Record a normal ¹H NMR spectrum and note the ¹H frequency offset of the methyl group attached to the double bond at δ_H = 1.8. Define and calibrate a RE-BURP pulse for p3 in the ¹H channel (see Exp. 7.2). Change to the 2D mode of the spectrometer software. You have to set:

initial value for t₁ evolution: 3 μs
td2	1 k data points in F₂
td1	128 data points in F₁
sw2	200 ppm
sw1	50 Hz
o1	middle of ¹³C NMR spectrum
o2	on resonance of the methyl group at δ_H = 1.8
p1	90° ¹³C transmitter pulse
p2	180° ¹³C transmitter pulse
p3	selective 180° RE-BURP ¹H decoupler pulse [40 ms length at 46 dB]
d1	2 s
increment for t₁ evolution = 1/[2·sw1]
ds	2
preacquisition delay: as small as possible
ns	8

5. Processing

Apply zero-filling in F₁ to 256 real data points. Use π/2 shifted sinusoidal windows in both dimensions. Apply complex Fourier transformation corresponding to the N-type signal selection using the quadrature-off mode in F₁. Phase correction is not necessary, since the data are processed in the magnitude mode.

6. Result

The figure shows the result obtained on an AMX-500 spectrometer for the signals of C-2 and C-3. The coupling of these carbon nuclei with the protons of the methyl group C-4 can be seen in F₁ from the splitting of the quartets. Note that in this case the geminal coupling constant ²J(C-3,H-4) of 7.1 Hz is slightly larger than the vicinal coupling constant ³J(C-2,H-4) of 6.7 Hz. A reliable assignment of these spin couplings by any other means would be very difficult. Compare the signal patterns with the result obtained in Experiment 10.2.

7. Comments

This method can be thought of as the selective version of the heteronuclear 2D J-resolved technique. The selective 180° pulse consisting of p3 and p4 acts only on the chosen protons. At the end of the t₁ period the spin-echo is modulated only by this selected spin coupling. The spin coupling to all other protons is not refocused and is therefore not observable in the final spectrum.