6.3.2 Spectroscopy (NMR)

In OCR questions the assessed ¹³C NMR spectrum is proton decoupled, so each distinct carbon environment gives one carbon signal and you do not analyse proton splitting on those carbon peaks.

Use symmetry before you count. Carbons related by molecular symmetry can be equivalent even when they are drawn in different places, so the number of peaks can be smaller than the number of carbon atoms.

Read the δ value in ppm and compare it with the ¹³C regions on the OCR Data Sheet. A carbonyl carbon, an aromatic/alkene carbon and an ordinary saturated carbon occupy very different regions, so shift data can remove many candidate structures quickly.

Use the supplied ranges as evidence, not as an exact address. The detailed chemical environment affects δ, so combine the shift with the molecular formula and the rest of the spectrum.

Start with the number of signals, then assign likely carbon types from their shifts. Use the molecular formula to decide which combinations are possible and check that your proposed symmetry gives exactly the observed number of carbon environments.

A candidate is only viable if every ¹³C peak has somewhere to come from. If your structure predicts an extra carbon environment or misses a carbonyl-region signal, reject it and try another connectivity.

Each set of chemically equivalent hydrogens gives one ¹H NMR signal. Count environments by asking which H atoms sit in genuinely identical surroundings; symmetry often makes several H atoms equivalent.

Ignore reference/solvent signals when the question is asking about the compound itself. Exchangeable O–H and N–H protons can also give their own signals before D₂O exchange.

Chemical shift tells you about the electronic environment around the proton. Use the OCR Data Sheet to place a signal in a sensible region: protons beside electronegative atoms, C=O or an aromatic ring are shifted differently from ordinary alkyl protons.

O–H and N–H positions are more variable, so do not force one of those peaks into a narrow remembered value. Use D₂O exchange and the other structural evidence when an exchangeable proton is possible.

The integration ratio tells you the relative number of H atoms producing each signal. Simplify the areas to a whole-number ratio, then compare the total with the molecular formula if one is supplied.

A 6H signal often means two equivalent CH₃ groups, not a six-hydrogen chain. OCR’s 2025 report picked up exactly this mistake: use integration together with symmetry and splitting before deciding what fragment the signal represents.

For a simple aliphatic signal with one set of neighbours, n equivalent neighbouring protons give n + 1 peaks: 0 → singlet, 1 → doublet, 2 → triplet, 3 → quartet, and OCR can take the rule further when the structure requires it. Equivalent protons do not split one another.

Use splitting to build fragments, then check integration and shift. OCR expects you to recognise aromatic protons from their chemical shifts, but not to analyse aromatic splitting patterns. O–H and N–H coupling is not used as a dependable n + 1 clue in this level of analysis.

For each signal, write down three clues: shift → environment, integration → number of H, splitting → neighbouring H. Turn those clues into small fragments, then join fragments in a way that matches the molecular formula.

Finally re-predict the spectrum from your proposed structure. OCR’s 2025 report praised answers that organised the NMR evidence clearly and then actually used those fragments in the final molecule; partial fragments that never reach a consistent structure leave marks behind.

Work from structure to spectrum. Mark equivalent carbons first, count the distinct environments, then place each one in an approximate ¹³C shift region using the OCR Data Sheet.

As a check, your predicted spectrum needs one signal for each distinct carbon environment and no extras. Symmetry is usually the quickest place to catch an over-count.

Mark each proton environment on the structure, count its H atoms for the integration, identify neighbouring non-equivalent H for n + 1 splitting, then estimate the shift from the Data Sheet.

For ethyl ethanoate, CH₃COOCH₂CH₃, you expect three main proton environments: COCH₃ = 3H singlet, OCH₂ = 2H quartet, and the terminal CH₃ = 3H triplet. The OCH₂ signal is furthest downfield of those three because its carbon is bonded to oxygen.

Tetramethylsilane (TMS) provides the reference signal assigned δ = 0 ppm. Chemical shifts for the sample are measured relative to that reference.

TMS gives one sharp signal because all its H atoms are equivalent. When you interpret a spectrum, do not count the TMS reference as a proton environment in the unknown compound.

A normal proton-containing solvent would produce a large ¹H NMR signal that could obscure signals from the sample. A deuterated solvent such as CDCl₃ replaces most of those solvent ¹H nuclei with deuterium, so that interference is greatly reduced.

Keep the jobs separate: the deuterated solvent lets you dissolve the sample without a dominant ordinary proton signal; TMS sets δ = 0.

Record the proton spectrum, add D₂O, shake, then record it again. Exchangeable O–H and N–H protons are replaced by deuterium, so their ordinary ¹H signal disappears or is strongly removed from the second spectrum.

Disappearance tells you the peak came from an exchangeable proton; it does not by itself prove whether it was O–H or N–H. OCR’s 2025 report also reminds us that O–H signals can occur over a variable range, so D₂O evidence is especially useful.

Treat the techniques as constraints on one molecule. Elemental analysis can establish composition or an empirical formula; the molecular ion in mass spectrometry gives Mᵣ; IR identifies key bonds/functional groups; ¹³C NMR gives carbon environments; ¹H NMR adds proton environments, ratios and neighbours.

Build a candidate, then test it against every dataset and the molecular formula. Do not bend one piece of evidence to rescue a favourite structure. OCR’s 2024 and 2025 reports both rewarded clear, staged analysis followed by one final structure that fits all the evidence. OCR does not require interpretation of mass spectra of organic halogen compounds.