3.7 Raman Spectroscopy

Raman spectroscopy reveals molecular vibrations through light scattering. It complements infrared spectroscopy, offering insights into molecular structure and symmetry. The Raman effect occurs when light interacts with a molecule's electron cloud, causing changes in polarizability.

This technique is valuable for studying non-polar molecules and aqueous solutions. By analyzing Raman spectra, scientists can determine molecular geometry, bond strengths, and intermolecular interactions. It's a powerful tool for understanding molecular behavior and structure.

Principles of Raman Spectroscopy

Raman Effect and Selection Rules

• Raman spectroscopy is based on the inelastic scattering of light by molecules, resulting in a change in the frequency of the scattered light compared to the incident light
• The Raman effect arises from the interaction between the electric field of the incident light and the polarizability of the molecule, which is related to the ease with which the electron cloud of the molecule can be distorted
• The selection rule for Raman spectroscopy states that a vibrational mode is Raman-active if the polarizability of the molecule changes during the vibration
• The intensity of a Raman band is proportional to the square of the change in polarizability during the vibration

Molecular Symmetry and Raman Spectra

• Raman spectroscopy can provide information about the symmetry of a molecule, as the number and symmetry of Raman-active modes are determined by the molecular point group
• The Raman spectrum consists of Stokes and anti-Stokes lines, which correspond to the energy of the scattered light being lower or higher than the incident light, respectively
• The Stokes lines are typically more intense than the anti-Stokes lines because the ground vibrational state is more populated at room temperature
• The polarization of Raman bands (the ratio of the intensity of the scattered light with polarization perpendicular and parallel to the incident light) can be used to distinguish between totally symmetric and non-totally symmetric vibrational modes

Raman vs Infrared Spectroscopy

Principles and Selection Rules

• Both Raman and infrared (IR) spectroscopy provide information about the vibrational modes of molecules, but they are based on different physical principles
• IR spectroscopy is based on the absorption of infrared light by molecules, while Raman spectroscopy is based on the inelastic scattering of visible or near-infrared light
• The selection rules for IR and Raman spectroscopy are different: IR-active modes require a change in the dipole moment during the vibration, while Raman-active modes require a change in the polarizability
• Some vibrational modes may be active in both IR and Raman spectroscopy (CO2 asymmetric stretch), while others may be active in only one or the other (CO2 symmetric stretch is Raman-active but IR-inactive), depending on the symmetry of the molecule

Complementary Nature and Applications

• Vibrational modes that are symmetric with respect to the center of symmetry are Raman-active but IR-inactive, while modes that are antisymmetric are IR-active but Raman-inactive (mutual exclusion principle)
• IR spectroscopy is typically more sensitive to polar functional groups (O-H, N-H), while Raman spectroscopy is more sensitive to non-polar groups (C=C, C-C) and symmetric vibrations
• Raman spectroscopy can be used to study samples in aqueous solutions, as water is a weak Raman scatterer, while IR spectroscopy is limited by the strong absorption of water in the infrared region
• The complementary nature of IR and Raman spectroscopy allows for a more comprehensive characterization of molecular structure and dynamics

Molecular Structure from Raman Spectra

Vibrational Energies and Molecular Geometry

• The position (Raman shift) of a Raman band corresponds to the energy of the vibrational mode, which depends on the masses of the atoms involved and the strength of the chemical bonds
• The number and symmetry of Raman-active modes can be used to determine the molecular point group, which provides information about the symmetry elements present in the molecule
• The relative intensities of Raman bands can provide information about the relative polarizability changes associated with different vibrational modes
• The presence of overtones and combination bands in the Raman spectrum can provide additional information about the anharmonicity of the vibrational potential energy surface

Intermolecular Interactions and Raman Spectra

• Raman spectroscopy can be used to study the effects of intermolecular interactions, such as hydrogen bonding, on the vibrational modes of molecules, as these interactions can cause shifts in the positions and changes in the intensities of Raman bands
• The formation of hydrogen bonds (water, alcohols) can lead to a red-shift (lower frequency) and broadening of the Raman bands associated with the involved functional groups
• Raman spectroscopy can provide insights into the strength and directionality of intermolecular interactions, which are crucial for understanding the properties and behavior of molecular systems (supramolecular assemblies, biological macromolecules)