Electron Spin Resonance (ESR), also called Electron Paramagnetic Resonance (EPR), is a spectroscopic technique used to study materials with unpaired electrons, such as free radicals, transition metal ions, and defects in solids. It works by detecting the resonant absorption of microwave radiation by these unpaired electron spins in the presence of a magnetic field.

🌍 Definition

• ESR/EPR: A branch of absorption spectroscopy that measures the magnetic properties of unpaired electrons.
• Principle: When placed in a magnetic field, unpaired electron spins split into energy levels. Microwave radiation induces transitions between these levels, producing a resonance signal.

🔑 Key Features

• Magnetic Field + Microwave Radiation: Electrons absorb energy when the microwave frequency matches the energy difference between spin states.
• Selectivity: Only paramagnetic species (with unpaired electrons) are detectable.
• Sensitivity: Can detect very small concentrations of radicals or transition metal ions.
• Non-Destructive: Leaves the sample intact, unlike chemical assays.

📚 Applications

• Chemistry:
  • Identifying free radicals in reactions.
  • Studying transition metal complexes.
• Biology & Medicine:
  • Detecting reactive oxygen species in cells.
  • Investigating metalloproteins (e.g., hemoglobin, cytochromes).
• Materials Science:
  • Characterizing defects in semiconductors.
  • Studying radiation damage in crystals.
• Physics:
  • Quantum computing research (single-spin detection).
  • Magnetic resonance studies of solid-state systems.

🛠 Instrumentation

• Magnet System: Provides a strong, stable magnetic field.
• Microwave Source: Typically klystrons or Gunn diodes.
• Resonant Cavity: Holds the sample and enhances signal.
• Detector: Measures absorption of microwave radiation.

✨ Summary

ESR is a powerful spectroscopic method for probing unpaired electrons, widely used in chemistry, biology, materials science, and physics. It reveals the presence, environment, and dynamics of radicals and paramagnetic species, making it essential for both fundamental research and applied science.

Sources: Microbe Notes overview, ETH Zurich ESR introduction, Chem LibreTexts ESR section, Priyam Study Centre ESR notes, ScienceVivid ESR introduction.