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Why do greenhouses gases need to have dipoles?

  1. Feb 9, 2014 #1
    Greenhouse gases absorb and then re-emit the heat energy of infrared rays which have been partially absorbed and then reflected off the Earth from the sun, which is how the greenhouse effect works. I have heard, however, that in order for a gas in the atmosphere to be considered a greenhouse gas in the first place, it must have at least a permanent dipole or something that can induce a temporary dipole.

    Nobody in my chemistry class knows why and neither does my professor. We have all been looking for explanations as to why, but so far none of them are complete or satisfactory.

    For example, one explanation I found was that the changing electric charge in a dipole was the necessary condition for a molecule to interact with the electric field, which is what electromagnetic waves travel in. But this isn't consistent because, for example, oxygen gas (which, although irrelevant, isn't a greenhouse gas) doesn't have any dipoles whatsoever yet it can interact with UV light to break down into oxygen radicals and produce ozone. This makes the theory I found, therefore, inconsistent.

    Can anybody provide a consistent and complete explanation as to why greenhouse gases require a permanent dipole or something that can induce a temporary dipole?

    Thank you.
  2. jcsd
  3. Feb 9, 2014 #2
    Oxygen is a good example.
    Dioxygen is not a greenhouse gas because it can only absorb ultraviolet. It cannot absorb infrared.

    ALL substances can absorb ultraviolet. This is because all substances consist of electrons and nuclei which have different charges (they are held together by opposite charges). Electromagnetic field can therefore separate the charges of nuclei and electrons, causing transition dipole moment. But this is only possible if the field has sufficient frequency, compared to the orbital frequency of electrons. In substances where all electrons are very strongly held and there are no low lying excited states, only very high frequency far UV waves can be absorbed, like helium, dinitrogen, calcium fluoride etc. In substances where low lying electron excited states are available like oxygen, near UV or even visible light can be absorbed.

    But this does not mean infrared can be absorbed!

    Now, another possibility to oscillate is nuclei. These are much more massive than electrons, so oscillate at a low frequency. So they might absorb infrared.

    But in dioxygen both oxygen atoms are equal. So even if O-O bond oscillates, there is no dipole moment. Thus dioxygen can absorb ultraviolet, but not infrared.

    Infrared is absorbed mainly by compounds where dipoles can be induced. Like carbon dioxide: no dipole moment because the two C-O bonds are opposite and equal, and the dipole moments cancel. But infrared radiation can cause charge separation to move carbon in one direction and the two oxygens in opposite direction. This induces a dipole and allows absorption.
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