d3: set to effective gradient duration [1.05 ms]
d4: set to d2 minus effective gradient duration
g1–g3: sinusoidal-shaped field gradients, ca. 0.1 T/m strength, with gradient loop counters, ring-down delays (50 μs), lock blanking and gradient coil blanking switches according to actual instrumentation used. Gradient duration and strength according to the table:

gradient	g1	g2	g3
duration [ms]	1	1	1
strength [%]	80	30	20.1

90° ¹³C decoupler pulse and attenuation for GARP sequence [70 μs, 11.5 dB]
increment for t₁: 1/[4·sw1]
ds: 16
ns: 8

5. Processing

Apply zero-filling to 1024 real data points in F, to obtain a matrix of 1kx1k real data points. Use Gaussian multiplication [gb = 0.2, lb = –4 Hz] in F, and 7/3-shifted squared sinusoidal window in F. Phase correction and base-line correction may be necessary for both dimensions. Reference the ¹³C dimension using the =-scale procedure described in the introduction to this chapter using the DSS signal set to 6; = 0.

6. Result

The figure shows the 2D spectrum obtained on a DRX-600 spectrometer with a multinuclear inverse z-gradient triple-resonance probe-head (fixed third channel on ¹³C). A huge number of correlation signals can be seen. The relevant chemical shift ranges for further, more advanced 3D experiments on proteins can be easily extracted.

7. Comments

The pulse sequence is divided into four sections separated by dotted vertical lines. First we find in a a standard INEPT transfer from proton to ¹³C yielding the coherence 21, /c, after pulse p8 as described for the standard HSQC experiment 10.17.

The next section b is only for optimization. The dephasing gradient g! is applied in a [delay–180° pulse–gradient] bracket to prevent phase distortion during the finite duration of the gradient. The pulses p10 and p11 and the gradient g2 produce a gradient zz-filter (see Exp. 11.8). The 90° pulse p10 on ¹³C creates 2/ H, Ic, and the following gradient pulse g2 dephases all residual magnetization that is still in the xy-plane. This is especially important for the residual water signal. Pulse p11 on ¹³C restores the anti- phase magnetization 2/ H, Icy .