Infrared Spectroscopy

Typically when a molecule is exposed to infra-red (IR) radiation, it absorbs specific frequencies of radiation. The frequencies which are absorbed are dependent upon the functional groups within the molecule and the symmetry of the molecule. IR radiation can only be absorbed by bonds within a molecule, if the radiation has exactly the right energy to induce a vibration of the bond. This is the reason only specific frequencies are absorbed.

Infrared spectroscopy focuses on electromagnetic radiation in the frequency range 400-4000cm-1, where cm-1 is known as wavenumber (1/wavelength), which is equivalent tofrequency. To generate the infrared spectrum, radiation containing all frequencies in the IR region is passed through the sample. Those frequencies which are absorbed appear as adecrease in the detected signal. This information is displayed as a spectrum of % transmitted radiation plotted against wavenumber.

Infrared spectroscopy is very useful for qualitative analysis (identification) of organic compounds because a unique spectrum is produced by every organic substance with peaks corresponding to distinct structural features. Also, each functional group absorbs infrared light at a unique frequency. For example, a carbonyl group, C=O, always absorbs infrared light at 1670-1780 cm-1, which causes the carbonyl bond to stretch.

A carbonyl group always absorbs infrared radiation in this frequency range because thebond between the carbon atoms is constantly stretching and contracting within a range of bond lengths. This “vibration” occurs as if the bond was a spring connecting the two atoms and it always occurs within a certain frequency range, 1670-1780 cm-1. When a molecule is irradiated with infrared radiation, a vibrating bond will absorb energy of the same frequency as its vibration, making the bond vibrate even faster.

In addition to identifying the molecule using IR spectroscopy, often the strength of the bonds within the molecule can be estimated by comparing their stretching ferquency. Just as with a spring, a large vibration frequency corresponds to a long bond, ie a weak bond and a small frequency corresponds to a short bond, ie a strong bond.

The technique is also useful for quantitative analysis. For example the concentration of a solution can be estimated if the specific absorption of the solute is known in a spectral region where the solvent is transparent.

Dispersive IR

Dispersive spectrometers expose a sample to several narrow bands of infrared radiation frequencies at a time. This is done using a diffraction grating to disperse the radiation from the energy source and by rotating the grating the particular frequencies required can be selected. Typically the instrument scans through a large range of frequencies by rotating the grating. Frequencies that have not been absorbed by the sample reach the detector at 100% transmittance, while the remainder of frequencies that have been absorbed reach the detector at much lower percent transmittance. This data is used to construct a spectrum.

Dispersive IR instruments are normally double beam, which means that the source beam is split into two beams. One beam passes through the sample cell, the other passes through the reference cell (usually air). The reference beam subtracts out any variation in signal over time.

Fourier Transform IR (FTIR)

Fourier-Transform spectrometers simultaneously expose a sample to the entire range of IR frequencies. Frequencies not absorbed by the sample constitute the “sample beam,” which reaches the detector at 100% transmittance. As this takes place, another beam of infrared radiation reaches the detector. This “reference beam” has not passed through the sample. The sample beam and the reference beam are allowed to interfere with each other as they reach the detector and the resulting data is used to construct an interferogram. This interferogram is converted to a normal spectrum by performing a Fourier Transformation (FT).

Once data has been collected by the spectrometer, it is sent to the computer as an interferogram. The computer determines the frequency and intensity of each component wave of the interferogram, by performing a Fourier Transform of the data. These frequencies and intensities make up the peaks of an IR spectrum.

Spectral Analysis

Once the IR spectrum is complete, it is available for analysis. Each compound has a distinct set of strong absorptions, or peaks, which the chemist can use to identify the compound present. Each functional group within the molecule has a distinct absorption. For a complete list see the functional group absorptions.

The peak is the name given to the position in the spectrum at which an absorption occurs. The horizontal scale is measured in cm-1 where cm-1 is known as wavenumber (1/wavelength), which is equivalent to frequency. For instance, in the acetophenone spectrum above, the carbonyl group has an absorption peak at 1700 cm-1. This peak has a minimum point at about 5 percent transmittance on the vertical scale, which means the carbonyl group absorbed almost all the infrared radiation at 1700 cm-1. The carbonyl group bond vibrations contain a very large oscillating dipole moment which can absorb IR radiation of the same frequency.

Common Functional Group IR Absorption Band Positions

Functional Group Absorption Band Position (cm-1)
C-H 2850-2960
=C-H 3020-3100
C=C 1650-1670
=C-H 3300
C=C 2100-2260
C-Cl 600-800
C-Br 500-600
C-I 500
O-H 3400-3640
C-OH 1050-1150
N-H 3310-3500
C-N 1030,1230
C=O 1670-1780
COOH 2500-3100
C=N 2210-2260
NO2 1540