Topic: NMR

Reading Beyond A-Level

In the high-resolution ${}^1$H NMR spectrum of a pure, dry sample of ethanol, the signal for the hydroxyl ($\text{-OH}$) proton often appears as a broad singlet, seemingly violating the $\text{n+1}$ splitting rule you would expect from the adjacent $\text{-CH}_2$ group. Furthermore, if the ethanol sample is shaken with a drop of deuterium oxide ($\text{D}_2\text{O}$) and the spectrum is re-analysed, the $\text{-OH}$ signal disappears completely.

Investigate and explain these two phenomena. Your analysis should cover:

• The concept of **rapid proton exchange** and how this chemical process, occurring on the NMR timescale, effectively decouples the $\text{-OH}$ proton from its neighbours.
• The mechanism of the **”$\text{D}_2\text{O}$ shake”**, explaining how **isotopic exchange** with deuterium leads to the disappearance of the labile proton signal from the ${}^1$H NMR spectrum, confirming its identity.