Molecular Spectroscopy

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Molecular Spectroscopy

• Qiang Cui
• Chem 562
• Spring, 2004

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Powerful analytical tool
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Atomic spectroscopy
n4
n3
n2
Ef-Eihn
n1
N. Bohr (1885-1962) Nobel prize 1922
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Vibrational Rotational Spectra
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Energy levels in a molecule
E(J)J(J1)/2I
R
J3
J2
J1
J0
NOTE the typical energy ranges associated with
vibration and rotation!
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Rotational levels in diatomics
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True IR (vibrational) spectra
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Vibrational levels in a molecule
R
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True IR (vibrational) spectra
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Vibrational levels in a molecule
R
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Rovibrational spectroscopy
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Rovibrational states
Ro-vibrational transition
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Rovibrational spectroscopy
Note the direction of axis here!!
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Normal modes in polyatomics

• Conserve C.O.M. properties
• Independent of each other
• Characterize bonding patterns and functional
  groups
• High-frequency modes Localized
• Low-frequency modes Delocalized (collective/soft
  motions)

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Revealing functional groups
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Even low-resolution IR is powerful!
The functional-group region (40001400 cm1) is
similar for both compounds but the fingerprint
region (1400600 cm1) differs for the two
compounds.
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Theoretical studies for assignments
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Raman Scattering
Chandrashekhara Venkata Raman (1888-1970)
Nobel prize in Physics (1930)
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Raman Spectroscopy. I.
N2
Stokes
Anti-Stokes
S
O
S
O
n
of scattered light
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Raman Spectroscopy
v1
v
v-1
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Raman Spectroscopy
N2
Should be much weaker Than the Stokes vib-Raman
Stokes
Anti-Stokes
Stokes
Anti-Stokes
S
O
S
O
DJ-2
DJ2
DJ-2
DJ2
n
of scattered light
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Potential energy surfaces
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Electronic Spectroscopy
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Franck-Condon principle
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Vibronic progression
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What happens after excitation?
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Vision involves excited state!

• Absorbs at the right frequency (spectral
  tuning)
• Isomerize at the right bond

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Vision involves excited state!

• Absorbs at the right frequency (spectral
  tuning)
• Isomerize at the right bond

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Vision involves excited state!

• Absorbs at the right frequency (spectral
  tuning)
• Isomerize at the right bond

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Vibronic progression
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Absorption vs. Fluorescence
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Use of fluorescence Single molecule spectroscopy
http//www.k2.ims.ac.jp/F1movies/F1Step.htm
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Use of fluorescence Single molecule manipulation
http//www.k2.ims.ac.jp/Knotmovies/KnotFA.htm
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Use of fluorescence GFP