In simple terms
A friendly intro before the formal notes — no formulas yet.
Decoding Molecules with Magnets
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.
Imagine a group of people singing the same note. If some are inside a soundproofed room (shielded) and others are outside (less shielded), they'll sound slightly different to a listener. ¹H NMR 'listens' to these tiny differences in the 'notes' sung by hydrogen atoms to map out their positions in a molecule.
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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).
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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.
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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.
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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.
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Full topic notes
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1. Chemical Shift (δ) and Shielding
The position of a signal along the x-axis of an NMR spectrum is its chemical shift, measured in parts per million (ppm). This position is determined by the proton's local electronic environment. The electrons orbiting a nucleus generate a small magnetic field that opposes the main external magnetic field of the spectrometer. This is called shielding. Protons in electron-rich environments are more shielded and require a lower frequency to resonate, appearing 'upfield' (to the right, low δ). Conversely, if a proton is near an electronegative atom (like O, N, or a halogen), electron density is withdrawn from it. This 'deshields' the proton, causing it to resonate at a higher frequency, 'downfield' (to the left, high δ).
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.
2. Integration: The Proton Count
A ¹H NMR spectrum can not only tell us about the types of proton environments but also how many protons are in each. The area under each signal is directly proportional to the number of protons creating that signal. The spectrometer calculates these areas and represents them as a stepped line called an integration trace. The relative height of each step gives the simplest whole-number ratio of the protons in the different environments.
3. Spin-Spin Splitting and the n+1 Rule
Signals in a ¹H NMR spectrum are often not single peaks but are split into clusters called multiplets. This splitting is caused by the magnetic field of non-equivalent protons on adjacent carbon atoms influencing the local field of the proton being observed. The splitting pattern can be predicted by the n+1 rule, where 'n' is the number of equivalent protons on the adjacent atom(s). This rule is a powerful tool for deducing which groups are next to each other in the molecular structure.
Number of peaks in a multiplet = n + 1
Singlet (1 peak): n=0. No adjacent non-equivalent protons.
Doublet (2 peaks): n=1. Adjacent to a CH, NH or OH group.
Triplet (3 peaks): n=2. Adjacent to a CH₂ group.
Quartet (4 peaks): n=3. Adjacent to a CH₃ group.
Protons on the same carbon do not split each other if they are equivalent.
Splitting is generally not observed over more than three bonds.
4. D₂O Exchange for Labile Protons
Protons attached to highly electronegative atoms like oxygen (-OH in alcohols and carboxylic acids) or nitrogen (-NH in amines and amides) are known as labile protons. They are acidic and can undergo rapid exchange with other labile protons in the sample, such as trace amounts of water. This rapid exchange means they often don't couple with neighbouring protons, appearing as broad singlets. To confirm the presence of an -OH or -NH group, a simple test is performed. The sample is shaken with a few drops of deuterium oxide (D₂O), and the spectrum is run again. The labile H atoms are replaced by D atoms (e.g., R-OH + D₂O ⇌ R-OD + HOD). Since deuterium (²H) does not produce a signal in a ¹H NMR spectrum, the peak corresponding to the labile proton will disappear. This is a definitive confirmation.
When deducing a structure from a ¹H NMR spectrum, always use a systematic approach. First, count the number of signals to find the number of proton environments. Second, use the integration ratio to determine the relative number of protons in each environment. Third, analyse the splitting patterns using the n+1 rule to establish which groups are adjacent. Finally, use the chemical shifts to identify the type of environment. Write down your reasoning for each piece of evidence before proposing a final structure.
Worked examples
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The ¹H NMR spectrum of a compound with the molecular formula C₃H₆O shows two signals. Signal A at δ = 2.1 ppm has an integration value of 3. Signal B at δ = 9.8 ppm has an integration value of 1. Deduce the structure of the compound.
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Analyse the information:
A compound has the molecular formula C₄H₁₀O. Its ¹H NMR spectrum consists of two signals: a singlet at δ = 3.2 ppm (integration 3H) and a singlet at δ = 1.2 ppm (integration 9H). Deduce the structure of the compound.
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Analyse the data:
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What is chemical shift (δ)?
The position of a signal in an NMR spectrum, relative to a reference standard (TMS). It is measured in parts per million (ppm) and depends on the proton's local electronic environment.
Key takeaways
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Reference Standard: Tetramethylsilane, Si(CH₃)₄ (TMS), is the universal reference, with its signal set to δ = 0 ppm.
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Upfield (low δ): High shielding. Typically alkyl groups (C-H). E.g., R-CH₃ at δ ≈ 0.9 ppm.
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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.
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Practice Questions on ¹H NMR Spectroscopy
Practice Questions on ¹H NMR Spectroscopy
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