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NMR detects suitable nuclei in different chemical environments. Carbon-13 spectra usually show one signal for each distinct carbon environment in the standard decoupled presentation. Symmetry can make several carbons equivalent. Proton chemical shifts depend on the local electron environment; electronegative atoms and unsaturated groups often shift nearby protons downfield. Tetramethylsilane supplies a reference and deuterated solvents reduce unwanted proton signals. Use the supplied shift table rather than guessing an exact value from memory.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Count carbon and proton environments
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does proton integration primarily indicate?
Relative numbers of contributing protons. Compare the integrated areas.
After watching 2: How many signals does a symmetric molecule necessarily have?
One per distinct environment. Symmetry can make multiple nuclei equivalent.
What you'll be able to do
NMR detects suitable nuclei in different chemical environments. Carbon-13 spectra usually show one signal for each distinct carbon environment in the standard decoupled presentation. Symmetry can make several carbons equivalent. Proton chemical shifts depend on the local electron environment; electronegative atoms and unsaturated groups often shift nearby protons downfield. Tetramethylsilane supplies a reference and deuterated solvents reduce unwanted proton signals. Use the supplied shift table rather than guessing an exact value from memory.
A proton spectrum gives chemical shift, integration and splitting. Integration indicates relative numbers of equivalent protons contributing to signals, not simply the height of each line. In simple first-order cases, n equivalent neighbouring hydrogens split a signal into n+1 lines. The neighbouring hydrogens must be considered in the appropriate coupled environment; more complex spectra require care. Exchangeable OH or NH protons often behave differently and can be identified with a D2O exchange experiment. Do not apply n+1 indiscriminately to every peak.
Start with molecular formula or molecular-ion mass, then count carbon environments, identify likely functional groups and assemble fragments from proton data. An ethyl group often gives a triplet and quartet with a 3:2 integration ratio, but chemical shifts locate it within the molecule. Check every signal against the complete proposed structure. A convincing answer accounts for the number, shifts, integrations and splitting together, and explains why a plausible structural isomer would give a different pattern.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A simple ethyl group gives 3H triplet and 2H quartet signals. Explain both splittings.
The splitting and 3:2 integration support an ethyl fragment.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.