Count environments
Count distinct signal groups. Each normally represents one hydrogen environment.
Module 3
NMR spectroscopy: nuclei report their chemical environments
In a strong magnetic field, ¹H or ¹³C nuclei absorb radiofrequency energy. Their exact resonance depends on the surrounding electrons and nearby atoms, producing a in ppm.
How the NMR machine builds the spectrum
The old single graphic has been replaced with clearer stage cards. Select a stage or run the full sequence. The explanation box underneath tells the full story without crowding the diagram.
Ready. Select a stage or run the full sequence. The focus is on the classroom story: alignment → RF pulse → FID → spectrum.
Core idea to remember
NMR is not just “peaks appear”. First, nuclei in a magnetic field absorb radiofrequency energy. Then, as they relax, the machine records a decaying signal and converts it into the final spectrum.
Different local electron environments shield nuclei by different amounts, so each distinct environment appears at a different δ value in ppm.
Interactive molecule and spectrum
Ethanol
Select a signal. The nuclei in that chemical environment will be highlighted.
High-resolution ¹H NMR
Proton-decoupled ¹³C NMR
Use the same ¹H NMR reasoning sequence every time
This follows the method used in the uploaded spectroscopy notes. Work from the overall spectrum to individual environments, then assemble the fragments.
Use integration
Use the ratio to estimate how many equivalent H are in each environment. Do not use the tallest line as the H count.
Use splitting
For simple first-order signals, use to estimate equivalent neighbouring hydrogens.
Use chemical shift
Compare δ with the VCE data book and chemical logic to identify the likely local environment.
Assemble and check
Connect the fragments, then check the proposed structure against every ¹H signal and the ¹³C, IR and MS evidence.
Worked example: colour-coded ethyl ethanoate
Use colour matching to connect each environment in the molecule with the correct signal.
Molecule
- Purple: terminal CH₃
- Orange: CH₃ next to C=O
- Blue: OCH₂
- Red: ester carbonyl carbon
The same colours appear on the interactive molecule and the peaks.
How to read it
¹H NMR: blue OCH₂ appears downfield near 4.1 ppm and is a quartet; purple terminal CH₃ is a triplet near 1.25 ppm; orange CH₃ next to the carbonyl is a singlet near 2.05 ppm.
¹³C NMR: red C=O appears farthest left at high δ, while the oxygen-bearing carbon also appears downfield compared with the simple alkyl carbons.
Symmetry explorer: predict ¹³C environments
Propanone, CH₃COCH₃, contains three carbon atoms but only two carbon environments. Select each carbon to see which atoms are equivalent.
—C(=O)—
Prediction: select a carbon. Equivalent carbons share a letter and produce one ¹³C signal.
Result: 3 carbon atoms → 2 chemically distinct carbon environments → 2 proton-decoupled ¹³C signals.
¹H versus ¹³C NMR
| Feature | ¹H NMR | ¹³C NMR used in VCE |
|---|---|---|
| What a signal counts | A hydrogen environment | A carbon environment |
| Useful extra information | Integration and splitting | Position, especially carbonyl carbons |
| Typical presentation | Often high-resolution multiplets | Usually proton-decoupled single lines |
| Signal area | Used as a hydrogen ratio | Usually not treated as a reliable carbon count |
O–H and N–H caution: exchangeable protons often give broad, variable signals and may not show normal n + 1 splitting. Do not force the simple rule onto every spectrum.
Question bank: 4 sets of 5
The bank is split into four mixed sets of five questions. Each set combines instrument and interpretation content. Answer choices are shuffled once when the attempt begins, then remain fixed while you work.