Electron Paramagnetic Resonance
Electron Paramagnetic Resonance Spectroscopy · Also known as: EPR spectroscopy, ESR, electron spin resonance
Electron Paramagnetic Resonance (EPR), also called Electron Spin Resonance (ESR), is a spectroscopic technique that detects and characterizes unpaired electrons in molecules and materials. Discovered by Zavoiskii in 1945, EPR measures the absorption of microwave radiation by paramagnetic species in a magnetic field, providing information about electron spin states, local electronic environment, and molecular dynamics.
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When to use it
Apply EPR to detect and characterize any species with unpaired electrons: organic radicals, transition metal ions, defects in solids, and metal-protein complexes. EPR is powerful for following reaction kinetics and radical formation in photochemistry, photosynthesis, and oxidative stress studies. Requires paramagnetic species (EPR-silent diamagnetic molecules cannot be studied directly).
Strengths & limitations
- High selectivity: only paramagnetic species are detected; diamagnetic background is invisible
- Detailed structural information: hyperfine coupling reveals atomic-scale electronic environment
- Direct measurement of electron spin states and molecular dynamics
- Compatible with cryogenic temperatures, enabling study of short-lived transients and low-temperature materials
- Only paramagnetic species are observable; most organic molecules lack unpaired electrons
- Signal intensity is weak compared to NMR; requires sensitive detection and often signal averaging
- Overlapping hyperfine patterns can obscure structure in complex systems
- Quantification requires careful calibration and standard references
Frequently asked
Why is EPR called ESR in some contexts?
ESR (Electron Spin Resonance) and EPR (Electron Paramagnetic Resonance) are synonymous terms. EPR is now the preferred name in the spectroscopic community. ESR emphasizes the magnetic resonance of electron spin, while EPR emphasizes detection of paramagnetic species. Both terms refer to the same physical phenomenon.
What is the difference between EPR and NMR?
Both are magnetic resonance techniques, but NMR detects nuclear spins in non-magnetic fields while EPR detects electron spins in strong magnetic fields. EPR operates at higher microwave frequencies (GHz) compared to NMR (MHz). EPR requires paramagnetic species; NMR detects all nuclei with non-zero spin.
Can EPR quantify the number of unpaired electrons?
Yes, by double integrating the EPR absorption spectrum and comparing to a standard of known spin concentration. Proper temperature control and field calibration are essential. Spin quantification is often needed for biological applications.
Sources
- Zavoiskii, E. K. (1945). Paramagnetic relaxation of liquid solutions for perpendicular fields. Zhurnal Eksperimental'noi i Teoreticheskoi Fiziki, 15(6), 378-380. link ↗
- Aasa, R., & Vänngård, T. (1975). Parameter hyperfine interactions in high-spin ferric complexes with applications to biochemical electron paramagnetic resonance. The Journal of Magnetic Resonance, 19(3), 308-315. link ↗
How to cite this page
ScholarGate. (2026, June 3). Electron Paramagnetic Resonance Spectroscopy. ScholarGate. https://scholargate.app/en/spectroscopy/electron-paramagnetic-resonance
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
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