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 ↓


Electron paramagnetic resonance spectroscopy


Electron paramagnetic resonance (EPR) spectroscopy, also known as electron spin resonance (ESR) spectroscopy, is a form of magnetic resonance spectroscopy. Think of it as a sibling of nuclear magnetic resonance (NMR) spectroscopy, but instead of focusing on the nucleus, it focuses on unpaired electrons. This technique is particularly interesting in the fields of chemistry and physics because it allows scientists to study substances with unpaired electrons. This can include radicals, transition metal complexes, and defects in solids.

Fundamentals of EPR spectroscopy

EPR spectroscopy is based on the interaction between electromagnetic radiation and the magnetic moments of unpaired electrons in a magnetic field. Unpaired electrons have a property called "spin," and this spin creates a magnetic moment. In the presence of an external magnetic field, these unpaired electrons can exist in one of two states: aligned with the field or opposite to the field. These states have different energies.

When we apply microwave radiation of the appropriate frequency to a sample in a magnetic field, transitions between the spin states of unpaired electrons can occur. Resonance conditions occur when the energy of the microwave radiation matches the energy difference between the two spin states. The energy difference is given by the equation:

    ΔE= gμBB

Where:

  • ΔE is the energy difference.
  • g is the g-factor, a dimensionless quantity that characterizes the interaction of magnetic moment and magnetic field.
  • μ B is the Bohr magneton, a physical constant related to the magnetic moment of the electron.
  • B is the strength of the magnetic field.

The EPR spectrum is a plot of the absorption of microwave radiation as a function of the strength of the magnetic field. Each signal in the spectrum corresponds to a different site in the sample that has unpaired electrons.

Visual example

+-------------+ Microwave Radiation +-------------+
| Unpaired |=============================>| Spin  |
| Electron | (Energy Absorption) | Transition     |

EPR spectroscopy instrumentation

The basic components of an EPR spectrometer include a source of microwave radiation, a resonance cavity or waveguide, a magnet to provide the magnetic field, and a detector to measure the absorption of the microwaves by the sample. The resonance cavity holds the sample and aligns it with both the microwave and magnetic fields.

Experimental setup diagram

  ,
 ,
 | Microwave source |====| Cavity with sample
 ,
      ,
 Microwave Detector

Applications of EPR spectroscopy

EPR spectroscopy has many applications in various scientific fields. In chemistry, it is used to study metal complexes and radical reactions. In physics, it can help investigate low-dimensional systems and measure solid-state defects. Here are some examples:

Example: Investigation of transition metal complexes

EPR helps identify the oxidation state of metals in a complex and its surrounding ligand field environment. For example, a copper complex may show signs of Cu(II), which has an unpaired electron.

Example: Study of radical reactions

Radicals are usually short-lived, but with EPR, you can catch them in action. For example, short-lived radicals in polymerization reactions can be detected and studied to better understand the reaction mechanism.

Example: Investigating biological systems

EPR is used in biology to study active centers in metalloproteins and enzymes that undergo redox reactions. One example of this is to investigate the manganese cluster in photosystem II of plants, which is important for the water-splitting reaction in photosynthesis.

Factors affecting EPR spectra

EPR spectra can be affected by several factors such as hyperfine splitting, g-factor anisotropy, and zero-field splitting.

Hyperfine splitting

Hyperfine splitting occurs when the magnetic moment of unpaired electrons interacts with nearby nuclear spins, causing a single EPR line to split into multiple lines. This interaction provides valuable information about the number and type of nuclei surrounding the unpaired electron.

g-factor anisotropy

The g-factor is not always constant; it can vary depending on the orientation of the molecule relative to the magnetic field. This can cause line broadening or splitting in EPR spectra, especially in solid samples where the molecular orientation varies.

Zero-field splitting

For systems with more than one unpaired electron, such as in some metal complexes, electron-electron interactions can cause a shift in energy levels, resulting in a zero-field splitting (ZFS). The ZFS can be substantial and observed without the need for an external magnetic field.

The challenge of interpretation

Interpreting EPR spectra can sometimes be challenging due to complexities such as overlapping signals, noise, and multiple unpaired electron sites in the sample. Advanced techniques, including computational methods, often aid in understanding complex spectra.

Example: Overlapping signals

In cases where multiple paramagnetic centers are present, their signals may overlap, making it difficult to identify the individual contribution of each center. Deconvolution techniques can be applied to resolve these signals.

Conclusion

Electron paramagnetic resonance spectroscopy is a powerful tool for exploring chemical, physical, and biological systems characterized by unpaired electrons. Despite its analytical challenges, EPR remains invaluable for understanding molecular and electronic structures. Ongoing advances in EPR technologies and analytical methods continue to expand its capabilities and applications, making EPR spectroscopy an essential technique in scientific research.


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Electron paramagnetic resonance spectroscopy

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