PHD
  1. Inorganic chemistry  1.1. Coordination chemistry  1.1.1. Ligand field theory  1.1.2. Crystal field theory  1.1.3. Molecular Orbital Theory for Coordination Compounds  1.1.4. Stability of Coordination Compounds  1.1.5. Electronic spectra of coordination compounds  1.1.6. Isomerism in Coordination Compounds  1.1.7. Kinetics and mechanism of coordination reactions  1.2. Organometallic Chemistry  1.2.1. Metal Carbonyls  1.2.2. Metal alkyl and aryl  1.2.3. Fischer and Schrock carbonaceans  1.2.4. Oxidative Addition and Reductive Elimination  1.2.5. Metal Hydrides  1.2.6. Catalysis by organometallic compounds  1.2.7. Metallocenes and Sandwich Complexes  1.2.8. CH activation  1.3. Solid state chemistry  1.3.1. Crystal structures and lattices  1.3.2. Band theory and electrical properties  1.3.3. Defects in the crystal  1.3.4. Synthesis of solid state materials  1.3.5. Magnetic and optical properties of solids  1.3.6. Superconductivity  1.3.7. Solid state ionics  1.4. Bio-inorganic chemistry  1.4.1. Metalloproteins and enzymes  1.4.2. Metal ion transport and storage  1.4.3. The role of metals in medicine  1.4.4. Bioinspired catalysis  1.4.5. Metal–DNA interactions  1.4.6. Metal Complexes in Medical Science  1.5. Lanthanides and Actinides  1.5.1. Electronic configuration and oxidation states in lanthanides and actinides  1.5.2. Spectral and Magnetic Properties in Lanthanides and Actinides  1.5.3. Separation and Extraction of Lanthanides and Actinides  1.5.4. Coordination chemistry of f-block elements  1.6. Main Group Chemistry  1.6.1. Chemistry of boron compounds  1.6.2. Chemistry of Silicon and Germanium Compounds  1.6.3. Chemistry of Phosphorus and Sulfur Compounds  1.6.4. Organosilicon chemistry  1.6.5. Noble Gas Chemistry
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition and substitution reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic Reactions  2.1.7. Photochemical reactions  2.2. Stereoscopic  2.2.1. Chirality and optical activity  2.2.2. Structural analysis  2.2.3. Stereoelectronic effects  2.2.4. Asymmetric synthesis  2.2.5. Dynamic Stereochemistry  2.3. Organometallic Chemistry in Organic Synthesis  2.3.1. Organolithium and organomagnesium reagents  2.3.2. Transition metal catalyzed coupling reactions  2.3.3. Palladium-catalyzed reactions  2.3.4. Olefin Metathesis  2.3.5. Gold and silver catalysis  2.4. Natural products chemistry  2.4.1. Alkaloids and terpenoids  2.4.2. Steroids and Flavonoids  2.4.3. Total synthesis of natural products  2.4.4. Biosynthesis of natural products  2.5. Supramolecular Chemistry  2.5.1. Host-Guest Chemistry  2.5.2. Self-assembly and molecular recognition  2.5.3. Supramolecular Catalysis  2.5.4. Molecular Machines
  3. Physical Chemistry  3.1. Thermodynamics  3.1.1. Laws of Thermodynamics  3.1.2. Chemical Potential and Equilibrium  3.1.3. Statistical thermodynamics  3.1.4. Non-equilibrium thermodynamics  3.2. Quantum Chemistry  3.2.1. Schrödinger equation and its applications  3.2.2. Molecular orbital theory  3.2.3. Density functional theory  3.2.4. Post-Hartree–Fock methods  3.3. Chemical kinetics  3.3.1. Reaction rate theory  3.3.2. Mechanism and rate laws  3.3.3. Catalysis and enzyme kinetics  3.3.4. Reaction dynamics  3.4. Spectroscopy and molecular structure  3.4.1. Infrared Spectroscopy  3.4.2. UV-visible spectroscopy  3.4.3. Nuclear Magnetic Resonance Spectroscopy  3.4.4. Mass Spectrometry  3.4.5. Raman Spectroscopy  3.5. Surface and Colloid Chemistry  3.5.1. Absorption and Catalysis  3.5.2. Surfactants and micelles  3.5.3. Colloidal Stability  3.5.4. Nanomaterials and Interfaces
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.1.4. Ion Chromatography  4.2. Electroanalytical techniques  4.2.1. Voltammetry and Polarography  4.2.2. Conductometry and potentiometry  4.2.3. Colometry  4.3. Spectroscopic Methods  4.3.1. Atomic absorption spectroscopy  4.3.2. Fluorescence Spectroscopy  4.3.3. X-ray diffraction  4.3.4. Electron paramagnetic resonance spectroscopy  4.4. Chemometrics  4.4.1. Data processing and statistical methods  4.4.2. Multidisciplinary analysis  4.4.3. Machine Learning in Analytical Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.2. Quantum chemistry methods  5.3. Computational drug design  5.4. Machine Learning in Chemistry  5.5. Ab initio molecular dynamics
  6. Biophysics and Medicinal Chemistry  6.1. Drug discovery and design  6.2. Enzyme kinetics and inhibition  6.3. Chemical biology approach  6.4. Protein-ligand interactions  6.5. Bioorthogonal chemistry
  7. Materials chemistry  7.1. Polymer chemistry  7.1.1. Polymerization mechanism  7.1.2. Functional Polymers  7.1.3. Biodegradable Polymer  7.1.4. Conducting and semiconducting polymers  7.2. Nano Chemistry  7.2.1. Nanoparticles and nanostructures  7.2.2. Carbon-based nanomaterials  7.2.3. Quantum dots  7.3. Energy and Environmental Chemistry  7.3.1. Battery and Fuel Cell Chemistry  7.3.2. Green chemistry and sustainable materials  7.3.3. Photocatalysis

PHD → Physical Chemistry → Spectroscopy and molecular structure ↓


Infrared Spectroscopy


Infrared (IR) spectroscopy is a powerful analytical technique used in chemistry to identify and study chemicals. This method of spectroscopy exploits the fact that molecules absorb specific frequencies of infrared light, which cause vibrational transitions within molecules. These absorptions, which occur at specific frequencies, make IR spectroscopy an integral tool in the field of chemical analysis for determining molecular structure.

Basic principles

Infrared spectroscopy detects the interaction of infrared radiation with matter. The basic idea is that when infrared light is shined on a sample, certain wavelengths of light are absorbed by the sample. These absorptions correspond to different vibrational states in the molecules.

The infrared spectrum is a graph showing the absorbance (or transmittance) of light versus its frequency. The IR spectrum typically ranges from about 4000 cm -1 to 400 cm -1, where frequency is measured in wave numbers, which is the reciprocal of wavelength, and represents the number of wave cycles in one centimetre.

Wave number (cm -1) = 1 / wavelength (cm)

Molecular vibrations

Molecular vibrations are motions that change the shape of a molecule through atomic displacement. There are two main categories of molecular vibrations:

  • Stretching: It involves change in the length of the bond.
  • Bending: This involves a change in the bond angle.

In general, stretching vibration requires more energy than bending motion.

For simple diatomic molecules, AB, two primary types of stretching can be observed in the infrared:

  • Isometric stretch
  • Asymmetric stretch
isometric stretch

In more complex molecules, vibrational modes may involve groups of moving atoms rather than just two, yielding more complex IR spectra and allowing finer details in structural determination.

Sample preparation and equipment

Infrared spectroscopy can be used with solids, liquids, or gases. Sample preparation depends on the state of matter, the physical properties of the sample, and the data desired from the analysis. Generally, solid samples must be pressed into pellets, usually with potassium bromide (KBr), or made into thin films, while liquid samples can be placed between two IR translucent salt plates, often made of sodium chloride (NaCl).

The main components of an infrared spectrometer are a light source, a monochromator to select specific wavelengths, a sample cell, and a detector. Fourier transform infrared (FTIR) spectroscopy, which produces an interferogram using an interferometer, is the modern technique of choice because of its signal-to-noise advantages and speed of data acquisition.

Applications and analysis

One of the fundamental applications of IR spectroscopy is to identify functional groups in organic compounds. Different functional groups absorb specific frequencies of IR radiation:

  • CH stretch: about 2900 cm -1
  • C=O stretching: about 1700 cm -1
  • OH stretch: about 3300 cm -1
Oh stretch C=O stretch CH stretch

By analyzing the IR spectrum of a sample, chemists can understand the presence of particular functional groups and infer structural information about the compound. Complex spectra may require further analysis using a database of known spectra to infer the structure of the sample.

Examples and case studies

Consider the spectrum of ethanol. The IR spectrum of ethanol shows characteristic peaks for OH stretching (about 3300 cm -1), CH stretching (about 2900 cm -1), and CO stretching (about 1050 cm -1). The structure of ethanol can be confirmed by comparing this spectrum with known compounds.

Complex molecules and IR spectra

More complex molecules such as proteins or polymers have correspondingly more complex IR spectra. For example, the peptide bonds in proteins give rise to several absorption bands collectively known as the amide bands. These bands provide information about the secondary structure of the protein, such as alpha-helices and beta-sheets.

Polymers, which are long chains of repeating units, show peaks depending on the type of bonds present in their structure. For example, polyethylene shows strong C-H stretching and bending bands.

Advancements and future directions

Modern advances in infrared spectroscopy have focused on improving resolution and sensitivity. Techniques such as attenuated total reflectance (ATR) and diffuse reflectance have expanded the capabilities of conventional IR spectroscopy by reducing sample preparation procedures and allowing direct analysis of solid samples.

Another emerging area is the integration of computerized databases and spectral analysis software, which helps to quickly and accurately identify unknown compounds using pattern recognition and automated peak determination.

Conclusion

Infrared spectroscopy is a fundamental analytical tool in physical chemistry. Its ability to identify functional groups and extract information about molecular structure makes it indispensable in both research and industrial applications. As technology advances, the field of infrared spectroscopy will continue to grow, providing deeper insights into molecular behavior and new applications in a variety of scientific disciplines.


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Infrared Spectroscopy

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