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Chapter 30 — Chapter 30 — Spectroscopy-2 NMR
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📘Easy summary
NMR spectroscopy is based on the magnetic properties (nuclear spin) of nuclei. Atoms with odd mass numbers usually show signals; nuclei such as ¹H and ¹³C behave like tiny magnets. With no magnetic field all nuclei have random orientation and the same energy. In a strong magnetic field they align with or against the field, giving a low-energy and a high-energy spin state. Irradiating with radio waves lets low-energy nuclei absorb energy and move to the higher state (spin flipping); on returning they emit energy, which is detected and converted into an NMR spectrum. The exact flipping energy depends on the molecular environment, so H atoms in different environments (e.g. CH₃ and OH of methanol) absorb different frequencies. Nuclei with even atomic and mass numbers (e.g. ¹²C) have no net magnetic field.
⭐Key points
- Based on nuclear spin.
- Strong field → two spin states.
- Radio waves cause spin flipping.
- Energy depends on molecular environment.
🔬Diagram
🌿Real-life example
In CH₃OH the CH₃ hydrogens and the O–H hydrogen absorb different radio frequencies.
💡Exam tip
- ¹²C is NMR inactive; ¹H and ¹³C are active.
✅Quick check
1. NMR relies on:
2. Radiation used for spin flipping:
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