2.9). Here 85 dB attenuation was used. As in Experiment 4.9, a reference signal that does not introduce an NOE effect is irradiated in a separate measurement. TMS is normally used for this purpose. Depending on the spectrometer used, an automatic program may be loaded which organizes the stepping of the irradiation frequency through the multiplets. For dilute samples the spectra are accumulated in an interleaved mode. You have to set:

decoupler attenuation for presaturation [85 dB]
td	32 k
sw	10 ppm
o1	middle of spectrum
o2	lists of frequencies within the multiplets to be irradiated
p1	90° ¹H transmitter pulse
d1	0.1 s
d2	7.2 s total pre-irradiation time; in the example two multiplets each with 6 lines were chosen, each line was irradiated for 400 ms, and the process was performed three times. In addition, the whole sequence for two irradiated multiplets and one reference line was repeated four times and the data averaged.
ds	4
ns	8

5. Processing

Use processing for NOE difference spectroscopy as described in Experiment 4.9.

6. Result

The figure shows the result obtained on an AMX-500 spectrometer. Spectrum a shows the normal spectrum in the region between δ_H = 6.0 and 1.0. In spectrum b proton 12 at δ_H = 4.2 was irradiated. Intensity enhancements can be seen for three signals, namely for proton 13 at δ_H = 1.18, one of the protons 11 at δ_H = 3.05, and a slight effect for one of the protons 23 at δ_H = 3.98. In spectrum c one of the protons 23 at δ_H = 4.05 was irradiated. Note that with this technique the NOE effect of the nearby resonance of the other proton 23 at δ_H = 3.98 can be observed without distortion; the olefinic proton 22 also shows an effect.

7. Comments

See Experiments 4.8 and 4.9 for some short remarks on the mechanism of the NOE effect. Compared with Experiment 4.9 the experiment described here requires more preparation and instrument adjustments, but is less prone to artefacts. An even more advanced version with selective pulses and gradient selection is described in Experiment 11.10.