[tomek]

What is the source of those very broad peaks in proton NMR? I have always attributed them to the presence of paramagnetic impurities (eg. oxygen) or adventitious inorganic salts in the sample. What is the most common source of these peaks in your opinion? I include one such spectrum for illustration (notice the broad 4 ppm peak).

[Babcock_Hall]

Exchange broadening is another reason why some peaks are broad.  My recollection is that the water peak in DMSO is usually broad.  What is your solvent?

[Corribus]

Broadened OH or NH can show up in this region. Adding a drop of D₂O can help.

[tomek]

The solvent in this case was DMSO. But the peak is too broad for it to be coming from HDO and the chemical shift is 4 ppm. All protons from a molecule are accounted for. This is an extra peak. In some other cases I have observed extremely broad and flat peaks.

[Corribus]

If there is any trace water in your DMSO and your analyte has any OH or NH (or other functional group capable of hydrogen bonding), proton exchange with the trace water can significantly broaden the NH or OH peak. This is why adding a small amount of D₂O can help.

[Babcock_Hall]

If that is not your water peak, then where is your water peak?

[tomek]

I would normally expect water peak in DMSO to be around 3.3 pm. I've never seen water in DMSO at 4 ppm but as Corribus suggested, addition of D₂O helped. The peak shape was improved and it moved to 3.6 ppm. Take a look. Thank you for advice.
PS. What threw me off a bit was that the compound is a phenol and in the previous spectrum, the -OH was present at 10 ppm. If it exchanged then it should be just the broad peak seen at 4 ppm, no?

[phth]

The solvent in this case was DMSO. But the peak is too broad for it to be coming from HDO and the chemical shift is 4 ppm. All protons from a molecule are accounted for. This is an extra peak. In some other cases I have observed extremely broad and flat peaks.

Probably just a specific amount of water / HDO and changing the ratio changes the peak shape. Could try ampoule DMSO for less water or another NMR solvent that is cheap like d3-MeCN

[Babcock_Hall]

Perhaps in the original spectrum the broad peak was the weighted average of water and the signal from ArOH in your molecule.  Moving to higher temperature might sharpen the peak by increasing the rate of exchange.

[rolnor]

Try CDCl3 instead