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IR Spectroscopy

  1. May 30, 2008 #1
    Quote from Organic Chemistry, L.G. Wade, Jr.:
    "Not all molecular vibrations absorb IR radiation."
    "One of the component of an electromagnetic wave is E field. This field alternately stretches and compresses a polar bond."
    "If this alternate stretching compressing of the bond occurs at the frequency of the molecule's natural rate of vibration, energy may be absorbed."
    "If a bond has zero dipole moment, the E field doesn't interact with the bond."

    Things that I am confuse with:
    1. If there's no interaction with EM wave, does a molecule still vibrate? If so, what will be the frequency of the molecule? Is it temperature dependent? Or is it the lowest frequency possible?

    2. Is the vibrational energy quantized? Eg. A molecule has 3 different mode of vibration. The frequency of each different mode of vibration is 10Hz, 20Hz and 30Hz. Does this means that the molecule can ONLY vibrate in ONE of these three frequency? i.e. Frequency of 10.1 Hz, 15 Hz, 22 Hz etc. is disallowed?

    3. What happens if the EM wave radiated on the molecule is not the same as the neutral vibrational frequency? It wouldn't be absorbed, but will the E field interacts with the molecule? eg. Increasing the frequency of the vibration frequency?

    4. Would the result of IR spectroscopy influenced by surrounding temperature?

    5. Why is EM wave absorbed when it is the same as the natural frequency? Is the Em wave energy absorbed in order to change the ground state vibrational energy to another natural frequency?

    Thanks for your time.
  2. jcsd
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