Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Physical Chemistry → Spectroscopy ↓


Raman Spectroscopy


Raman spectroscopy is an analytical technique widely used in physical chemistry to study vibrational, rotational and other low frequency modes in systems. It is based on the inelastic scattering of monochromatic light, usually coming from a laser in the visible, near-infrared or near-ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photon being shifted up or down. This shift in energy gives information about the vibrational modes of the molecules.

Fundamentals of Raman Effect

The Raman effect is named after Indian physicist C.V. Raman, who first discovered it in 1928. When light interacts with matter, most photons are scattered elastically, meaning that the scattered photon has the same energy (and therefore wavelength) as the incident photon. This is known as Rayleigh scattering. However, a small fraction of the light is scattered inelastically, at a slightly different energy than the incident photon. This phenomenon is known today as the Raman effect.

Stokes and anti-Stokes scattering

In Raman spectroscopy, inelastic scattering of light is classified into two categories, determined by whether light energy is gained or lost:

  • Stokes scattering: If the scattered photon has less energy than the incoming photon, the change in energy corresponds to a gain in vibrational energy in the molecules. This is called Stokes scattering.
  • Anti-Stokes scattering: Conversely, if the scattered photons have more energy than the incident photons, the molecules lose vibrational energy. This is known as anti-Stokes scattering.
Incident photon energy: E_0
Stokes scattered photon energy: E_0 - E_vib
Anti-Stokes scattered photon energy: E_0 + E_vib

Energy diagram

Energy-level diagrams are a useful way of representing Raman scattering:

ground state Virtual states Anti-Stokes Stokes

Here, the blue line represents the transition to the excited virtual state. The red and purple lines represent the transitions corresponding to energy loss and gain, respectively.

Selection rules and molecular information

Not all vibrations are Raman active. For a molecule to be Raman active, its polarizability must change as it vibrates. Polarizability is a measure of how easily the electron cloud of a molecule can be distorted by an external electric field.

  • Symmetrical stretching: These are usually Raman active as they usually cause a significant change in polarization.
  • Asymmetric stretching or bending: These often have little effect on the polarization and cannot always be seen in the Raman spectrum.

Raman spectroscopy is complementary to infrared spectroscopy, which is highly dependent on changes in the dipole moment. As a result, Raman and infrared spectroscopy often provide complementary information about molecular vibrations.

Example

For a simple diatomic molecule:

HCl

In the case of infrared spectroscopy, we measure vibrational modes that change the dipole moment. However, in Raman spectroscopy, we see changes in polarization, which provides insight into different vibrational modes that may not be apparent in infrared spectra.

Equipment

Raman spectrometers generally consist of the following major components:

  1. Laser source: Used to provide the monochromatic light required for scattering.
  2. Sample stage: where the sample is put to test.
  3. Dispersive element: Typically a diffraction grating, used to disperse the scattered light into its component wavelengths.
  4. Detector: captures the scattered light, usually a CCD detector.

Laser source

The laser is an important component of the Raman spectrometer. The selection of the laser wavelength can affect the Raman spectrum due to fluorescence interference. Lasers commonly used for Raman spectroscopy include:

  • Argon ion laser (488 nm, 514.5 nm)
  • Diode Laser(780nm, 830nm)

Sample preparation

Preparation of samples depends on the state of the material being analyzed. Solids, liquids, and gases can be analyzed using Raman spectroscopy, each with its own preparation considerations. Solid samples may need to be kept in powder form, while liquids can be kept in special cells. Gas samples must be kept within cells that have transparent windows for laser penetration.

Applications of Raman spectroscopy

Raman spectroscopy has a wide range of applications in various scientific disciplines:

  • Chemistry: Identification of molecules and compound characterization.
  • Materials science: Investigation of the crystal structures of materials, such as polymers and carbon nanotubes.
  • Biology: The study of biomolecules, including proteins and lipids.
  • Pharmaceuticals: Analysis of drug compounds and active pharmaceutical ingredients.

Its non-destructive nature makes Raman spectroscopy particularly valuable for delicate and sensitive samples in a variety of fields, including archaeology and art restoration.

Case study: Analysis of carbon nanotubes

Carbon nanotubes are a quintessential example of materials that can be analyzed using Raman spectroscopy. Different Raman peaks correspond to different properties of carbon nanotubes, such as diameter and chirality. The G-band and D-band are particularly important; they provide information about electronic properties and structural defects, respectively.

G-band: ~1580 cm-1
D-band: ~1350 cm-1

Data interpretation

Interpreting Raman spectra involves identifying peaks that correspond to specific molecular vibrations. This process often requires comparison to known spectra or computational modeling. Despite the complexity, the resulting data provide detailed information about the material structure and state.

Analysis of the sample spectrum

Consider a hypothetical Raman spectrum with these peak shifts:

Peak 1: 500 cm-1 - probably skeletal vibration
Peak 2: 1000 cm-1 - CH tilt
Peak 3: 1500 cm-1 - C=C stretching

Each peak corresponds to different molecular vibrations, providing detailed information about the molecular structure.

Benefits and limitations

Like any analytical method, Raman spectroscopy has its strengths and limitations:

Benefit

  • The non-destructive nature allows for the analysis of valuable samples.
  • Provides complementary data for IR spectroscopy.
  • Minimal sample preparation is required.
  • This works well in aquatic environments, as the water disperses very little.

Boundaries

  • Fluorescence interference can obscure Raman signals.
  • Low sensitivity compared to other spectroscopic techniques.
  • Limited ability to analyze aromatic structures.

Current trends and future directions

Technological advances continue to broaden the applications of Raman spectroscopy. Innovations such as surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have pushed the limits of detection and the boundaries of spectral resolution, promising a bright future for the integration of Raman spectroscopy into a variety of scientific fields.


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Raman Spectroscopy

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