9701 · 37.4
Proton (¹H) NMR spectroscopy — FAQ
Frequently asked questions for 9701 Proton (¹H) NMR spectroscopy. Direct answers first, then deeper explanation — then practise with marking.
Why don't protons in an -OH group usually cause splitting in adjacent C-H protons?
Due to rapid acid-catalysed exchange, the -OH proton is not bonded to a single oxygen atom for long enough to maintain a consistent spin state relative to its neighbours. This rapid exchange 'averages out' the coupling interaction, so the -OH peak typically appears as a broad singlet and does not split adjacent proton signals (nor is it split by them).
What solvent is used for ¹H NMR and why?
The solvent used for ¹H NMR must not contain any protons (¹H atoms) itself, as it is present in a much larger quantity than the sample and would produce an enormous signal, overwhelming the spectrum. Therefore, deuterated solvents are used, where the hydrogen atoms have been replaced by their isotope, deuterium (²H or D). Common examples include chloroform-d (CDCl₃), deuterium oxide (D₂O), and acetone-d₆ ((CD₃)₂CO).
Can two protons on the same carbon atom split each other?
Usually, no. If the two protons on a CH₂ group are chemically equivalent (which they most often are), they will not split each other. However, in some complex, rigid molecules (like certain cyclic compounds), protons on the same carbon can become non-equivalent (diastereotopic) and can indeed split each other. This is generally considered beyond the scope of the A-Level syllabus.