The ENDOR experiment thus consists of 'sitting on' an ESR transition with microwave
power sufficient to cause some saturation, and at the same time sweeping through the NMR
frequencies. The ENDOR effect then causes small changes, usually of the order of 1% in the
intensity of the EMR line (a very sensitive EMR spectrometer is needed). The spectrum which
is observed for a single electron and single nucleus of
spin half consists of two lines separated by the electron-nucleus hyperfine interaction
(to a first order approximation). The lines are
centred on the nuclear magnetic resonance frequency of the particular nucleus at 0.34T.This enables the ENDOR measurement to :
- Measure hyperfine splittings more accurately
ENDOR lines have widths between about 3kHz and 1MHz, typically 10kHz. EMR lines have widths down to around 1MHz, typically 3MHz or more. This enables splittings to be obtained to 10-3% accuracy. - Measure smaller hyperfine splittings than EMR
This follows from the above - Identify coupled nuclei
The frequency of the NMR resonance identifies the coupled nucleus. In addition |gn| can be obtained to within 0.1% in favourable circumstances - In liquids ENDOR will simplify the spectra.
A radical with x sets of y non-equivalent protons gives 2x ENDOR lines, but (y+1)x EMR lines. Put simply, to first order, the ENDOR spectrum of a liquid has double the number of lines as hyperfine couplings. - Measure quadrupole splittings for nuclei with I>ω
Quadrupole splittings are not observable from first order EMR spectra