Raman Spectroscopy

1
Raman Spectroscopy

• Chem 325

2
Light Scattering

• Incident light beam interacting with a collection
  of molecules
• Some elastic collisions occur, light scattered
• Energy of the scattered light is identical with
  the incident light
• RAYLEIGH scattering
• Very small amount undergoes inelastic scattering
• There has been some energy transfer to or from
  the molecules!

3
Light Scattering

• Incident light of frequency ?º
• Elastically scattered light of frequency ?º

Incident light has GAINED energy
Incident light has LOST energy
Vibrational Energy
Raman Scattering
4
Raman Scattering
5
Raman Scattering

• Most of the light is transmitted, very little is
  scattered
• gt99 scattered light is Rayleigh
• Very weak Stokes scattering
• Very, very weak anti-Stokes scattering
• Stokes, anti-Stokes due to transfer of
  vibrational quantities of energy

6
Raman Scattering
Rayleigh
Stokes
º
A vibrational spectrum!
7
Raman Scattering

• A vibrational spectrum generated by Raman
  scattering of light.
• Scattering is more efficient if higher ? light is
  used for excitation.
• Use visible light.
• Need very high intensity the Raman effect is
  weak! Need sensitive detector.
• Historically high-intensity Hg arc lamp
• Now lasers (visible, Near-IR)

8
Raman Spectrometer

• CW or FT

9
IR
Raman
10
Raman Spectroscopy

• The Raman intensities may be quite different than
  those of the IR spectrum
• Often the opposite to those of IR
• Some IR bands may be missing from the Raman,
  and vice versa

11
Band Intensities
Rule For a vibrational mode to be IR active the
vibrational motion must cause a change in the
dipole moment of the molecule.
Rule For a vibrational mode to be Raman active
the vibrational motion must cause a change in the
polarizability of the molecule.
12
Raman Activity

• Polarizability
• The ease with which the electron cloud of the
  molecule can be distorted by an electric field.
• Specifically, the oscillating electric field of
  the incident light.

13
Raman vs IR

• Rule of Mutual Exclusion

If a molecule has a center of symmetry, then
Raman active vibrations are IR inactive, and vice
versa. If there is no center of symmetry then
some (but not necessarily all) vibrations may be
both Raman and IR active.
Weak or zero-intensity IR bands are often strong
in the Raman spectrum
14
Vibrational Modes

• Web animation of vibrational modes, IR versus
  Raman activity
• http//www.chem.purdue.edu/gchelp/vibs/index.htm
  l

15
Alkynes 4-Octyne
IR
Raman
16
Benzene
1038
IR
996
Raman
17
N-ethylpropanamide
IR
Raman
18
Polarization

• Incident laser light is plane-polarized
• Scattered light may become de- polarized

Depolarization occurs for the less symmetrical
vibrational modes
19
Depolarization

• Put a (plane) polarizing filter between sample
  and detector
• Acquire spectrum with polarizing filter parallel
  to laser plane polarization
• Rotate polarizing filter 90º, reacquire spectrum
• Compare relative intensities of bands in the two
  spectra

20
Depolarization

• Define depolarization ratio ?

0 lt ? lt 0.75, band is said to be polarized ?
0.75, band is said to be depolarized
21
Polarization

• Raman
• - totally symmetric vibrations produce polarized
  bands, 0 lt ? lt 0.75
• - non-totally symmetric vibrations produce
  depolarized bands, ? 0.75
• - polarization measurements can help identify
  (symmetry) type of vibration producing a band

22
IR
CH2Cl2
Raman
23
CH2Cl2
Polarized, symmetric vibration
24
(No Transcript)
25
Polarized, symmetric vibrations
26
Raman Spectroscopy

• Complementary to IR Spectroscopy
• Different activity factors
• Can see IR-inactive vibrations
• Can see low-frequency vibrations easier
• Generally, much simpler spectra
• Uses visible or NIR light
• Can use different solvents, especially water
• N.B. in vivo biological systems