Light Scattering Chem 325 Raman Spectroscopy • 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 Raman Scattering • Incident light of frequency νννº • Elastically scattered light of frequency νννº Incident light has Incident light has LOST energy GAINED energy Vibrational Raman Energy Scattering 1 Raman Scattering Raman Scattering Rayleigh • Most of the light is transmitted, very little is scattered Stokes • >99% scattered light is Rayleigh • Very weak Stokes scattering • Very , very weak anti-Stokes scattering ν1 • ν 2 Stokes, anti-Stokes due to transfer of vibrational ν quantities of energy 3 ν 4 º A vibrational spectrum! ν Raman Scattering Raman Spectrometer • A vibrational spectrum generated by Raman scattering of light. • Scattering is more efficient if higher ννν light CW or FT 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) 2 Raman Spectroscopy • The Raman intensities may be quite O different than those of the IR spectrum IR C • Often the opposite to those of IR H3C CH3 • Some IR bands may be ‘missing’ from the Raman, and vice versa Raman Band Intensities Raman Activity Rule : • Polarizability For a vibrational mode to be IR active the vibrational – The ease with which the electron cloud of the motion must cause a change in the dipole moment of molecule can be distorted by an electric field. the molecule. – Specifically, the oscillating electric field of the incident light. Rule : For a vibrational mode to be Raman active the vibrational motion must cause a change in the polarizability of the molecule. 3 Raman vs IR Vibrational Modes • Rule of Mutual Exclusion If a molecule has a center of symmetry , then Raman active vibrations are IR inactive, and vice • versa. Web animation of vibrational modes, IR versus Raman activity If there is no center of symmetry then some (but not necessarily all) vibrations may be both http://www.chem.purdue.edu/gchelp/vibs/index.html Raman and IR active. Weak or zero-intensity IR bands are often strong in the Raman spectrum Alkynes: 4-Octyne Benzene 1038 IR H3C IR CH3 996 Raman Raman 4 N-ethylpropanamide Polarization • Incident laser light is plane-polarized O • Scattered light may become de- polarized N CH3 IR CH3 H Depolarization occurs for Raman the less symmetrical vibrational modes Depolarization Depolarization • Put a (plane) polarizing filter between sample and • Define depolarization ratio ρ detector ρρ • Acquire spectrum with polarizing filter parallel to I laser plane polarization ρ === ⊥⊥⊥ I |||||| 0 < ρρρ < 0.75, band is said to be polarized • Rotate polarizing filter 90º, reacquire spectrum ρρρ = 0.75, band is said to be depolarized • Compare relative intensities of bands in the two spectra 5 Polarization • Raman: - totally symmetric vibrations produce polarized bands, 0 < ρρρ < 0.75 IR - non-totally symmetric vibrations produce depolarized bands, ρρρ = 0.75 CH 2Cl 2 - polarization measurements can help identify (symmetry) type of vibration producing a band Raman CH Cl CH2CL2_POL2.ESP 2 2 CH3 Polarized, symmetric vibration 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) 6 CH3 Raman Spectroscopy • Complementary to IR Spectroscopy – Different activity factors TOLUENE_25_05_2006_POL.ESP – Can ‘see’ IR-inactive vibrations – Can ‘see’ low-frequency vibrations easier – Generally, much simpler spectra – Uses visible or NIR light Polarized, symmetric vibrations – Can use different solvents, especially water • N.B. in vivo biological systems 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) 7.