Predict proton NMR shifts and splitting from a structure
Predict each proton environment's typical shift and n + 1 pattern from molecular connectivity.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a CH₃ group next to CH₂ usually produce under the n + 1 rule?
What you will learn
- Group equivalent proton environments
- Use the Cambridge shift table
- Predict simple neighbouring-proton patterns
Assign environments before predicting peaks
Mark chemically equivalent proton sets in the structure. Place each set using the supplied Cambridge chemical-shift ranges: alkane, next to carbonyl or electronegative atom, alkene, aromatic, aldehyde and exchangeable O–H/N–H environments.
The table gives typical ranges, not exact predictions. O–H and N–H shifts are especially variable with solvent and concentration.
Count adjacent equivalent protons
For each carbon-bound set, identify equivalent protons on adjacent carbon atoms and apply n + 1 for the simple patterns in the syllabus. A CH₃ next to CH₂ generally gives a triplet; CH₂ next to CH₃ generally gives a quartet.
Do not count protons on the same carbon. Exchangeable O–H/N–H signals can be broad or variable and may not show the simple pattern expected for carbon-bound protons.
Pause and explain
Which region is typical for alkyl protons next to an electronegative atom in the Cambridge table?
Worked example
Predict the main proton environments and splitting for CH₃CH₂Cl, ignoring exchangeable protons.
Show the worked solution
- CH₃ is next to CH₂: two neighbours give a triplet.
- CH₂Cl is next to CH₃: three neighbours give a quartet.
- Use the electronegative-atom shift range for CH₂Cl.
Answer Two environments: CH₃ (3H, usually lower-shift alkyl region, triplet) and CH₂Cl (2H, about 3.2–4.0 ppm, quartet).
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict the splitting of the CH₃ and CH₂ signals in CH₃CH₂OH using the simple n + 1 rule for carbon-bound protons.
Give me a hint
Count neighbours on the adjacent carbon for each set.
Compare my reasoning
- CH₃ is next to two CH₂ protons.
- CH₂ is next to three CH₃ protons.
- Apply n + 1 to each.
CH₃ is a triplet; CH₂ is a quartet. The O–H signal is exchangeable and is not assigned by this simple neighbour count.
Look for these in your work
- Neighbour counts correct
- Patterns correct
- O–H treated cautiously
Which proton set in CH₃OCH₃ is most affected by attachment to an electronegative atom, and what happens to its chemical shift?
Give me a hint
Both methyl groups are attached through oxygen.
Compare my reasoning
- The two methyl groups are equivalent.
- Their protons are next to oxygen.
- Use the higher typical shift range for alkyl next to an electronegative atom.
The equivalent O–CH₃ protons are shifted downfield relative to ordinary alkane protons, typically into the 3.2–4.0 ppm range in the supplied table.
Look for these in your work
- Equivalence identified
- O adjacency noted
- Shift region selected
A predicted spectrum gives an aromatic proton at 1.4 ppm. What should be checked?
Give me a hint
Compare the predicted shift to the supplied aromatic range.
Compare my reasoning
- Aromatic H is typically around 6.0–9.0 ppm.
- 1.4 ppm is in the alkane range.
- Check whether the signal assignment or structure is wrong.
The assignment is inconsistent: aromatic protons are typically 6.0–9.0 ppm, whereas 1.4 ppm is in the alkane region; recheck the structure or assignment.
Look for these in your work
- Reference range used
- Mismatch identified
- Correction action stated
Common mistakes
- Using the carbon-13 environment count for proton signals
- Forgetting equivalent protons can form one environment
- Counting non-adjacent hydrogens for splitting
- Treating typical shift ranges as exact
What can you explain now?
What does a CH₃ group next to CH₂ usually produce under the n + 1 rule?
Compare with the explanation
A triplet
Two adjacent protons give 2 + 1 = 3 lines.
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Make it stick
How do structure, typical shifts and neighbouring carbon-bound protons predict a simple proton NMR spectrum?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.