Electron Spin Resonance
Definition and meaning of Electron Spin Resonance in chemistry.
Electron spin resonance (ESR), also called electron paramagnetic resonance (EPR), is a spectroscopic technique that detects and characterizes chemical species containing unpaired electrons by measuring their resonant absorption of microwave radiation in an applied magnetic field.
In more detail
In a magnetic field, the spin magnetic moment of an unpaired electron splits into two energy levels (the electronic Zeeman effect); resonant absorption occurs when the microwave photon energy equals the gap, hν = gμBB, where g is the electron g-factor. Interaction of the electron spin with nearby nuclear spins produces hyperfine splitting patterns in the spectrum, which reveal how the unpaired electron's density is distributed over the surrounding atoms. Because only paramagnetic species (free radicals, many transition metal and lanthanide ions, and certain defects) give a signal, ESR is highly selective and widely used to identify radicals and probe metal-ion coordination environments.
Key facts
| Field | Physical Chemistry |
|---|---|
| Resonance condition | hν = gμBB |
| Detects | Species with unpaired electrons (free radicals, transition-metal complexes, defects) |
| Typical radiation used | Microwaves (X-band ≈ 9-10 GHz) |
ESR spectroscopy readily detects the stable free radical DPPH (2,2-diphenyl-1-picrylhydrazyl), which gives a single sharp resonance line and is used as a standard reference for calibrating spectrometer g-values and sensitivity.
Frequently asked questions
How does ESR differ from NMR?
ESR probes transitions between electron spin states and requires unpaired electrons (paramagnetic samples), operating at microwave frequencies, whereas NMR probes nuclear spin transitions in diamagnetic or paramagnetic samples alike, operating at lower radiofrequencies.
Why can't ESR study most stable organic molecules?
Most stable organic compounds have all electrons paired, giving no net electron spin and therefore no ESR signal; only paramagnetic species such as radicals, radical ions, and many transition-metal complexes are ESR-active.