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Study Notes: Interpreting Infrared Spectra

IR spectrometry is very similar in instrumentation to UV-vis spectroscopy but uses a different light source and range. The infrared portion of the electromagnetic spectrum is lower in energy than the radiation associated with the visible region. The infrared covers frequencies from about 1011 to 1014 Hz which corresponds to the natural vibrational frequencies of molecules.

The instrumentation consists of an infrared beam that is split into two identical beams (i.e. the reference beam and the analytical beam). The analytical beam is passed through the sample so that molecular vibrational absorption can take place. The intensities of the two beams are compared over the range 2.5 to 16 µm and plotted as a function of wavenumber (reciprocal wavelength in cm-1).Skip flash movie

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The radiation source is a silicon carbide rod that is heated to between 1000 and 2000°C to emit in the infrared. The beam is split into two equal intensity beams by a mirror system. The sample beam passes through a cell containing the sample substance. Some light is absorbed and the remainder is transmitted along a path to the rotating sector mirror. The reference beam passes through another cell, identical in construction to the sample cell (minus the sample). The transmitted portion passes to the rotating mirror. The rotating mirror alternately reflects the sample beam and transmits the reference beam along the same path to the monochromator diffraction grating, which selects the scanning frequencies, and finally to the thermocouple detector. The nett effect is that light passes alternately through the sample and blank at high frequency. The attenuator is automatically adjusted to produce equal intensities of the sample and reference beams so that the attenuation current measures the absorption intensity.

Intensity is plotted as a function of wavenumber over the range 4000 - 625 cm-1 as a series of downward facing peaks (i.e. a measure of absorption).

IR spectrum of ethanol

The position, width and height of the peaks can be used to assist the identification of structural features of molecules. Infrared analysis provides a fast and relatively cheap method of organic functional group identification. Solids, liquids, solutions and gases can all be examined. NaCl optics are used in the instrumentation since glass, which absorbs strongly in the IR region, would obscure many of the analytical peaks.

Gases are placed in evacuated cells of 10 cm path length. Multiple internal reflections can give an effective path length through the sample of up to 40 metres. As little as 30 µg of a volatile compound can be scanned in 4 seconds.

Liquids are pressed between two optically smooth NaCl disks. Neat liquid samples of 1 - 10 mg in a 0.01 mm film are required to avoid too strong an absorption. Solutions of 0.05 - 10% (0.1 - 1 mL) in a film up to 1 mm give good results. Solutions in CCl4 (non-polar) solvent can also be used although CCl4 does mask absorptions below 1300 cm-1. Water cannot be used with NaCl plates since they are soluble (and must not be handled with the fingers). Aqueous solutions can be used between AgCl plates.

Solids (2 - 5 mg) may be mulled by grinding with a few drops of Nujol oil although Nujol does interfere just below 3000 cm-1 and around 1500 - 1400 cm-1. The solid can also be mixed with powdered NaCl (or KBr) to be pressed into a disk.

The Fingerprint region extends from 1000 to 650 cm-1. These absorptions are due mainly to C-H vibrations. this section of the spectrum is very difficult to interpret due to its complexity.

The Functional group region (4000 - 1100 cm-1) will often show absorptions that are characteristic of particular functional groups:

alcohol, phenol ROH 3650 - 3580 -O-H absorption in vapour.
    3550 - 3200 in solution (H-bonding).
    1200 - 1050 for C-O absorption.

IR spectrum of methanol


ether ROR' ~ 1125 C-O-C absorption.

IR spectrum of ethyl ether


ketone RCOR' ~ 1715 carbonyl (C=O).

IR spectrum of 2-butanone


aldehyde RCHO 1740 - 1720 carbonyl (C=O).
    2830 - 2695 for C-H.
amide RCON(R')R'' ~ 1650 1° carbonyl (C=O).
    ~ 1680 2° carbonyl (C=O).
    ~ 3500 weak -N-H for 1°.
    and ~ 3400 weak -N-H for 1°.
    ~ 3300 multiplet -N-H for 2°.
ester RCOOR' 1750 - 1735 carbonyl (C=O).
    1300 - 1000 for C-O.

IR spectrum of ethyl acetate


carboxylic acid RCOOH 1720 - 1710 carbonyl (C=O).
    3300 - 2500 broad -O-H band.
    1440 - 1395 -O-H band.
    1315 - 1280 C-O.
1° amine RNH2 ~ 3500 weak -N-H
    and ~ 3400 weak -N-H

IR spectrum of leucine


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