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Vibration and Rotation Spectroscopy
Raman
400 -4000 cm-1
Solid ,liquid, gas
Raman solvent:
Fingerprint
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Vibration and Rotation Spectroscopy
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Vibration and Rotation Spectroscopy
CO stretching 1662 cm-1
CO stretching 1715 cm-1
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Vibration and Rotation Spectroscopy
Coupling C-O and O-X
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Vibration and Rotation Spectroscopy
After coordination: C=O force constant reduced by draining π
electron density out of C=O then C=O shift to lower
energy.
If N atom coordinated to X: C=O shift to higher energy
because lone pair participate in coordination and then force
constant of C-N increase.
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Vibration and Rotation Spectroscopy
C=O frequency decrease after coordination
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Vibration and Rotation Spectroscopy
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Vibration and Rotation Spectroscopy
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Vibration and Rotation Spectroscopy
CN frequency increase after coordination
Π back-bonding
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Vibration and Rotation Spectroscopy
3300 cm-1 1600 cm-1
1300 cm-1 825 cm-1
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Vibration and Rotation Spectroscopy
CN frequency decrease after coordination via. N atom more than S atom
760-780 cm-1 C-S 690-720 cm-1
440-450 cm-1 NCS bending 400-440 cm-1
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Vibration and Rotation Spectroscopy
Free N2 : IR inactive Raman active (2331 cm-1)
N2 : IR 2130 cm-1
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Vibration and Rotation Spectroscopy
Vaska’s complex
Free O2 : IR inactive Raman active (1555 cm-1)
N2 : IR 857 cm-1
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Vibration and Rotation Spectroscopy
D3h
C2v Cs
E' asymmetric stretching
v3 1400 cm-1
1460 cm-1 1280 cm-1
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Vibration and Rotation Spectroscopy
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Vibration and Rotation Spectroscopy
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Vibration and Rotation Spectroscopy
How many vibration modes would you expect to see for acetylene (C2H2) and
peroxide (H2O2). Discuss why this kind of analysis sometimes predicts more,
and sometimes less, vibrations bands than are actually observed.
Acetylene is linear so we would use the equation:
3N-5 = 12-5 = 7 expected frequencies
Peroxide is not linear so you use the equation:
3N-6 = 12-6 = 6 expected frequencies
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Vibration and Rotation Spectroscopy
sometimes observe less vibrational modes than expect because:
1. Sometimes the symmetry of the molecules is such that the molecule’s dipole
does not change, and if the dipole does not change, you cannot absorb IR
energy.
2. Sometimes the frequencies of two vibrations overlap so you cannot
distinguish between them.
3. Sometimes the absorptions are so low you cannot observe the bands
4. Sometimes the vibrations are simply outside of the regions you are detecting
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Vibration and Rotation Spectroscopy
You may observe more vibrations than you predict because:
1. Sometimes vibrational modes combine to create overtone bands
2. Sometimes a photon can excite a combination band in which the single photon
is exciting two different vibration modes simultaneously
3. Sometimes the absorptions are so low you cannot observe the bands
4. Sometimes the vibrations are simply outside of the regions you are detecting
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Vibration and Rotation Spectroscopy
Linear Circular Elliptical
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Vibration and Rotation Spectroscopy