Electron Nuclear Double Resonance
Definition and meaning of Electron Nuclear Double Resonance in chemistry.
Electron nuclear double resonance (ENDOR) is a magnetic resonance technique that combines electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) to measure the hyperfine coupling between an unpaired electron and the magnetic nuclei near it, with far greater resolution than EPR alone.
In more detail
In an ENDOR experiment, a paramagnetic sample sits in a strong magnetic field while microwave radiation partially saturates an EPR transition and a second, swept radiofrequency field is applied simultaneously. When the radiofrequency matches a nuclear spin transition within the electron's hyperfine manifold, it desaturates the EPR signal, producing a detectable ENDOR response. Because nuclear transitions are far sharper than electron transitions, this uncovers hyperfine and nuclear quadrupole coupling constants that are unresolved or overlapping in a conventional EPR spectrum, revealing how the unpaired electron's spin density is distributed over surrounding nuclei.
Key facts
| Field | Physical Chemistry |
|---|---|
| Abbreviation | ENDOR |
| Combines | EPR (microwave) + NMR (radiofrequency) |
| First demonstrated by | George Feher, 1956 |
ENDOR was used on X-irradiated single crystals of malonic acid to resolve individual proton hyperfine couplings of the trapped radical, one of the classic systems on which the technique was developed and validated by George Feher in 1956.
Frequently asked questions
How does ENDOR improve on ordinary EPR spectroscopy?
EPR spectra of radicals with many coupled nuclei often show broadened, overlapping hyperfine lines that are hard to interpret. ENDOR adds a swept radiofrequency field that probes nuclear spin transitions directly, giving much narrower lines and separating hyperfine couplings that EPR cannot resolve.
What can ENDOR be used to study?
Any system with unpaired electrons and detectable EPR transitions, including organic free radicals, transition metal and rare-earth complexes, defect centers in solids, and metalloproteins, to map how the electron spin interacts with surrounding nuclei.