Sample: 80% CHCl₃ in [D₆]acetone; do not use a degassed and sealed sample, since that would make the relaxation time of the CHCl₃ nuclei exceedingly long.

Obtain normal ¹H and ¹³C spectra of the sample and note the offsets. The spectrometer is set to ¹³C operation. First the desired pulse shape of the decoupler soft pulse and its length must be selected. For this experiment a Gaussian shaped pulse with 1024 data points and 30 ms length was chosen. You have to set:

decoupler attenuation for soft pulse, initial value 80 dB, to be varied
td	4 k
sw	500 Hz
o1	on ¹³C resonance
o2	on ¹H resonance
p1	90° ¹³C transmitter pulse
p2	¹³C spin-lock pulse for ¹³C decoupling at 15 dB transmitter attenuation, length equal to p4, effective phase must be the same as for the hard pulse p1, adjust if necessary.
p3	leave at zero for the determination of the correct attenuation of the shaped pulse. For phase determination, set to hard 90° ¹H decoupler pulse.
p4	Gaussian shaped ¹H decoupler pulse, 30 ms
d1	10 s
d2	1/[2J(C,H)] = 2.36 ms, calculated from ¹J(C,H) = 212 Hz
rg	receiver gain for correct ADC input
ns	1

Since it is not allowed to change the pulse-duration in this experiment you have to determine the signal-to-noise ratio of the signal as a function of the decoupler attenuation. With a very high decoupler attenuation (about 80 dB) adjust the phase of the C,H doublet to an antiphase pattern. Then repeat the experiment with different decoupler attenuations until you get a spectrum with zero intensity of the doublet, which corresponds to the soft 90° decoupler pulse. To obtain the correct phase of the soft decoupler pulse one introduces another hard 90° decoupler pulse p3 before the soft pulse in the pulse program. If both have the same phase, they are additive and yield an antiphase pattern with opposite phases to those adjusted before.

5. Processing

Use standard 1D processing as described in Experiment 3.2.

6. Result

The figure shows spectra obtained on an AMX-500 spectrometer. a is the initial spectrum with high decoupler attenuation; b is the spectrum obtained with a shaped 90° decoupler pulse where both signals disappear, and c was obtained with an additional hard 90° decoupler pulse p3, where both hard and soft pulses have the same effective phase.