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Infrared Spectroscopy

Key Concepts

Infrared (ir) spectroscopy is used to identify particular bond types and functional groups in organic molecules by measuring a substance's absorption of infrared radiation at different frequencies.
Wavenumber is the number of wavelengths per centimetre (cm^-1). Organic groups show absorbance in the range ~1000 to 3000 cm^-1.
Infrared (ir) spectra show a 'fingerprint' region between ~1000 and 400 cm^-1 which is specific to a particular compound. The ir spectrum of an unknown compound can be compared to known spectra in order to identify the unknown compound.

Infrared Spectrometer

Atoms in molecules are continually vibrating.
This vibrational energy is quantised.
When molecules absorb infrared radiation they become hot and the atoms and molecules vibrate more vigorously.
Molecules absorb infrared radiation at particular wavelengths depending upon which chemical bonds are present in the molecule.

The Infrared (ir) Spectrometer

infrared radiation is produced by electrically heating a filament which is divided by mirrors into 2 beams, a reference beam and a sample beam.
In the sampling area, a segmented rotating disk allows each beam to pass through alternately.
The reference beam and the sample beam are combined into a beam of alternating segments.
The detector measures the heat energy and the recorder records the results as a plot of percent absorption (or transmittance) as a function of wavenumber (cm^-1) or wavelength (μm).

Infrared (ir) Spectra

Particular chemical bonds absorb energy at a particular wavelength (wavenumber) regardless of the compound the bond is in.

Some examples are given below:

molecular vibration wavenumber (cm^-1) wavelength (μm)

C-Cl 700-800 14.3-12.5

C-C 750-1100 13.3-9.1

C-O 1000-1300 10-7.7

C=C 1610-1680 6.2-6.0

C=O 1670-1750 6.0-5.7

O-H (acids) 2500-3300 4.0-3.0

C-H 2850-3300 3.5-3.0

O-H (alcohols) 3200-3550 3.1-2.8

N-H (primary amines) 3350-3500 3.0-2.9