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March07_2005_2pp

March07_2005_2pp - March 7 2005 Change Happens Probs added...

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1 March 7, 2005 ¾ Change Happens! ¾ Probs added to Prob Set #3 ¾ Schedule Adjustment ¾ Exam #2: Wed., March 16 th , 7pm ¾ Review Session: Sunday? Monday? Tuesday? 2 Origins of IR Absorption ¾ Due to transitions between vibrational and/or rotational energy states of molecules ¾ A molecule can absorb IR photons if: 9 there is a change in the dipole moment of the molecule during a vibrational or rotational motion AND 9 The frequency associated with the photon matches the frequency of the vibrational motion So, almost all molecules absorb in the IR (except for homonuclear diatomics)
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3 Modeling Molecular Vibrations Simple Harmonic Oscillator (SHO) Model Consider two objects connected by a spring: The potential energy of the system will vary with the displacement (y) and the force constant (k) of the spring: E = ½ky 2 4 More SHO It can be shown that there is a natural vibration frequency ( ν m ) for the system: ν m = (½ π )(k/ µ ) 1/2 where the reduced mass ( µ ) is given as: µ = (m 1 m 2 )/(m 1 + m 2 ) ν m
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