Unit 2 Section 4 Infrared Spectroscopy

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The Basics

In IR spectroscopy a beam of IR radiation is passed through a sample of a chemical.

The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy.

Bonds between different atoms absorb different frequencies of IR radiation.

Bonds in different places in a molecule absorb different frequencies too - so the O-H group in an alcohol and the O-H group in a carboxylic acid absorb different frequencies.

Wavenumber is the measure used for the frequency (it's just 1/wavelength).

An IR spectrometer produces a graph that shows you what frequencies of radiation the molecules are absorbing. So you can use it to identify the functional groups in a molecule.

The peaks show you where radiation is being absorbed - the 'peaks' on IR spectra are upside-down. 

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The Fingerprint Region

The region between 1000 cm^-1 and 1550 cm^-1 on the spectrum is called the fingerprint region.

It's unique to a particular compound. You can check this region of an unknown compound's IR spectrum against those of known compounds. If it matches up with one of them then you know it is that molecule.

IR spectroscopy can also be used to find out how pure a compound is, and identify any impurities.

Impurities produce extra peaks in the fingerprint region.

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Infrared Absorption and Global Warming

Some of the EM radiation emitted by the sun is in the form of IR radiation.

Molecules of greenhouse gases, like carbon dioxide, methane and water vapour, have bonds that are really good at absorbing IR energy - so if the amounts of them in the atmosphere increase, it leads to global warming.

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