Pulse Width and Gradient Considerations

Nitrogen-15 pulse lengths can have a significant impact on how H— heteronuclear shift correlation data are acquired. Since has a much greater spectral width than C, if a molecule is expected to have widely disparate types of functional groups in its structure, it may be necessary to acquire the HMBC data in two segments. 

For older gradient triple resonance cryogenic NMR probes, pulses tended to be relatively long, e.g., 30—40 ps, which do not lend themselves to uniformly inverting resonances across wide spectral windows. Practically, for probes with 90° pulses in this range, it would be best to acquire a pair of HMBC spectra of 250 ppm each, with about a 50 ppm overlap, e.g., perhaps 0—250 and 200—450 ppm (see Fig. 5). Using 

The standards shown have been the most commonly utilized although others such as nitric acid, form-amide, and potassium nitrate have also been reported [5]. 

The impact of the N 90° pulse width is shown in Fig. 6. The two H— N HMBC spectra shown were acquired using methyl orange (3) as a model compound for the study. The spectrum in the top panel was acquired with the N 90° pulse = 35 ps, while the spectrum in the bottom panel was acquired identically using the same sample in the same probe with the N 90° pulse = 25 ps. Adiabatic 180° pulses were employed to acquire both spectra, and both spectra were plotted with an identical 

Nitrogen-15, of course, has a different gyromagnetic ratio (/n) than carbon-13, and consequently, the gradient ratios used in NMR pulse sequences employed to observe long-range HMBC correlations 

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