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 ↓


Vibrational Spectroscopy


Vibrational spectroscopy is an important analytical tool used in physical chemistry to detect vibrational transitions in molecules. These transitions occur when molecules absorb certain frequencies of light, causing the molecules to be excited from one vibrational energy level to another. The two primary types of vibrational spectroscopy are infrared (IR) spectroscopy and Raman spectroscopy, each of which provides unique insights into molecular vibrations and structure.

Introduction to vibrational spectroscopy

Molecular vibrations involve the periodic motions of atoms within a molecule. These motions can be stretching (changes in bond length) or bending (changes in bond angle). Understanding these vibrations helps to elucidate molecular structures, identify functional groups, and analyze molecular interactions.

Each molecule has a unique vibrational signature depending on its structure. This signature can be compared to a fingerprint that helps identify and analyze unknown samples. Vibrational spectroscopy provides a non-destructive method of examining substances, making it an important tool in chemistry, physics, and biology.

Fundamentals of molecular vibrations

The atoms in any molecule are in a state of vibrational motion. Without external energy, these vibrations are in a very low energy state known as zero-point energy. By applying energy such as a light photon, these vibrational states can be excited to higher energy levels.

Types of Vibrations: Vibrations in molecules can generally be classified into two categories:

  • Stretching vibrations: This involves changes in the length of bonds between atoms. Stretching can be symmetric or asymmetric, depending on how the bonds move in relation to each other.
  • Bending vibrations: This involves a change in the angle between two bonds. Types of bending vibrations include scissoring, shaking, vibrating, and twisting.
        CO₂ for cutting with scissors Example:
             O=C=O

Infrared (IR) spectroscopy

Infrared spectroscopy is based on the absorption of IR radiation by molecules, causing excitation from lower to higher vibrational states. The IR spectrum is usually displayed as percent transmittance or absorbance as a function of wave number (cm -1 ).

In IR spectroscopy, molecules absorb specific frequencies that correspond to the vibration frequency of bonds. This process results in a spectrum with peaks corresponding to different vibrations. These peaks can be used to determine the functional groups present in the molecule.

        Simple IR spectra representation:
        | intensity
        ,      
        | | | (CH stretch)
        ,
        ,
        | | | | (C=O, NH, OH)
        ,
           4000 3000 2000 1500 500 Wavenumber (cm⁻¹)

Raman spectroscopy

Raman spectroscopy is a complementary technique to IR spectroscopy. It involves the inelastic scattering of light (Raman scattering) that occurs when monochromatic light interacts with molecular vibrations. When light strikes a molecule, most of the photons are scattered elastically (Rayleigh scattering), but a small fraction is scattered inelastically, with energy shifts corresponding to vibrational transitions.

Unlike IR spectroscopy, Raman is more sensitive to symmetric vibration modes and non-polar bonds. It is particularly valuable for the analysis of aqueous solutions because it is not affected by water absorption, which can be a limitation in IR spectroscopy.

        Raman spectra sketch:
        | intensity 
        ,
        ,      
        ,
        | / / /  / background noise
        ,
        ,
        -500 0 500 1000 1500 2000 (Shift in cm)

Comparison between IR and Raman spectroscopy

Both IR and Raman spectroscopy provide information about molecular vibrations, but they differ fundamentally in the way they detect these changes:

IR spectroscopy Raman spectroscopy
This involves the absorption of infrared light. This involves the scattering of light.
Sensitive to polar bonds and asymmetric vibrations. Sensitive to nonpolar bonds and symmetric vibrations.
Water may interfere with the measurement. Not affected by water, suitable for aqueous solution.

Applications of vibrational spectroscopy

Vibrational spectroscopy via both IR and Raman is applied in various scientific fields.

  • Chemical Analysis: Determining the identity and characterization of chemical compounds by revealing functional groups and molecular structure.
  • Biochemistry: The study of biological molecules such as proteins, nucleic acids, and lipids.
  • Pharmaceuticals: Quality control, ensuring purity of compounds and analyzing polymorphism in drugs.
  • Materials Science: Investigating the surface properties and structural formations of materials.
  • Environmental science: monitoring of pollutants and detection of hazardous substances.

Theoretical background

To understand the principle behind vibrational spectroscopy, one must consider the quantized nature of molecular vibrations. According to quantum mechanics, the energy of a vibrating molecule is given by:

        E_v=(v+1/2)hν

where v is the vibration quantum number, h is the Planck constant, and ν is the vibration frequency.

The selection rule for vibrational transitions is Δv = ±1 , which means that transitions usually occur between adjacent energy levels. However, overtone transitions with Δv = ±2, ±3... can occur, but are less intense.

Interpretation of spectra

Accurate interpretation of vibrational spectra is an important skill. Peaks in spectra correspond to different vibrational modes and are often characteristic of specific molecular bonds.

For IR spectra, certain regions can indicate specific types of chemical bonds:

  • 3650-3200 cm -1 - OH stretch
  • 3500-3300 cm -1 - NH stretch
  • 3000-2850 cm -1 - CH stretch (alkanes)
  • 1750-1650 cm -1 - C=O stretching
  • 1650-1450 cm -1 - C=C stretch

Conclusion

Vibrational spectroscopy, which includes both IR and Raman techniques, provides a comprehensive method for investigating molecular structures and properties. Its ability to identify and characterize functional groups and molecular geometries makes it indispensable in various fields of scientific research and industry.

As modern technology continues to develop, advances in spectroscopic techniques will deepen our understanding of molecular dynamics, and spur innovation in chemistry and related sciences.


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