4. Acquisition

Time requirement: 20 min Sample: 3% strychnine in CDCl₃ Record a normal ¹H spectrum of the sample and optimize the spectral width. Change to the 2D mode of the spectrometer and load the phase-sensitive gs-NOESY pulse program. You have to set:

td2: 2 k data points in F, td1: 256 data points in F, sw2: 10 ppm swl: 10 ppm o1: middle of ¹H NMR spectrum p1, p2, p3: 90° ¹H transmitter pulse dl:2s initial value for ¢; evolution: 3 μs increment for f, evolution: 1/[2-sw1] g1: sinusoidal-shaped field gradient with 5% truncation, 250 ms duration and ca. 0.05 T/m strength, with gradient loop counters, ring-down delays (100 μs), lock blanking and gradient coil blanking switches according to actual instrumentation used. Note that the duration of the gradient pulse replaces the usual mixing time and its strength must therefore be adjusted appropriately to rather weak values.

5. Processing

Apply zero-filling in F, to 512 words in order to have a matrix of 512x512 real data points. Use an exponential window in F with lb = 2 Hz and a 1/2-shifted squared sine bell in F,. Phase correction is usually only necessary in the F2 dimension.

6. Result

The figure shows an expansion of the spectrum obtained on an AMX-500 spectrome- ter using a BGU (10 A) gradient unit and a multinuclear z-gradient probe-head. Note that the spectrum looks almost identical to the result of Experiment 10.20, which was obtained with the full phase cycle requiring 16 transients.

7. Comments

The second r.f. pulse in the NOESY sequence creates –z magnetization, which is frequency-labeled with the proton chemical shift. This pathway is shown in the coher- ence diagram. In addition, however, this pulse can generate zero-, double-quantum- and antiphase coherences, since H,H spin coupling is also evolved during . The gradient pulse that replaces the mixing time dephases all these components except the