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الموضوع: IR Number of vibrational modes

  1. #1
    عضو مميز الصورة الرمزية مالك محمد
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    Aug 2011
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    افتراضي IR Number of vibrational modes

    In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes in the permanent dipole.
    A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules where N =
    Number of atoms in them, linear molecules have 3N – 5 degrees of vibrational modes whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.
    Simple diatomic molecules have only one bond and only one
    vibrational band. If the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum, but only in the Raman spectrum. Unsymmetrical diatomic molecules, e.g. CO, absorb in the IR spectrum. More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e. big molecules have many peaks in their IR spectra.
    The atoms in a CH2 group, commonly found in
    organic compounds, can vibrate in six different ways: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting:
    Symmetrical
    stretchingAntisymmetrical
    stretchingScissoring
    RockingWaggingTwisting
    (These figures do not represent the "
    recoil" of the C atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter H atoms).
    Special effects

    The simplest and most important IR bands arise from the "normal modes," the simplest distortions of the molecule. In some cases, "overtone bands" are observed. These bands arise from the absorption of a photon that leads to a doubly excited
    vibrational state. Such bands appear at approximately twice the energy of the normal mode. Some vibrations, so-called 'combination modes," involve more than one normal mode. The phenomenon of Fermi resonance can arise when two modes are similar in energy, Fermi resonance results in an unexpected shift in energy and intensity of the bands.


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  2. #2
    مشرفة الاقسام الاكاديمية الصورة الرمزية تمارا احمد
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    Jul 2011
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    افتراضي رد: IR Number of vibrational modes

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  3. #3
    عضو مميز الصورة الرمزية قمر بلحاج
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    افتراضي رد: IR Number of vibrational modes

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