4. Acquisition

Time requirement: 5 min Sample: 3% strychnine in CDCl₃ Record a normal ¹H NMR spectrum of the sample. Determine the 90° pulse-width for the hard ¹H transmitter pulse, select a Gaussian pulse shape for the soft pulse, and determine the correct attenuation corresponding to a 180° pulse at 50 ms duration (see Exp. 7.1). Determination of the phase difference between the hard and the soft pulse is not necessary. Load the pulse program for gs-SELCOSY. You have to set:

td: 32 k sw: 10 ppm o1: on resonance of selected signal. If the software allows offsets for selective pulses, one can also put o1 in the middle of the ¹H NMR spectrum. p1, p4: 90°¹H transmitter pulse p2, p3: 180° Gaussian shape ¹H transmitter pulse, 50 ms length, transmitter attenuation corresponding to 180° excitation [64 dB] d1: 2s d2: 30-60 ms, adjusted to ~ 1/[2/(H,H)] g1–g6: sinusoidal-shaped field gradients with 5% truncation, 2 ms duration and ca. 0.1 T/m strength, with gradient loop counters, ring-down delays (100 μs), lock blanking and gradient coil blanking switches according to actual instrumentation used. Gradient strength ratio: 40 : 40:7: 7:20:-20 ds: 2

5. Processing

Use standard ¹H processing as described in Experiment 3.1. Note that the signals of the irradiated protons are unperturbed and that the signals of the coupling partners show the active coupling in antiphase. For the method shown here you have to adjust the phase for each multiplet individually because of the linear phase shift across the spectrum caused by the finite duration of the pulsed field gradients.

6. Result

The figure shows the result obtained on an AMX-500 spectrometer with a BGU (10 A) gradient unit and an inverse gradient probe-head with self-shielded gradients. In a an expanded portion of the normal ¹H NMR spectrum is shown, in b (d2 = 50 ms) H-12 was selected, giving the responses of both H-11 and (weakly) of H-13, and in ¢ (d2 = 50 ms) H-15β was selected, giving the responses of H-15α, H-14, and H-16. Note that the signals of H-14 and H-16 have yet to be correctly phased for analysis. Compare the result with that of Experiment 7.5 and note that all artefacts have disappeared.