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IR spectroscopy

  1. Nov 18, 2013 #1
    When analyzing the absorption pattern for different compounds when radiated by IR radiation, we see the wavelengths of the IR lights that is absorbed. These wavelengths correspond to the frequency at which the bonds in the molecule vibrate. My question is why a compound, for example an alcohol, has a peak in absorption for wavenumbers ~3300 cm-1 and no where higher. Is the concept of threshold frequency no longer in play? Why not? I understand that this would cause a greater stretching in the bonds than normal, but why can't radiation of higher energy be absorbed?

    Any explanations would be great! Thanks! :)
  2. jcsd
  3. Nov 19, 2013 #2


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    Staff: Mentor

    If you are asking why you can't excited a bond by more than one quantum at a time, you actually can, but the corresponding line will be much weaker. For a harmonic oscillator, electric dipole selection rules show that you can only excite by one quantum at a time. In a real molecule, bonds are of course not harmonic oscillators, but close enough that the selection rule holds to a good approximation. From the ground vibrational state, the transition dipole moment will be close to zero except for a transition to the first excited vibrational state, which is essentially the only spectral line you will see in an IR spectrum.
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