NMR and IR spectroscopy: study guide

Key ideas

Spectroscopy reads a molecule’s structure from how it absorbs radiation or fragments. Infrared shows which functional groups are present, NMR shows the hydrogen environments, and mass spectrometry gives the mass.

• IR: a broad band at 3200–3600 cm⁻¹ is O–H, a strong band at 1670–1760 cm⁻¹ is C=O, and 2200–2260 cm⁻¹ is C≡N or C≡C. A very broad O–H at 2500–3300 cm⁻¹ marks a carboxylic acid.
• ¹H NMR: the number of signals equals the number of different hydrogen environments. A signal with n neighbor hydrogens splits into n + 1 lines, and the integral gives the number of hydrogens.
• Chemical shifts: alkyl C–H about 1 ppm, H–C–O about 3.5–4 ppm, aromatic H about 7 ppm, aldehyde H about 9–10 ppm and acid O–H about 10–12 ppm.
• Mass spectrum: the molecular ion M⁺ gives the molar mass. By the nitrogen rule, an odd M means an odd number of nitrogens, and Cl or Br show M and M+2 peaks in the ratios 3:1 and 1:1.

Key equations

• n + 1 rule
• IR: O–H 3200–3600, C=O 1670–1760, C≡N 2200–2260 cm⁻¹
• M⁺ = molecular mass
• N rule: odd M ⇔ odd number of N

Worked example

Ethyl ethanoate, CH₃COOCH₂CH₃, shows a strong IR band near 1740 cm⁻¹ (C=O ester) and no O–H band. Its ¹H NMR has three signals: a singlet for CH₃CO at about 2.0 ppm (3H), a quartet for OCH₂ at about 4.1 ppm (2H) and a triplet for CH₃ at about 1.2 ppm (3H).

Common mistakes

• Counting hydrogens on the same carbon but in different environments as equivalent, or the reverse.
• Applying the n + 1 rule to hydrogens on the same carbon. Only neighbors on adjacent carbons split a signal.
• Confusing the base peak, the tallest one, with the molecular ion peak.

Practice

Test yourself with practice questions on this topic: NMR and IR spectroscopy