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Friday, October 12, 2007

Nyquist Fold-Backs in HMBC Spectra

The digital filters of modern NMR spectrometers have not only improved the signal-to-noise specification quoted by instrument companies but they have also eliminated Nyquist fold-backs for signals outside of the spectral window. This is not the case however for the indirect dimension of 2D experiments. In the left panel of the figure below is a properly recorded HMBC spectrum of 3-heptanone. In the right panel is a spectrum acquired with the 13C spectral window set too small. The carbonyl correlations are folded into the low frequency end of the 13C axis.
If you inadvertently collect a spectrum with a Nyquist fold-back you can still calculate the correct chemical shift, as the signal will be the same number of ppm away from the wrong end of the axis as it is outside of the correct end of the axis.

2 comments:

Stetty said...

Hi Glenn,

first of all I want to express real deep gratitude and respect for your attitude towards sharing knowledge in your blog - I found out about it only yesterday and have been reading every free minute since then!!

My qestion to nyquist: Is there any way to prevent the nyquist fold-backs in the indirect dimension in HMBC or HSQC. It would allow to set SW to what you really want to see/ask and enjoy the high resolution in this small window in a short time.

All the best, Christian

Glenn Facey said...

Christian,
Thank you for the question. The only thing that comes to mind to eliminate Nyquist foldbacks in the indirect dimension of a 2D spectrum would be the use of the appropriate band selective shaped pulses in the pulse program. My feeling however is that this would deteriorate the sensitivity to the point where there may not be any advantage. There may also be other ways that I am not aware of.

Glenn