J-Modulated Spectra. This is the most primitive form of spectral editing. By placing a suitable delay time between the pulse and the beginning of the acquisition, spectra are obtained in which C and CH2 groups are positive, and CH and CH3 are negative.
In this experiment, after the pulse there is a short delay, during which the decoupler is turned off, and the 13C NMR spectrum becomes modulated by the CH coupling frequency. After the delay the decoupler is turned on, and the FID is recorded. If the delay is 1/J then the quaternary and CH2 carbons are positive, and the CH and CH3 signals are negative. If the delay is 1/2J all peaks except quaternary are nulled.
