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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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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