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 → Analytical chemistry → Spectroscopic Methods ↓


Electron paramagnetic resonance spectroscopy


Electron paramagnetic resonance (EPR) spectroscopy, also known as electron spin resonance (ESR), is a powerful analytical technique used primarily in chemistry and physics to study materials with unpaired electrons. Unpaired electrons are found in free radicals, transition metal ions, and defects in solids. Understanding EPR spectroscopy involves delving deeper into the interactions between magnetic fields and electron spins, providing insights into molecular structure, dynamics, and electronic distribution.

Fundamentals of electron paramagnetic resonance

At the core of EPR spectroscopy is the interaction of an external magnetic field with the magnetic dipole moment of the unpaired electron. This interaction causes the splitting of the magnetic energy levels associated with the spin states of the electron. The EPR technique applies microwave radiation to the sample, and resonance occurs when the energy of the microwave photons matches the energy difference between the split spin states.

The resonance condition can be given by the following equation:

hν = gμ B B 0

Where:

  • h is the Planck constant (6.626 × 10 -34 J·s).
  • ν is the frequency (in Hz) of the microwave radiation.
  • g is the g-factor, a dimensionless quantity specific to the system.
  • μ B is the Bohr magneton (9.274 × 10 -24 J/T).
  • B 0 is the strength of the external magnetic field (in Tesla).

In a typical EPR experiment, the magnetic field is changed while the frequency used is kept constant, and resonance occurs when the field strength satisfies the resonance condition.

Components of EPR spectrometer

EPR spectrometers typically consist of the following components:

  • Magnet: Produces a strong, uniform magnetic field.
  • Microwave source: Produces microwave radiation of a specific frequency.
  • Resonator: This contains the sample and amplifies the interaction of the microwave radiation with the electron spin.
  • Detector: Measures changes in microwave absorption by the sample.
  • Recorder: Captures the EPR signal for analysis.

Visualization of EPR transitions

Visualization of the EPR process can often be better understood using a simplified energy diagram:

energy E(Spin Up) E (spin down) haw

In the diagram, two energy levels are shown: one for the electron spin that is aligned with the field (low energy) and the other opposite (high energy). The transition between these levels is what is detected in the EPR experiment.

Hyperfine splitting

In many cases, EPR spectra show additional features known as hyperfine splitting. This splitting is caused by the interaction between the magnetic fields of unpaired electrons and nearby nuclear spins. The hyperfine interaction modifies the energy levels, resulting in multiple resonance lines.

The hyperfine interaction can be represented by the Hamiltonian:

[hat{H_{text{hf}}} = hat{S} cdot hat{A} cdot hat{I}]

Where:

  • hat{S} is the electron spin operator.
  • hat{A} is the hyperfine coupling constant tensor.
  • hat{I} is the nuclear spin operator.

For isotropic systems, the infinitesimal coupling constant A becomes a scalar. The number of lines and their intensity can provide information about the number and type of nuclei associated with unpaired electrons.

Applications of EPR spectroscopy

EPR spectroscopy is a versatile tool and has many applications, including:

  • Identification of free radicals: EPR can detect and characterize free radicals in chemical reactions and biological systems.
  • Study of metalloproteins: EPR helps in probing the electronic structure of metal centres in proteins.
  • Characterization of paramagnetic materials: EPR provides information about the local environment and symmetry of paramagnetic ions in solids.
  • Dosimetry: EPR is used to measure the dose of ionizing radiation in materials such as tooth enamel.

Example of detection of free radicals

Let us consider a common example: the detection of the hydroxyl radical (·OH), which is a reactive species in many chemical processes.

In an EPR experiment, the spectrum of the hydroxyl radical may appear as a set of lines due to the minute splitting caused by the interaction of the unpaired electron with the nuclear magnetic moment of the proton.

The EPR spectrum will give details such as the g-factor and hyperfine coupling constant, which can be used to confirm the identity of the radical and understand its reactivity.

Conclusion

Electron paramagnetic resonance spectroscopy is a key technique in understanding the magnetic properties of materials with unpaired electrons. It provides insights into molecular structure, dynamics, and electron distribution, proving invaluable in disciplines such as chemistry, biology, and materials science. By mastering the fundamentals and applications of EPR, researchers can gain the detailed information needed to push the boundaries of scientific knowledge.


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Electron paramagnetic resonance spectroscopy

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