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Asked by Joe to Andrew on 11 Nov 2014.
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Andrew McKinley answered on 11 Nov 2014:
Carbon dioxide is indeed “IR Active” – gaseous carbon dioxide absorbs very strongly around 2300 cm-1.
This is due to the particular vibration of carbon dioxide called an “asymmetric stretch”:
O =C====O O==C==O O====C=O
There is also a “bend” which is IR active also – harder to show in ASCII text, but hold your arms out to the side so they are parallel to the ground: your hands are oxygen atoms, your body is the carbon atom. The bend is then when you flap your arms up and down (both up, then both down). The bend is also something we call “degenerate” – that means there are two ways for it to vibrate, but each has the same energy! The first bend is ‘flapping up and down’, the second bend – with the same energy – is ‘flapping front to back’.
There is also a “symmetric stretch”:
O=C=O O==C==O O====C====O
but this stretch is not active towards infra-red radiation.
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Comments
Joe commented on :
Since some of your work is based on IR, does this affect it?
Joe commented on :
Also, ASCII is awesome.
Andrew commented on :
@Joe Any work which is done involving IR will be affected by CO2, but only if it absorbs in the same band as CO2 – not a lot does actually, so we can easily identify the CO2 vibrations, and eliminate them from the spectrum. Usually we do this by ‘correcting’ – i.e. measure the spectrum without our sample in place (we call this “recording a background”) and we then measure the spectrum of our sample. We then subtract the background from our sample spectrum (this is exactly the mathematical operation it sounds like! We take each data point in the sample spectrum, and subtract the absorption value of the corresponding background data point), and this (should) give us the uncorrupted sample spectrum.