Raman Spectroscopy - PowerPoint by malj

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									     Chem 325



Raman Spectroscopy
                  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!
                     Light Scattering
    • Incident light of frequency º
    • Elastically scattered light of frequency º



Incident light has                        Incident light has
LOST energy                               GAINED energy



  Vibrational                                Raman
    Energy                                  Scattering
Raman Scattering
                   Raman Scattering

•   Most of the light is transmitted, very little is scattered
•   >99% 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
Raman Scattering
                       Rayleigh
Stokes




                           1
                2
         3
4

 A vibrational spectrum!        º
             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)
           Raman Spectrometer



CW or FT
              O


              C
 IR
        H3C       CH3




Raman
          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
                 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.
                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.
                       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
            Vibrational Modes




• Web animation of vibrational modes, IR
  versus Raman activity
http://www.chem.purdue.edu/gchelp/vibs/index.html
        Alkynes: 4-Octyne



                      H3C
  IR
                            CH3




Raman
        Benzene



                  1038
  IR




                         996




Raman
        N-ethylpropanamide




                                   O


                                       N   CH3
  IR                         CH3       H




Raman
                 Polarization

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




                          Depolarization occurs for
                          the less symmetrical
                          vibrational modes
                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
                 Depolarization

• Define depolarization ratio 

                      I
                   
                      I 

     0 <  < 0.75, band is said to be polarized
      = 0.75, band is said to be depolarized
                Polarization

• Raman:
- totally symmetric vibrations produce polarized
   bands, 0 <  < 0.75

- non-totally symmetric vibrations produce
  depolarized bands,  = 0.75

- polarization measurements can help identify
  (symmetry) type of vibration producing a band
  IR




        CH2Cl2

Raman
CH2CL2_POL2.ESP
                  CH2Cl2




                         Polarized, symmetric vibration
           3000   2500     2000        1500    1000       500
                         Wavenumber   (cm-1)
CH3
                                               CH3




TOLUENE_25_05_2006_POL.ESP




                 Polarized, symmetric vibrations


            3000             2500     2000      1500    1000   500
                                    Wavenumber (cm-1)
            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

								
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