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Tuesday, October 7, 2008

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

Nuclei with positive gyromagnetic ratios, such as 13C, exhibit positive NOE's with nearby protons. When observing 13C directly with proton decoupling, the intensity of the resonances will be increased to an extent dependant on the magnitude of the NOE's and the amount of time over which they are allowed to build up. When quantitative results are not sought after, it is always best to collect the NMR data with decoupling during both the acquisition time and the relaxation delay so that the effect of the NOE's on the intensity of the lines is maximized. The figure below compares the 13C NMR spectra for formamide collected with inverse gated decoupling (left) and full decoupling (right).
The above spectra were collected at 11.7 tesla with 4 scans, using a recycle delay of 30 seconds and an acquisition time of 2.3 seconds. This effect of positive NOE's on the resonance intensity should be compared to that for negative NOE's.

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