ESR Spectroscopy: Detecting and Identifying Unpaired Electrons
A technique that sees only species with unpaired electrons, which makes it uniquely selective for radicals and certain metal complexes.
The basic phenomenon
An electron has spin, and in a magnetic field its two spin orientations differ in energy. The gap is proportional to the field:
Absorption occurs when the microwave frequency matches that gap. In practice the frequency is held constant and the magnetic field is swept, which is the reverse of how NMR spectra are usually presented.
The g value
For a free electron g is close to 2.0023. In a real molecule it deviates, because spin–orbit coupling mixes in some orbital angular momentum.
The size and direction of that deviation is informative. Organic radicals, where spin–orbit coupling is weak, have g values very close to the free electron value. Transition metal complexes deviate much more, because heavier atoms have stronger spin–orbit coupling. So the g value alone often identifies what kind of species is present.
In an anisotropic environment g depends on orientation, and the resulting pattern in a frozen solution or a solid gives information about the symmetry of the site.
Hyperfine splitting
The unpaired electron interacts with nearby nuclei that have magnetic moments, splitting the signal. This is the most structurally informative part of the spectrum, because the pattern reveals which nuclei the electron reaches.
| Nuclei | Lines produced | Intensity pattern |
|---|---|---|
| One nucleus with I = ½ | 2 | 1 : 1 |
| Two equivalent nuclei with I = ½ | 3 | 1 : 2 : 1 |
| Three equivalent nuclei with I = ½ | 4 | 1 : 3 : 3 : 1 |
| n equivalent nuclei with I = ½ | n + 1 | Binomial coefficients |
| One nucleus with I = 1 | 3 | 1 : 1 : 1 |
The general rule is 2nI + 1 lines for n equivalent nuclei of spin I. Non-equivalent sets split independently, multiplying the number of lines — so a spectrum with a large number of lines usually indicates several inequivalent sets.
What the splitting tells you
The magnitude of the hyperfine coupling to a given nucleus measures how much unpaired electron density sits at that nucleus. So the pattern maps the delocalisation of the unpaired electron across the molecule, which is precisely what one wants to know about a radical.
A radical showing coupling to nuclei far from where the unpaired electron was nominally placed is direct evidence of delocalisation — a favourite exam inference.
Applications
- Detecting radical intermediates in reaction mechanisms, confirming that a proposed radical pathway operates.
- Characterising transition metal complexes with unpaired electrons, where g anisotropy reports on the coordination geometry.
- Spin trapping, in which a short-lived radical is converted to a longer-lived one that can be observed, extending the technique to species too fleeting to detect directly.
- Studying defects in solids, including colour centres, which contain trapped unpaired electrons.
Comparison with NMR
| ESR | NMR | |
|---|---|---|
| Observes | Unpaired electron spins | Nuclear spins |
| Frequency region | Microwave | Radiofrequency |
| Sensitivity | Much higher | Lower |
| Applicable to | Only paramagnetic species | Most molecules |
The much larger electron magnetic moment is why ESR is more sensitive and why it operates at far higher frequency for the same field.
Frequently asked questions
Why can ESR not be used on ordinary organic molecules?
Because their electrons are paired, so there is no net electron spin to observe. Only paramagnetic species give signals.
What does a g value close to the free electron value indicate?
Weak spin–orbit coupling, typical of organic radicals where the unpaired electron sits on light atoms.
How does hyperfine coupling reveal structure?
The coupling to each nucleus is proportional to the unpaired electron density there, so the pattern shows how the electron is distributed across the molecule.
What is spin trapping for?
To observe radicals too short-lived to detect directly. They are converted into stable radicals whose spectra still identify the original species.
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