Sample: 10% CHCl₃ in [D₆]acetone, do not use a degassed and sealed sample since that would make the relaxation times of carbons and protons exceedingly long.

Obtain both offsets for the ¹H and ¹³C signals of the sample with the spectrometer in the normal mode and tune both coils to the sample. Older spectrometers are then changed to inverse set-up, and an inverse probe-head (inner coil ¹H, outer coil ¹³C) is placed into the magnet. Since the inverse set-up varies widely dependent on the year of the spectrometer's construction it cannot be discussed here. Load the correct pulse program to obtain an inverse ¹H NMR spectrum with ¹³C decoupling. You have to set:

td	4 k
sw	500 Hz
o1	on resonance of ¹H signal
o2	on resonance of ¹³C signal, very important for Exp. b
p1	90° ¹H transmitter pulse in inverse configuration (Exp. 2.4)
p2	¹³C decoupler pulse in inverse configuration, exp. a: 0 μs as starting value, to be varied; exp. b: 100 μs
d1	10 s
d2	1/[2 J(C,H)] = 2.33 ms for ¹J(C,H) = 215 Hz
rg	receiver gain for correct ADC input
¹³C decoupler attenuation	exp. a: [0 dB]; exp. b: to be varied [70 μs]
ns	1

Exp. a: Record a first spectrum with p2 = 0 and adjust the phase of the big signal stemming from the protons bound to ¹²C in dispersion; look for a clean antiphase pattern of the ¹³C satellites. Then repeat the experiment with increasing pulse-durations p2 until you get a zero for the satellites, which corresponds to the 90° decoupler pulse.

Exp. b: In a second set of experiments use a high decoupler attenuation for GARP [13 dB] and vary it so that p2 is in the region of 70 μs (depending on the magnetic field strength B₀ of the instrument used).

5. Processing

Use standard 1D processing (see Exp. 3.1) applying an exponential window with a line-broadening factor lb = 1 Hz.

6. Result

The figure shows the results of a 0° decoupling pulse (a) and of a 90° decoupling pulse (b) obtained on an AMX-500 spectrometer with an inverse probe-head. Note the disappearance of the satellites in (b).

7. Comments

The product operator formalism is exactly the same as given in Experiment 2.3, only the notation for C and H spins has to be interchanged. If this method is used to deter-