Nuclear Overhauser effect negative

The isotope N, with a natural abundance of 99.9%, has nuclear spin 7 = 1 and gives broad signals which are of little use for structural determinations. The N nucleus, with I = 1/2, is therefore preferred. However, the low natural abundance of about 0.4% and the extremely low relative sensitivity (Table 1) make measurements so difficult that N NMR spectroscopy was slow to become an accepted analytical tool. A further peculiarity is the negative magnetogyric ratio since, in proton decoupled spectra, the nuclear Overhauser effect can strongly reduce the signal intensity. DEPT and INEPT pulse techniques are therefore particularly important for N NMR spectroscopy. 

Meyer et al. recently reported transfer NOE (nuclear overhauser effect) (trNOE) (88), which detects the strong negative trNOE effect of receptor-bound molecules compared to the weak, positive trNOE of unbound compounds without the need of receptor labeling the method was used to identify an E-selectin antagonist from an artificially assembled 10-member library of saccharides (89). 

The differentiation between the 15N chemical shifts in dyes existing practically completely in their hydrazone forms is easy because —NaH— signals are shifted upfield and they are detected as strong negative signals because of the negative nuclear Overhauser effect (Table 18), 88... 

Until now the double-quantum coherence spectroscopy has not often been used for silicon frameworks. In double-quanmm coherence spectroscopy the silicon nucleus is almost ten times as sensitive as the C nucleus because of the higher natural abundance of Si (4.7% compared with 1.1% C). Disadvantages of the Si nucleus are the long relaxation times T i) and the negative nuclear Overhauser effect (NOE). In practice, this means that a very long recording time is necessary unless a relaxation reagent such as Cr(acac)3 is used. 

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