Raman Spectroscopy Overview

Raman Spectroscopy Overview

• Raman spectroscopy is a technique to study the frequency change of light interacting with matter.
• It provides information about molecular vibrations, useful for sample identification and quantification.

Light Scattering

• Light scattering is a process where molecules interact with electromagnetic radiation, redirecting it from its original path.
• Scattering can be elastic (Rayleigh scattering) or inelastic (Raman scattering).
  • Rayleigh scattering accounts for 99.999% of scattered light, with scattered photons having the same energy as the incident photons.
  • Raman scattering accounts for <0.001% of scattered light with shifted photon frequencies different from the incident photons.

Raman Effect

• Discovered by Sir C.V. Raman in 1928.
• A small fraction of scattered light has a different wavelength than the incident light, the shift in frequency depending on the molecule's structure.
• The scattering with frequency change is called Raman scattering.

Raman Scattering (Quantum Mechanical View)

• Quantum mechanically, Raman scattering corresponds to excitation to a virtual state, lower in energy compared to an electronic transition (real state), with nearly coincident de-excitation and vibrational energy change.
• Polarization changes are needed to form the virtual state required in Raman scattering.

Stokes and Anti-Stokes Radiation

• Stokes Radiation: Photons collide with a molecule in the ground state, causing energy loss (hν_in - ΔE) during scattering; this is more intense.
• Anti-Stokes Radiation: Photons collide with a molecule in an excited state, causing energy gain (hν_in + ΔE) during scattering; this is less intense.
• Both Stokes and anti-Stokes radiation produce similar spectra but have different intensities.

Raman Effect Mechanism

• Raman effect arises when a photon interacts with a molecule's electric dipole.
• Monochromatic light scattered by gas, liquid, or solid molecules are studied using lasers in the NIR, Visible or UV range.
• Raman spectrum is a plot of the shifted radiation intensity versus frequency (wavenumber).
• Raman spectra are plotted relative to the laser frequency, using a scale where the Rayleigh band lies at zero.

Raman Spectrum Analysis

• Band positions in the Raman spectrum correlate with energy levels of functional group vibrations.
• Compounds with strong IR bands correspond to weak Raman bands and vice versa; this difference is due to the electrical nature of the vibration.
• Polar bonds (C-O, N-O, O-H) result in a strong IR absorption band but a weak Raman scatter.
• Relatively neutral bonds (C-C, C-H, C=C) result in weak IR but strong Raman scatter.

Selection Rules for Raman Activity

• For a vibration to be Raman active, the molecule's polarizability must change during the vibration.
• Change in polarizability can be either a change in magnitude or direction of the polarizability ellipsoid.
• Using group theory and character tables, the symmetry labels (x, y, z or products of x, y, z) of the normal modes can predict if a mode is active in IR or Raman spectroscopies.

Vibrational Raman Spectra (CO₂)

• Symmetric stretch (v1): IR inactive, Raman active.
• Asymmetric stretch (v2): IR active, Raman inactive.
• Bending (v3): IR active, Raman inactive.

Raman Spectrometer Operation

• A sample (solid, liquid, or gas) is illuminated with a laser.

• Some light is scattered by molecules in the medium.

• Most scattered light has the same frequency as the incident light (Rayleigh scattering).

• The shifted light (Raman scattered light) is separated from the Rayleigh scattered light using a wavelength selector and detected.

• The basic Raman spectrometer design uses a dispersive element to separate the Raman light from the incident light.

• FT-Raman spectrometers utilize Michelson interferometers for Raman spectral acquisition.

Raman Spectroscopy Applications

• Raman spectroscopy provides insights into molecular shape and structure (linear/non-linear, symmetric/asymmetrical).
• Ideal for studying aqueous phase samples, as the water's absorption is less intense in the Raman spectrum.
• Useful, but not limited to, identifying or quantifying samples.

Advantages of Raman Spectroscopy

• Water doesn't interfere and can be used as a solvent.
• Can study aqueous solutions

Limitations of Raman Spectroscopy

• Instrument cost is high.

Mutual Exclusion Rule

• A molecule with a center of symmetry cannot display modes that are both IR and Raman active.

Comparison of Raman and IR Methods

• | Property | Raman | IR | |---|---|---| |Interaction mechanism| Scattering | Absorption | |Polarizability change requirement| necessary| necessary| |Dipole change requirement | Not necessary | Necessary | |Solvent use | Water (often) | Water (not often)| |Cost| Expensive | Relatively inexpensive|