decoupling, record a normal ¹³C NMR spectrum and note the offset of CHCl₃. Finally load a pulse program for c.w. decoupling. You have to set:

decoupler attenuation, to be varied
td	4 k
sw	500 Hz
o1	on resonance of ¹³C signal
o2	50 Hz offset from the ¹H signal
p1	45° ¹³C transmitter pulse
d1	2 s
ns	1

First record the ¹³C doublet of CHCl₃ using 500 Hz spectral width and the transmitter offset adjusted directly on resonance; the decoupler must be switched off for this first experiment. Measure the spin coupling constant from the separation of the two doublet lines. Adjust the decoupler offset 50 Hz from the chloroform ¹H resonance and record the ¹³C spectra with different decoupler attenuations. For certain decoupler offsets and attenuations you will obtain inner and outer doublets; the outer ones have a larger splitting than the normal CHCl₃ doublet. Measure the residual splitting within the outer or inner doublet and from this calculate the decoupler field strength γB₂ [Hz] using Equation (1).

γB₂ = [(J Δ / J_R)² + 0.25 (J_R² − J²) − Δ²]^1/2(1)

where J is the unperturbed spin coupling constant (214.8 Hz), J_R the residual splitting of the inner or outer doublet and Δ the offset of the decoupler frequency from the ¹H resonance. As an exercise perform the experiment with different decoupler offsets.

5. Processing

Use standard 1D processing as described in Experiment 3.2; use zero-filling to ensure enough data points to obtain accurate values for the residual splittings.

6. Result

A typical set of values for Δ = 50 Hz obtained on an ARX-200 spectrometer is given below, from which the graph was calculated:

Dec. att. [dB]	62	59	56	53	50	47	44	41	38
J_R [Hz]	91	90	84	77	61	50	38	29	21