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Monday, October 6, 2008

Negative NOE's and the Decision on Which Decoupling Mode to Use

Nuclei with negative gyromagnetic ratios, such as 15N and 29Si, exhibit negative NOE's with nearby protons. When observing these nuclei directly with proton decoupling, the intensity of the resonances will be decreased to an extent dependant on the magnitude of the NOE's. Usually it is best to collect the NMR data for these nuclei with inverse gated decoupling so that the effect of the NOE's on the intensity of the lines is minimized. There are cases however where the magnitude of the NOE is so large that the intensity of the resonance collected with full decoupling becomes negative and even stronger than that observed with inverse gated decoupling. In such cases it is advantageous to leave the decoupler on 100 % of the time. As the figure below shows, this is indeed the situation with the 15N resonance of formamide where the magnitude of the signal is smaller when inverse gated decoupling decoupling is used.
The above spectra were collected at 11.7 tesla with 4 scans, using a recycle delay of 120 seconds and an acquisition time of 2.3 seconds.

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