you may have present by comparing your stretching frequencies with the values listed in the data booklet. Eliminate the compounds that don't match these functional groups.
Look for additional functional group information by looking specifically for peaks you would expect to find if that functional group were present (i.e. if an aldehyde is one possibility, look to see if you have an aldehyde C-H stretch, etc.)
If you suspect a specific compound you can then look up the IR spectrum to match the fingerprint regions.
Usefulness
Typically, IR is useful only for providing evidence for structural features within a molecule. It is used as a supplement to other analytical techniques. As mentioned above, the fingerprint of a molecule is practically unique and may be used for almost certain identification, if a database of similar structures is available.
Similarly, when a new compound is constructed using synthetic steps, achievement of the 'correct' IR spectrum is evidence that the synthesis has been successful.