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Learn7405Paper 2 · Paper 2: Organic chemistryNuclear magnetic resonance spectroscopyNMR spectroscopy
Nuclear magnetic resonance spectroscopy · Subtopic 1 of 1

NMR Spectroscopy

NMR detects nuclei (¹H, ¹³C) in a magnetic field; each environment gives a signal at a chemical shift δ (ppm) relative to TMS (δ = 0). Samples are dissolved in CDCl₃ (no ¹H signal). ¹³C NMR: number of peaks = number of carbon environments; shift identifies type (C=O 160–220; C–O 50–90; aromatic 110–160). ¹H NMR: number of peaks = H environments; integration gives the ratio of H atoms; splitting follows the n + 1 rule (n = H on neighbouring carbons): singlet, doublet, triplet, quartet. O–H/N–H peaks disappear on shaking with D₂O. Combine with IR and mass spectra to identify unknowns.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 55 minRevision 1 · updated 2026-11-01
Section 1 of 14

Learning objectives

  • Count carbon environments and use ¹³C shifts.
  • Interpret ¹H shift, integration and splitting.
  • Explain the roles of TMS, CDCl₃ and D₂O.
  • Deduce structures from combined data.
Pause and think

Why does ethanol's CH₃ appear as a triplet and CH₂ as a quartet?

Section 2 of 14

Core idea

Start hereSeeing every hydrogen

NMR tells you how many kinds of hydrogen a molecule has, how many of each, and what is next door.

The big idea

Each environment gives a peak at a chemical shift. In ¹H NMR, peak area gives the number of H and splitting (n + 1) gives the number of neighbouring H.

Reading a ¹H NMR peak

Four pieces of information.

Worked exampleIdentify C₃H₆O₂

δ 1.2 (3H, t), δ 2.4 (2H, q), δ 11.5 (1H, s, gone with D₂O).

Goal
Structure.
Strategy
Assign each peak.
  1. t (3H) + q (2H)⇒CH3CH2−\text{t (3H) + q (2H)}\Rightarrow\mathrm{CH_3CH_2-}
    Ethyl pattern.
  2. δ 11.5, D2O exchange⇒COOH\delta\,11.5,\ \text{D}_2\text{O exchange}\Rightarrow\mathrm{COOH}
    Acid proton.
AnswerPropanoic acid, CH₃CH₂COOH.
✓ CheckC₃H₆O₂ ✓.
Key takeawayStandards TMS sets δ = 0; CDCl₃ solvent has no ¹H signal.

Summary

  • ¹³C: environments and shift.
  • ¹H: shift, integration, n + 1.
  • D₂O removes OH/NH peaks.
Key words
Chemical shift
Position of a peak (ppm) relative to TMS.
Spin–spin splitting
Peak split into n + 1 lines by n neighbouring H.
HistoryFrom physics to MRI. NMR was discovered by physicists in 1946. Chemists adopted it for structure; medicine turned it into MRI scanning, which images ¹H in body water.
Fun factBrain scans. An MRI scanner is an enormous ¹H NMR spectrometer. Different tissues have different water environments, giving contrast.
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