Rotational-echo double resonance

Rotational-echo double resonance, or REDOR, is a solid-state nuclear magnetic resonance method that reintroduces sensitivity to NMR dipolar coupling during magic angle spinning. Rotor-synchronized pulses acting on a second nuclear species interrupt the averaging of the interaction. Comparison with a matched reference echo reveals coupling-dependent dephasing; dipolar recoupling makes a normally suppressed interaction experimentally useful.^[ref]

Write $S_0$ for the full reference echo and $S$ for the echo measured with the dephasing pulse pattern. A common normalized readout isFor invented matched signals $S_0=100$ and $S=60$, the dephasing is0.40. This does not mean that40 percent of the nuclei are missing. The pulses change the coherent echo, and converting its attenuation into an internuclear distance requires an appropriate spin model and experimental calibration.

The early alanine record reproduced in Jacob Schaefer's retrospective is unusually concrete. It compares full and dephased carbon-13 echoes of nitrogen-15-labeled alanine, recorded by Terry Gullion on29May1987 at a carbon resonance frequency of50.3MHz. Dipolar evolution lasted ten rotor periods at3.2kHz spinning. The authors used an isolated-pair calculation and an isotropic powder average to choose pulse placements.^[ref]

An original timing calculation gives $10/(3200{\mathrm{s}}^{-1})=3.125\mathrm{ms}$ for those ten rotor periods. That is a duration derived from the reported settings, not a newly inferred pulse width. The retrospective explicitly says the sample weight and accumulated scan count were not recorded, so those details cannot be reconstructed from its spectrum.^[ref]

Because the dephasing experiment can involve many refocusing pulses, NMR pulse errors can mimic or obscure coupling-dependent changes. XY8 phase routing helped control off-resonance distortions in long REDOR pulse trains. Reference matching, pulse quality and a suitable model remain necessary even when the normalized dephasing is easy to calculate.^[ref]