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Why doesn't N2 absorb infrared radiation

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    (Original post by Hazel99)
    I would assume because of the triple bond present in this diatomic molecule is too high for a photon of infra-red to overcome?

    (Original post by AngryRedhead)
    I would assume because of the triple bond present in this diatomic molecule is too high for a photon of infra-red to overcome?
    For homonuclear diatomics you don't actually have to consider bond strength at all (and even though a bond might be strong IR absorptions deal with vibrational transitions which are still well in the IR range).

    To OP:

    It isn't really something you can understand at A Level.

    The transition moment for a transition between an upper and lower states (for which the vibrational wave functions are \psi_v^' and \psi_v^{''} respectively) is:

    \displaystyle \mathbf{R}_v=\int \psi_v^{'*} \hat{\mu} \psi_v^{''} \text{d}x

    Where \hat{\mu} is the dipole moment operator. The only thing you really need to worry about is that a homonuclear diatomic has no dipole moment no matter how the bond length changes, so this integral is zero and we call homonuclear diatomics IR inactive.

    Basically there has to be a change in dipole moment for IR to be absorbed by a molecule, which can never happen for a homonuclear diatomic.
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