Hydrogen Bonding

Protons that are involved in hydrogen bonding (i.e.-OH or -NH) are usually observed over a wide range of chemical shifts. This is due to the deshielding that occurs in the hydrogen bond. Since hydrogen bonds are dynamic, constantly forming, breaking and forming again, there will be a wide range of hydrogen bonds strengths and consequently a wide range of deshielding. This as well as solvation effects, acidity, concentration and temperature make it very difficult to predict the chemical shifts for these atoms.

Experimentally -OH and -NH can be identified by carrying out a simple D₂O exchange experiment since these protons are exchangeable.

• run the normal H-NMR experiment on your sample
• add a few drops of D₂O
• re-run the H-NMR experiment
• compare the two spectra and look for peaks that have "disappeared"