Electron Magnetic Resonance
Definition and meaning of Electron Magnetic Resonance in chemistry.
Electron magnetic resonance (EMR) is a spectroscopic technique, more commonly known as electron paramagnetic resonance (EPR) or electron spin resonance (ESR), that detects and characterizes chemical species containing unpaired electrons by measuring their resonant absorption of microwave radiation while immersed in an applied magnetic field.
In more detail
Placing a paramagnetic sample in a strong external field splits the electron spin's magnetic moment into two Zeeman energy levels. Sweeping the field strength while irradiating with fixed-frequency microwaves (typically ~9 GHz, X-band) induces transitions between these levels exactly when the resonance condition hν = gμ_BB is met, producing a characteristic absorption spectrum. The resulting g-factor, hyperfine splitting pattern, and linewidth reveal the electronic structure, oxidation state, and local chemical environment of the unpaired electron. Because it requires unpaired spins, EMR is uniquely suited to studying free radicals, transition-metal complexes, and defect centers that are invisible to NMR.
Key facts
| Common names | EPR (electron paramagnetic resonance), ESR (electron spin resonance) |
|---|---|
| Typical frequency band | ~9 GHz (X-band microwaves) |
| Detects | Species with unpaired electrons (radicals, transition-metal ions, defect centers) |
| Field | Analytical Chemistry |
EMR spectroscopy is routinely used to characterize the stable organic radical DPPH (2,2-diphenyl-1-picrylhydrazyl), whose single sharp resonance line serves as a standard for calibrating spectrometer field and g-factor.
Frequently asked questions
How does EMR differ from NMR?
EMR probes unpaired electron spins with microwave-frequency radiation in a magnetic field, while NMR probes nuclear spins with radiofrequency; because the electron's magnetic moment is about 658 times larger than a proton's, EMR is far more sensitive but only works on paramagnetic samples.
What information does the g-factor provide?
The g-factor describes how much the electron's resonance condition deviates from the free-electron value (2.0023), reflecting spin-orbit coupling and the electron's chemical and geometric environment, so it helps identify the radical or metal-ion species present.