IR Spectroscopy: Examining Absorption of Water, CO2 & Methane

[Scott Gray]

I've been doing IR spectroscopy to examine to the absorption spectrum of water and carbon dioxide, after which I took another absorption spectrum with a methane gas cell. The absorption spectrum without the methane shows absorption from carbon dioxide around 4000nm, however, this is not apparent on the spectrum with the methane which appears at a around . What else is it that contributes to the absorption in IR Spectroscopy?

Also why is it that the rotational line structure of Carbon Dioxide cannot be seen, does this have something to do with the moment of inertia of the carbon dioxide molecules?

Since carbon dioxide has no Q band (i.e. no transitions where the vibrational energy changes but the rotational energy doesn't) wouldn't it be expected that the wavelength at which it would occur if it happened to have one spike back up to match the observed intensity of the general curve? Since this doesn't happen there is some absorption resulting in a small spike but what else is absorbing this energy?

Thanks.

 

[SpectraCat]

I've been doing IR spectroscopy to examine to the absorption spectrum of water and carbon dioxide, after which I took another absorption spectrum with a methane gas cell. The absorption spectrum without the methane shows absorption from carbon dioxide around 4000nm, however, this is not apparent on the spectrum with the methane which appears at a around . What else is it that contributes to the absorption in IR Spectroscopy?

Not sure what you are asking here ... if you have replaced the CO2 with methane, why would you expect to still see bands from CO2? Perhaps I didn't understand what you were trying to say.

Also why is it that the rotational line structure of Carbon Dioxide cannot be seen, does this have something to do with the moment of inertia of the carbon dioxide molecules?

That rotational structure will be visible if you set the resolution of your instrument lower than about 1 cm-1 (I am assuming you are using an FTIR). If you can go down to 0.5 cm-1 (or lower), you should be able to resolve the peaks to the baseline. You are right to suspect the explanation has to do with the inertial moment of the molecules ... the rotational constant (B) of CO2 ( the inverse of the moment of inertia combined with some constants) is ~0.4 cm-1. Normally ro-vibrational peaks are separated by 2B, but in CO2, half of the peaks are missing due to nuclear spin statistics, so the peaks are separated by 4B, or ~1.6 cm-1.

Since carbon dioxide has no Q band (i.e. no transitions where the vibrational energy changes but the rotational energy doesn't) wouldn't it be expected that the wavelength at which it would occur if it happened to have one spike back up to match the observed intensity of the general curve? Since this doesn't happen there is some absorption resulting in a small spike but what else is absorbing this energy?

Thanks.

I am sorry but I really don't understand what you are trying to ask at all.

 

[Scott Gray]

In the first paragraph I mean that the first spectrum was taken of just air and the second contained a methane gas cell which the photons passed through as well as air. But the absorption for the CO2 in air can't be seen in the spectrum with the presence of the methane gas. What else here is contributing to this absorption?

Similarly for the spectrum of air the CO2 should have no Q band, however, there is still some absorption since the position where the Q band should occur provided it did have one does not spike but it appears as just a small hump (these plots are of the observed intensity against the wavelength not wavenumber).

 

[SpectraCat]

In the first paragraph I mean that the first spectrum was taken of just air and the second contained a methane gas cell which the photons passed through as well as air. But the absorption for the CO2 in air can't be seen in the spectrum with the presence of the methane gas. What else here is contributing to this absorption?

Similarly for the spectrum of air the CO2 should have no Q band, however, there is still some absorption since the position where the Q band should occur provided it did have one does not spike but it appears as just a small hump (these plots are of the observed intensity against the wavelength not wavenumber).

There are a couple of possibilities to explain what you see, but both of them are pretty mundane:

First, are you sure the spectrometer isn't set up to do some sort of background subtraction? FTIR's are oftern single beam spectrometers, so the protocol is to first take a background spectrum, and then subtract it from the sample spectrum, so that only the peaks unique to the sample spectrum appear.

The second possibility is that if the spectrometer is set up so that the pathlength through air is fairly short (i.e. the housing is purged with dry nitrogen, or evacuated), and most of that distance is taken up by the sample cell, then the intensity of the air-related bands would be strongly attenuated in the methane spectrum. You can appreciate the reason for this from Beer's law.

For you second question, my guess is that if you improved your resolution, you would see that there really is no Q-branch. However, if you resolve all the rotational lines to the baseline, and you are still seeing a peak at the vibrational origin, then I am not sure where that would be coming from. I seem to recall that a weak Q-branch may become allowed at high-pressure, due to collisions ... however that should not be appearing at atmospheric pressure.