[U. of Ottawa NMR Facility Blog] Improved 1H Resolution with 14N Decoupling

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Improved 1H Resolution with 14N Decoupling

The J coupling between 13C and quadrupolar nuclides can be resolved, for example, in the cases of the 13C NMR spectra of deuterated compounds, some cobalt complexes and some tetraalkyl ammonium salts. The ability to resolve the coupling depends on the relaxation rates among the Zeeman levels of the quadrupolar nuclide with respect to the reciprocal coupling constant. When the relaxation is slow, the J coupling can be resolved and when it is very fast, the 13C is a sharp singlet and said to be "self decoupled". When the relaxation rates among the Zeeman levels of the quadrupolar nuclide are on the same order of the coupling constant, the NMR resonance of the 13C will be broadened. This is a very common observation for the 13C resonances of nitrogen bearing carbons. It is also possible to see broadened 1H or 19F resonances due to coupling to 14N. Such is the case for the resonances of the proton on C6 and the fluorine on C2 in 2,3-difluoropyridine as can be seen from the figure below which clearly shows these resonances broadened compared to the resonances of 1H or 19F further removed from the nitrogen.

The broadening of the resonance of the 1H on C6 can be reduced by applying 14N decoupling during the acquisition time, thus providing much improved resolution. This is demonstrated in the figure below.

Source: University of Ottawa NMR Facility Blog