The NOE difference (NOEDIFF) experiment allows to detect and measure Nuclear Overhauser enhancements (NOEs) between ¹H through the space. The resulting changes in signal intensities are clearly visible and can be correlated to the inverse sixth power of the distance between the involved spins.

Easy implementation on AVANCE spectrometers.

In the NOEDIFF experiment, two spectra are recorded in an interleaved mode:

• Spectrum A: Conventional  ¹H spectrum using a long relaxation period and a 90º read pulse.
• Spectrum B: A NOE spectrum recorded as A in which selective irradiation of a selected  ¹H is performed during the relaxation period. A  ¹H spectrum will be obtained in which the irradiated proton is missing ( o showing residual intensity) and the intensities of all other protons can have changed depending if NOE has been transferred

These two spectra are subtracted (B-A) to give the final NOEDIFF spectrum.

The NOEDIFF experiment can be acquired, processed and plotted in a fully automated way. Minor changes from a predefined parameter set are required. Important parameters to consider are the offset and the power level of the irradiation. It is useful to have a calibration of the field strength delivered from the decoupler in order to optimize the required selectivity. Selective irradiation can be accomplished in two different ways: a) a single irradiation on the center of the multiplet using a moderate power level and b) a multiple irradiation on each line of the multiplet using a very-low power level.

For further details on practical implementation of NOEDIFF experiments on AVANCE spectrometers see Tutorial: NOEDIFF

The NOEDIFF spectrum only shows the proton resonances whose intensities have changed. Thus, the irradiated  ¹H signal appears as a strong negative resonance and all protons having NOE will show small positive/negative intensities. Artefacts due to improper subtraction procedure are usually present.

2D NOESY or 2D ROESY experiments can be more useful alternatives to trace out all NOE connectivities from a single spectrum. On the other hand, selective 1D versions of these experiments can also give better results: Selective 1D NOESY, Selective 1D ROESY,  Selective ge-1D NOESY and Selective ge-1D ROESY