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Name: Grace Y.
Status: student
Grade: 12+
Location: Outside U.S.
Country: Australia
Date: Winter 2009-2010


Question:
Hello, I would like to know (on a molecular level) why methane absorbs infrared better than carbon dioxide. Does it have something to do with the characteristic of the vibrational modes of methane? If so, what is it? Thank-you for your time! Grace



Replies:
A number of factors come into play.

First, of course, is the amount of gas present. I will ignore that because your question is not about that.

Second, the next question is how a molecular species interacts with electromagnetic radiation. Because neither CO2 nor CH4 has a permanent dipole moment, that is they are symmetric, they do not interact with microwave radiation.

Third, CO2 has 3 vibrations: a symmetric stretch, an asymmetric stretch, and a doubly degenerate bending mode -- up and down, in and out. The symmetric stretch (1354 cm^-1) does not interact with electromagnetic radiation. That leaves the asymmetric stretch ( 2396 cm^-1) and the two degenerate bending vibrations (673 cm^-1).

Fourth, while methane CH4 has more atoms than CO2, the arrangement of the atoms is highly symmetric. Only two vibrational modes absorb infrared radiation -- the ones where both the hydrogens and the carbon atom move. Those are the vibrations that present an oscillation of the dipole moment.

Fifth, things get even trickier because how much infrared radiation absorbed, or emitted, depends upon how easy it is to get the atoms to move, and what is their "inherent" mobility.

All of these factors come into play and it is difficult to attribute the absorption to any one of these factors.

Vince Calder



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