Shielding in Protons
Protons in different chemical environments are shielded by different amounts. The hydroxyl proton is not shielded as much as the methyl protons so the hydroxyl proton absorbs at a lower field than the methyl protons. We say that the proton is deshielded somewhat by the presence of the electronegative oxygen atom.
When protons are in different enviroments in the molecule and the molecule is exposed to a constant frequency, the protons will absorb the radiation at different magnetic field strengths. The NMR instrument will vary the magnetic field and plot a graph of the energy absorption as a function of the magnetic field strength.
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