decoupler attenuation and 90° pulse for CPD
sw	200 ppm
o1	middle of ¹³C NMR spectrum
o2	middle of ¹H NMR spectrum
p1	90° ¹³C transmitter pulse
p2	180° ¹³C transmitter pulse
d1	4 s
d2	1/¹J(C,H) = 7 ms, calculated from ¹J(C,H) = 140 Hz
preacquisition delay as short as possible
ns	16

5. Processing

Use standard 1D processing as described in Experiment 3.2. Adjust the phase for the signals of the methyl groups to be positive and of the carboxyl C-atom to be negative.

6. Result

The figure shows a J-modulated spin-echo spectrum obtained on an ARX-200 spectrometer.

SEFT carbon-13 spectrum of ethyl crotonate

7. Comments

As shown in the figure on the next page, CH_n spin vectors develop differently after a 90° pulse depending on how many hydrogens are bonded to the carbon atom. If the delay d2 is set to 1/[¹J(C,H)], CH and CH₃ vectors have opposite phase compared with C and CH₂. If the decoupler is switched on at the end of d2 the phases are “frozen” and the corresponding signals have positive or negative sign. The second d2 delay is needed to refocus phase errors caused by the chemical shift evolution. A disadvantage