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Infra-red radiation is electromagnetic radiation with a wavelength longer than red radiation. It occupies the region of the wavelength spectrum from 700 nm to 1000 nm. The instrumentation used to analyse the effect of infra-red (IR) radiation on substances is called an infrared (IR) spectrometer. |
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Infra-red radiation
Similar in character to ordinary light only with a longer wavelength and lower energy and frequency. It is sometimes referred to as heat radiation.
The electromagnetic spectrum has rather arbitary labels assigned. The radiation with longer wavelength than visible light is called infra-red and the radiation with shorter wavelength than visible light is called ultra-violet.
IR radiation covers the electromagnetic spectrum region with a wavelength range from 1 x 10-3 metres to visible light, 7 x 10-7 metres.
Quantum theory
The microscopic world of particles is governed by laws that seem to be in conflict with our everyday experience of the world. To our limited powers of observation energy appears to be continuous, after all, a car can have any velocity from zero to its maximum. However, this only 'seems' to be that case, as we are incapable of measuring the actual stepwise absorption of energy, it's too small.
In reality, all energy changes are stepwise, this is called quantum theory.
The energy needed to change from one state to the next is called a quantum of energy.
All motion is a form of energy and molecules vibrate, rotate and translate (move from one location to another). Molecules, and the relative positions of the atoms within the molecules are in constant motion. The consequence is that these dipoles produce magnetic fields that can interact with electromagnetic radiation.
Only certain vibrations are allowed and the difference between the energy of each vibrational or rotational state corresponds to an exact quantum of energy, no more no less.
The interaction between electromagnetic radiation and matter
Electromagnetic radiation consists of an oscillating electrical field and a corresponding magnetic field (at right angles) propagating through space at the speed of light. The radiation carries energy that is directly proportional to the frequency and inversely proportional to the wavelength.
Energy = Planck's constant x Frequency
Planck's constant = 6.63 x 10-34 J s
If electromagnetic radiation carries the same energy as the difference between two quantum levels of any kind it can be absorbed, changing the quantum state of the absorber in the process.
IR radiation has just the correct energy to change the quantum levels of molecular vibrations. It can only do so, however, if this causes a change in the overall polarity of the molecule.
Care must be taken with non-polar molecules. These can also absorb IR radiation providing that doing so causes a change in polarity. This is the case, for example, of carbon dioxide. However, symmetrical molecules such as oxygen cannot absorb IR radiation, they are said to be IR inactive.
The frequency of IR radiation absorbed by a molecule depends on the bond strengths and angles. The total nuimber of absorptions depends on the number of different vibrational modes available.
The IR 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 absorptions 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 convention 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 |
Analysing IR spectra
IR spectra are not particularly easy to analyse, nor do they give definitive information about structure. There are however, two different stages in an analysis.
- 1 Identification of absorptions
- 2 Fingerprinting
The first stage involves looking for characteristic absorptions and attempting to ascribe them to specific structural features. The second stage is usually carried out after a series of analyses leads to a possible conclusion.
Identifying absorptions
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.
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 C
N
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 CN.
Bond identification
Different bonds give rise to different stretching and bending vibrational frequencies, 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.
Bonds are not exactly the same strength in different types of molecule, so there is some variation in the absorbed frequency of radiation.
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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
Advantages and limitations of IR spectrometry
Advantages
IR spectrometry is fast and non-destructive. It provides information about possible structural features of an unknown compound.
It is used as a supplement to other analytical techniques. 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 known compound is constructed using a new synthetic method, achievement of the 'correct' IR spectrum is evidence that the synthesis has been successful.
Limitations
It is very difficult to assign specific absorptions to definite bonds.
