The GRECCO (Gradient-Enhanced Carbon COupling) experiment has been porposed to measure specific long-range carbon-carbon coupling constants. In this experiment, PFGs are used for coherence selection and, therefore, artifact free spectra are obtained without the presence of artefacts due to subtraction imperfections.

The best performance of this experiment is on AVANCE spectrometers equipped with pulsed field gradients incorporated in 13C-optimized probehead and selective excitation using shaped pulses. In inverse probeheads, the experimental sensitivity is decreased because 13C is detected.

The basic pulse sequence of the GRECCO experiment ( 95JACS9547 ) consists of the following parts:

• Initial selective polarization transfer from a selected proton to a selected carbon is accomplished using cross-polarization. Thus, after a 90º pulse, selective WALTZ16 pulse trains are simultaneously applied to both channels.
• Evolution of carbon-carbon coupling constants during a fixed delay optimized to 1/2*J(CC) (usually 5-10 Hz).
• Selective carbon inversion using a SPFGE scheme.
• 90º carbon pulse to polarization transfer to J-coupled carbon nuclei.
• Carbon detection under broadband proton decoupling.

The use of PFG in carbon detected experiments has been discussed in 94MRC665 .

The GRECCO experiment can be run with minor changes from a predefined parameter set. Important parameters to consider are:

Setting of the selected proton and carbon offsets.

Optimization of the selective cross-polarization transfer scheme. In the original paper, power level for both transmitter and decoupler channels were calibrated to have  a 90º pulse length of 1.8 ms).

Optimization of the defocusing period as a function of 1/2*J(CC)

Selectivity of the selective inversion 13C pulse: the user must define the offset, the shape, the duration and the power level needed for a defined excitation profile.

Set a large number of scans.

In the GRECCO spectrum appear the selected carbon as a strong signal and all its J-coupled carbons also appear as small anti-phase doublets. The use of gradients allows to obtain a clean, artefact-free spectrum from which careful analysis of the multiplets can be done.