The heteronuclear NOE (HETNOE) experiment is the heteronuclear version of the well known 1D NOEDIFF experiment. This experiment allows to trace out through-space ¹H-¹³C connectivities by detecting and measuring log-distance heteronuclear NOEs. The resulting changes in signal intensities are visible from the corresponding ¹³C spectrum and can be correlated to the inverse sixth power of the distance between the involved spins. The experiment is very useful to assign quaternary carbons.

Easy implementation on AVANCE spectrometers.

In the HETNOE experiment, two spectra are usually recorded in an interleaved mode ( 85MRC185 , 85MRC1072 , and 86MRC972 ):

• Spectrum A: A reference ¹H-decoupled ¹³C spectrum without ¹H NOE enhancement is recorded (no ¹H irradiation during the preparation period; see 13C spectrum) using a long relaxation period and a 90º read pulse.
• Spectrum B: A ¹H-decoupled ¹³C spectrum in which 1H resonance is recorded during the relaxation period. The intensities of ¹³C resonances will change depending if NOE from the selected ¹H has been transferred

These two spectra can be subtracted (B-A) if good signal-to-noise ratio has been achieved to give the final HETNOE spectrum.

Three-spin effects have been reported ( 86JCS647 , 88MRC511 , and 88MRC852 ) and multiple sequential saturation has been proposed to improve sensitivity and performance ( 92MRC466 ).

The HETNOE 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, duration and  power level of the ¹H irradiation. It is useful to have a calibration of the field strength delivered from the decoupler in order to optimize the required selectivity. It is also important to use a enough number of scans in order to have good S/N ratio.

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

The HETNOE difference spectrum only shows the ¹³C resonances whose intensities have changed. If S/N ratio is low, heteronuclear NOEs can be analyzed by comparing the ¹³C resonance intensities from the reference and the selective HETNOE spectra.

2D HOESY experiment can be more useful alternative to trace out all heteronuclear NOE connectivities from a single spectrum.

Theoretical and practical aspects of 1D and 2D Heteronuclear NOE experiments have been reviewed ( 87PROG223 ).