November 5, 2001, IU NMR Facility, prepared by UWZ

Homonuclear decoupling of ¹H NMR spectra, such as selectively decoupling methyl or methine groups, you can do by declaring homo='y'. For hardware reasons, this does not work for other nuclei, such as ³¹P. Instead, you need to use a special pulse sequence iu_homoXdec. This sequence has continuous wave decoupling at the center frequency (tof) explicitly built in. These instructions are written for ³¹P but can be used for other nuclei as well.

1. Set up your sample as usual (spinning, shimming etc.).
2. Set up a standard ³¹P experiment and take a spectrum.
3. Bracket with your cursors all peaks in the spectrum while roughly placing the peak you want to decouple in the center of the window. Type movesw to define these limits as your spectral window. Take a spectrum.
4. Set the cursor precisely on the line you want to decouple and type movetof. Take another spectrum to make sure this line is in the center of the spectrum and all your peaks are still in the spectral window.
5. Type iu_homoXdec to call up the pulse sequence.
6. Check that satmode='nnn' and homodm='nny' .
7. Set homodpwr to values between 20 and 40. Choose the lowest value that allows you to decouple your line or gives you the answer to your question. (See comments below)

1. Figure 2 shows the spectra as a function of decoupling powers. Zoomed into the region of interest you can see how for the higher powers, decoupling takes place (see Figure 3). In addition the doublet (excited line) and triplet lines (to be decoupled) are shifted by the Bloch-Siegert shift. The rf excitiation leading to a small Bloch-Siegert shift is not sufficient to decouple lines in itself, as you can see in Figure 4. You will have to balance the power needed for decoupling and the effects of the Bloch Siegert shift.
2. This pulse sequence also allows for presaturation, as may be needed for suppression of solvent peaks. For that set satmode='yyn'.
3. You have to leave homo='n'. Yes, this is 'n', because the regular decoupling channel is used for the protons. The homonuclear decoupling is written into the pulse sequence.

Example:

Figure 1: Full ³¹P spectrum of the Rh(PPh₃)₃Cl with one pulse excitation and ¹H decoupling.

Figure 2: ³¹P homonuclear decoupled NMR spectrum of the same material as a function of homodpwr.

Figure 3: Same as Figure 2 highlighting the regions of interest.

Figure 4: Test for the decoupling strength by direct radiation.