Infrared spectroscopy
From Free net encyclopedia
Image:IR spectrum of ethanol.gif Infrared spectroscopy (IR Spectroscopy) is the subset of spectroscopy that deals with the Infrared part of the electromagnetic spectrum. This covers a range of techniques, with the most common type by far being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify a compound and to investigate the composition of a sample.
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Theory
The infrared portion of the electromagnetic spectrum is commonly divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The far-infrared, (approx. 400-10cm-1) lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The mid- infrared (approx. 4000-400cm-1) may be used to study the fundamental vibrations and associated rotational-vibational structure, whilst the higher energy near-IR (14000-4000cm-1) can excite overtone or harmonic vibrations.
Infrared spectroscopy works because chemical bonds have specific frequencies at which they vibrate corresponding to energy levels. The resonant frequencies or vibrational frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, eventually by the associated vibronic coupling. In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. In particular, in the Born-Oppenheimer and harmonic approximations, i.e. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are determined by the normal modes corresponding to the molecular electronic ground state potential energy surface. Nevertheless, the resonant frequencies can be in a first approach related to the strength of the bond, and the mass of the atoms at either end of it. Thus, the frequency of the vibrations can be associated with a particular bond type.
Simple diatomic molecules have only one bond, which may stretch. More complex molecules may have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. The atoms in a CH2 group, commonly found in organic compounds can vibrate in six different ways, symmetrical and asymmetrical stretching, scissoring, rocking, wagging and twisting; as shown below:
| Image:Symmetrical stretching.gif | Image:Asymmetrical stretching.gif |
| Image:Scissoring.gif | Image:Twisting.gif |
| Image:Wagging.gif | Image:Rocking.gif |
In order to measure a sample, a beam of infrared light is passed through the sample, and the amount of energy absorbed at each wavelength is recorded. This may be done by scanning through the spectrum with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all wavelengths at once. From this, a transmittance or absorbance spectrum may be plotted, which shows at which wavelengths the sample absorbs the IR, and allows an interpretation of which bonds are present.
This technique works almost exclusively on covalent bonds, and as such is of most use in organic chemistry. Clear spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. The technique has been used for the characterization of very complex mixtures however.
Sample preparation
Gaseous samples require little preparation beyond purification, but a sample cell with a long pathlength (typically 5-10cm) is used as gases show relatively weak absorbances.
Liquid samples can be sandwiched between two plates of a high purity salt (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used). The plates are transparent to the infrared light and will not introduce any lines onto the spectra. Some salt plates are highly soluble in water, and so the sample, washing reagents and the like must be anhydrous (without water).
Solid samples can be prepared in two major ways. The first is to crush the sample with a mulling agent (usually nujol) in a marble pestle and mortar. If the solid can be induced to dissolve, or at least be crushed into a very fine powder, then the results will be good.
The second method is to mix a quantity of the sample with a specially purified salt (usually potassium bromide). This powder mixture is then crushed in a pellet press in order to form a pellet through which the beam of the spectrometer can pass. This pellet must be crushed to high pressures in order to ensure that the pellet is translucent, but this can be achieved without powered machinery. Potassium bromide does not absorb infrared light, so spectral lines will only appear from the analyte.
Typical method
Image:IR spectroscopy apparatus.jpeg A beam of infra-red light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained.
A reference is used for two reasons:
- This prevents fluctuations in the output of the source affecting the data
- This allows the effects of the solvent to be cancelled out (the reference is usually a pure form of the solvent the sample is in)
Summary of absorptions of bonds in organic molecules
Absorptions listed in wavenumbers. Image:IR summary version 2.gif
Detailed absorptions table of bonds in organic molecules
| Bond | Type of bond | Specific type of bond | Absorption range and intensity |
|---|---|---|---|
| C-H | alkyl | methyl | 1380 cm-1 (weak), 1260 cm-1 (strong) and 2870, 2960 cm-1 (both strong to medium) |
| methylene | 1470 cm-1 (strong) and 2850, 2925 cm-1 (both strong to medium) | ||
| methyne | 2890 cm-1 (weak) | ||
| vinyl | C=CH2 | 900 cm-1 (strong) and 2975, 3080 cm-1 (medium) | |
| C=CH | 3020 cm-1 (medium) | ||
| monosubstituted alkenes | 900, 990 cm-1 (both strong) | ||
| cis-disubstituted alkenes | 670-700 cm-1 (strong) | ||
| trans-disubstituted alkenes | 965 cm-1 (strong) | ||
| trisubstituted alkenes | 800-840 cm-1 (strong to medium) | ||
| aromatic | benzene/sub. benzene | 3070 cm-1 (weak) | |
| monosubstituted benzene | 700-750 cm-1 (strong) and 700±10 cm-1 (strong) | ||
| ortho-disub. benzene | 750 cm-1 (strong) | ||
| meta-disub. benzene | 750-800 cm-1 (strong) and 860-900 cm-1 (strong) | ||
| para-disub. benzene | 800-860 cm-1 (strong) | ||
| alkynes | 3300 cm-1 (medium) | ||
| aldehydes | 2720, 2820 cm-1 (medium) | ||
| C-C | acyclic C-C | monosub. alkenes | 1645 cm-1 (medium) |
| 1,1-disub. alkenes | 1655 cm-1 (medium) | ||
| cis-1,2-disub. alkenes | 1660 cm-1 (medium) | ||
| trans-1,2-disub. alkenes | 1675 cm-1 (medium) | ||
| trisub., tetrasub. alkenes | 1670 cm-1 (weak) | ||
| conjugated C-C | dienes | 1600, 1650 cm-1 (strong) | |
| with benzene ring | 1625 cm-1 (strong) | ||
| with C=O | 1600 cm-1 (strong) | ||
| aromatic C=C | 1450, 1500, 1580, 1600 cm-1 (strong to weak) - always ALL 4! | ||
| triple C-C | terminal alkines | 2100-2140 cm-1 (weak) | |
| disubst. alkines | 2190-2260 cm-1 (very weak, sometimes not visible) | ||
| C=O | aldehyde/ketone | saturated aliph./cyclic 6-membered | 1720 cm-1 |
| α,β-unsaturated | 1685 cm-1 (goes for aromatic ketones as well) | ||
| cyclic 5-membered | 1750 cm-1 | ||
| cyclic 4-membered | 1775 cm-1 | ||
| aldehydes | 1725 cm-1 (influence of conjugation like with ketones) | ||
| carboxylic acids/derivates | saturated carboxylic acids | 1710 cm-1 | |
| unsat./aromatic carb. acids | 1680-1690 cm-1 | ||
| esters and lactones | 1735 cm-1 (influence of conjugation and ring size like with ketones) | ||
| anhydrides | 1760 and 1820 cm-1 (both!) | ||
| halogenides | 1800 cm-1 | ||
| amides | 1650 cm-1 (associated amides) | ||
| carboxylates (salts) | 1550-1610 cm-1 (goes for aminoacid zwitterions as well) | ||
| O-H | alcohols, phenols | 3610-3670 cm-1 (concentrating samples broadens the band and moves it to 3200-3400 cm-1) | |
| carboxylic acids | 3500-3560 cm-1 (concentrating samples broadens the band and moves it to 3000 cm-1) | ||
| N-H | primary amines | doublet between 3400-3500 cm-1 and 1560-1640 cm-1 (strong) | |
| secondary amines | above 3000 cm-1 (medium to weak) | ||
| ammonium ions | broad bands with multiple peaks between 2400-3200 cm-1 | ||
| C-O | alcohols | primary | 1050±10 cm-1 |
| secondary | around 1100 cm-1 | ||
| tertiary | 1150-1200 cm-1 | ||
| phenoles | 1200 cm-1 | ||
| ethers | aliphatic | 1120 cm-1 | |
| aromatic | 1220-1260 cm-1 | ||
| carboxylic acids | 1250-1300 cm-1 | ||
| esters | 1100-1300 cm-1 (two bands - distinction to ketones, which do not possess C-O!) | ||
| C-N | aliphatic amines | 1020-1220 cm-1 (often overlapped) | |
| C=N | 1615-1700 cm-1 (similar conjugation effects to C=O) | ||
| nitriles (triple C-N bond) | 2210-2260 cm-1 (unconjugated 2250, conjugated 2230 cm-1) | ||
| isonitriles (R-N-C bond) | 2165-2110 cm-1 (2140 - 1990 cm-1 for R-N=C=S) | ||
| C-X (X=F, Cl, Br, I) | fluoroalkanes | ordinary | 1000-1100 cm-1 |
| trifluromethyl | two strong, broad bands between 1100-1200 cm-1 | ||
| chloroalkanes | 540-760 cm-1 (medium to weak) | ||
| bromoalkanes | below 600 cm-1 | ||
| iodoalkanes | below 600 cm-1 | ||
| N-O | nitro compounds | aliphatic | 1550 cm-1 (stronger band) and 1380 cm-1 (weaker band) - ALWAYS BOTH! |
| aromatic | 1520, 1350 cm-1 (conjugation usually lowers the wave number) |
The absorptions in this range do not apply only to bonds in organic molecules. IR spectroscopy is useful when it comes to analysis of inorganic compounds (such as metal complexes or fluoromanganates) as well.
Uses and applications
Techniques have been developed to assess the quality of tea-leaves using infrared spectroscopy. This will mean that highly trained experts (also called 'noses') can be used more sparingly, at a significant cost saving.
Infrared spectroscopy is widely used in both research and industry as a simple and reliable technique for measurement, quality control, and dynamic measurement. The instruments are now small, and can be transported, even for use in field trials. With increasing technology in computer filtering and manipulation of the results, samples in solution can now be measured accurately (water produces a broad absorbance across the range of interest, and thus renders the spectra unreadable without this computer treatment). Some machines will also automatically tell you what substance is being measured from a store of thousands of reference spectra held in storage.
By measuring at a specific frequency over time, changes in the character or quantity of a particular bond can be measured. This is especially useful in measuring the degree of polymerization in polymer manufacture. Modern research machines can take infrared measurements across the whole range of interest as frequently as 32 times a second. This can be done whilst simultaneous measurements are made using other techniques. This makes the observations of chemical reactions and processes quicker, more accurate, and more
Fourier transform Infrared spectroscopy
Fourier transform infrared (FTIR) spectroscopy is a measurement technique for collecting infrared spectra. Instead of recording the amount of energy absorbed when the frequency of the infra-red light is varied (monochromator), the IR light is guided through an interferometer. After passing the sample the measured signal is the interferogram. Performing a mathematical Fourier Transform on this signal results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy.
FTIR spectrometers are cheaper than conventional spectrometers because building of interferometers is easier than the fabrication of a monochromator. In addition, measurement of a single spectrum is faster for the FTIR technique because the information of all frequencies is collected simultaneously. This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. Because of its various advantages, virtually all modern infrared spectrometers are of the FTIR variety.
See also
- Fourier transform spectroscopy
- Near infrared spectroscopy
- Vibrational spectroscopy
- Rotational spectroscopy
- Spectroscopy
- Quantum vibration
- Raman spectroscopy
External links
- Tutorial
- The Science of Spectroscopy - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.
- Spectral calculator - Quickly and easily calculate and plot transmission or radiance spectra.
- High-resolution transmission molecular absorption database (HITRAN) browswer - Browse the HITRAN 2004 database , plot absorption lines by position or intensity.
- A useful gif animation of different vibrational modes: hereda:IR spektrometer
de:IR-Spektroskopie es:Espectroscopia infrarroja he:גלאי אינפרא אדום פסיבי ja:赤外分光法 nl:Infraroodspectroscopie pl:Spektroskopia IR pt:Espectroscopia de infravermelho