Infrared Spectroscopy
Technique Primer
Infrared Spectroscopy The IR spectrum of a compound reveals stretching and bending modes within the molecule. Conveniently, certain functionalities give rise to characteristic IR stretches (Figure 3). For example, -1 the O-H stretch occurs at around 3300 cm , the C=O -1 stretch at around 1740 cm , and the C=C stretch at -1 around 1600 cm . Therefore, compounds can be characterized by diagnostic absorbances, such as the carbonyl stretch in acetone (Figure 1), and reactions can be followed by observing the disappearance of one band and the appearance of another.
C=O stretch
Figure 1. IR spectrum of acetone (image from the Spectral Database for Organic Compounds) Samples for IR spectroscopy (Figure 2) are easily prepared by sandwiching a small amount of pure sample between two plates of sodium chloride, which is largely transparent to IR radiation.
Figure 3. Functional group regions in the infrared spectrum
Alternatively, a thin film can be deposited on a single plate by slowly dripping an organic solution of the compound onto the surface, allowing the solvent to evaporate.
Figure 2. IR Sample preparation using NaCl plates (image from organicchem.org) Of course, sodium chloride is soluble in water, so it is important to protect the plates from any sources of water. All washing and cleaning should be done with methylene chloride (or other similar solvent), and care should be taken to ensure that the sample is properly dried. Otherwise, the plates quickly become clouded and useless. Please note that infrared spectroscopy is non-destructive, which means you can recover your entire sample if desired.
Infrared Spectroscopy The IR spectrum of a compound reveals stretching and bending modes within the molecule. Conveniently, certain functionalities give rise to characteristic IR stretches (Figure 3). For example, -1 the O-H stretch occurs at around 3300 cm , the C=O -1 stretch at around 1740 cm , and the C=C stretch at -1 around 1600 cm . Therefore, compounds can be characterized by diagnostic absorbances, such as the carbonyl stretch in acetone (Figure 1), and reactions can be followed by observing the disappearance of one band and the appearance of another.
C=O stretch
Figure 1. IR spectrum of acetone (image from the Spectral Database for Organic Compounds) Samples for IR spectroscopy (Figure 2) are easily prepared by sandwiching a small amount of pure sample between two plates of sodium chloride, which is largely transparent to IR radiation.
Figure 3. Functional group regions in the infrared spectrum
Alternatively, a thin film can be deposited on a single plate by slowly dripping an organic solution of the compound onto the surface, allowing the solvent to evaporate.
Figure 2. IR Sample preparation using NaCl plates (image from organicchem.org) Of course, sodium chloride is soluble in water, so it is important to protect the plates from any sources of water. All washing and cleaning should be done with methylene chloride (or other similar solvent), and care should be taken to ensure that the sample is properly dried. Otherwise, the plates quickly become clouded and useless. Please note that infrared spectroscopy is non-destructive, which means you can recover your entire sample if desired.
