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IR energy and molecular vibrations

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IR energy and dipoles

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The infrared spectrometer

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Interpreting infrared spectra

An infrared spectrum is a plot of transmission of infrared radiation against wavenumber (1 / wavelength). Any wavelength that is absorbed by the sample will transmit less than the others, forming a dip or ‘peak’ in the graph.

0

20

40

60

80

100

3000 2000 1000

wavenumber (cm-1)

tran

smis

sio

n (

%)For instance, C–H

bonds absorb radiation at a wavenumber of 2950 cm-1, which produces a peak in an infrared spectrum as shown on the right.

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IR radiation and greenhouse gases

Nitrogen and oxygen, the most abundant gases in the atmosphere, do not absorb infrared energy. This is because the vibrations caused would not change the dipole of the molecules.

These gases absorb infrared radiation in the atmosphere, stopping it escaping into space: they are greenhouse gases.

However, methane, water and carbon dioxide do absorb strongly in the infrared region.

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Greenhouse gases

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Using infrared spectroscopy

When breathalyzed by police, motorists blow into a handheld device, which gives an indication of the amount of alcohol in their breath. However, the result is not accurate enough to be used as evidence.

At the police station, the motorist blows into a more accurate breathalyzer, containing an IR spectrometer. The breathalyzer calculates the percentage of alcohol in the breath by looking at the size of the absorption due to the C–H bond stretch in the alcohol.

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Infrared spectroscopy: summary