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Proton (¹H) NMR spectroscopy
¹H NMR spectroscopy uses a magnetic field to probe the different chemical environments of hydrogen atoms in a molecule. This gives us clues about the molecule's structure, like a unique fingerprint.
Need to know
What you need to know
- **Reference Standard:** Tetramethylsilane, Si(CH₃)₄ (TMS), is the universal reference, with its signal set to δ = 0 ppm.
- **Upfield (low δ):** High shielding. Typically alkyl groups (C-H). E.g., R-CH₃ at δ ≈ 0.9 ppm.
- **Downfield (high δ):** Low shielding (deshielded). Protons near electronegative atoms or π-systems. E.g., R-CHO at δ ≈ 9.7 ppm, or C₆H₅- at δ ≈ 7.3 ppm.
Explanation
Decoding Molecules with Magnets
- Chemical shift (δ) in ppm tells you the proton's electronic environment. High electron density 'shields' the proton, shifting its signal upfield (to a lower δ value), while nearby electronegative atoms 'deshield' it, shifting it downfield (to a higher δ value).
- The integration trace shows the relative area under each peak. This area is directly proportional to the number of equivalent protons causing the signal, giving you a ratio of hydrogens in each environment.
- The splitting pattern of a peak is determined by the 'n+1 rule', where 'n' is the number of non-equivalent protons on adjacent atoms. This reveals which proton groups are neighbours.
- Adding deuterium oxide (D₂O) causes protons on -OH or -NH groups to be exchanged for deuterium. These 'labile' proton peaks then disappear from the spectrum, confirming their presence.