8. Spectroscopy
Spectroscopy reads a molecule's structure from how it absorbs energy: IR shows which bonds are present, NMR shows the carbon skeleton and its hydrogens, and mass spectrometry gives the molecular weight.
Infrared (IR) Spectroscopy
Key absorptions (cm−1): O–H 3200–3600 (broad), C–H 2850–3000, C=O ~1715 (strong, sharp), C=C ~1650, C≡C ~2100–2260.
Example: A strong peak near 1715 cm−1 means a carbonyl group; a broad peak near 3300 cm−1 means an O–H (alcohol or carboxylic acid).
Proton NMR (1H NMR)
- Chemical shift (δ, ppm): where the signal appears — alkyl H ~0.9–1.5, H next to O ~3.3–4.0, aromatic H ~7–8, aldehyde H ~9–10.
- Integration: the area under a signal gives the relative number of hydrogens.
- Splitting (n + 1 rule): a signal splits into $n + 1$ peaks, where $n$ is the number of neighbouring hydrogens.
Example (ethanol): CH3 appears as a triplet (~1.2 ppm, 3H) — split by the 2 neighbouring CH2 hydrogens; CH2 appears as a quartet (~3.7 ppm, 2H); OH is a singlet (1H, variable position).
Carbon NMR and Mass Spectrometry
13C NMR gives one signal per unique carbon environment (acetone: 2 signals — the two equivalent CH3 carbons and the C=O carbon). In mass spectrometry, the molecular ion peak (M+) gives the molar mass.
Practice Problems
- An IR spectrum shows a strong sharp peak at ~1715 cm−1. What functional group is present?
- A 1H NMR signal integrates to 9H and appears as a singlet at 1.2 ppm. What group is it?
- How many peaks will the CH3 signal split into if it sits next to a CH2?
- How many signals does acetone show in 13C NMR?
Answers: 1. A carbonyl (C=O) 2. A tert-butyl group, (CH3)3C– 3. Three (a triplet, n + 1 = 2 + 1) 4. Two