4. Acquisition

Time requirement: 20 min

Sample: 10% CHCl₃ in [D₆]acetone with added Cr(acac)₃.

Set the instrument to normal ¹H NMR operation and load a pulse program as shown above. You have to set:

td	4 k
sw	5000 Hz
o1	1000 Hz off resonance from CHCl₃ signal
p1	10° ¹H transmitter pulse
d1	0.1 s
d2	gradient ring-down delay [300 ms–50 μs]
g1	positive pulsed field gradient, approximately 0.1 T/m, duration 1 ms, rectangular shape
ns	1

In order to determine the gradient ring-down delay d2 (see Exp. 11.4), go into the set-up mode of the instrument and display the FID on the screen. Choose a long gradient ring-down delay d2 of 300 ms and note the height and shape of the FID as a reference. According to the description provided by the manufacturer, shorten the ring-down delay until you observe a significant decrease in the FID. At this point vary the preemphasis time-constants and amplitudes. On some instruments there are three sets, which work with relatively long, medium, and short ring-down delays. The final aim is to observe a maximum FID at the shortest possible d2.

5. Processing

No processing required, since the FID is observed directly.

6. Result

The figure shows a typical FID obtained on an AMX-500 spectrometer with a BGU (10 A) gradient unit and an inverse gradient probe-head with self-shielded gradients with the parameters described above.