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Nuclear magnetic resonance (NMR) spectroscopy is arguably the most powerful technique for determining the structure of organic molecules. Proton (¹H) NMR tells us about the hydrogen environments in a molecule: how many different types of hydrogen there are, how many of each type, and which hydrogens are on adjacent carbons. This lesson covers the theory and practice of ¹H NMR interpretation.
The nucleus of ¹H (a proton) has a property called spin. In an external magnetic field, the nucleus can align either with the field (lower energy, α-state) or against it (higher energy, β-state).
When radio-frequency (RF) radiation of exactly the right frequency is applied, nuclei in the lower energy state absorb the energy and “flip” to the higher energy state. This absorption is detected and recorded as an NMR spectrum.
Not all protons in a molecule absorb at exactly the same frequency because they experience slightly different local magnetic fields. Electrons around each proton generate a small magnetic field that shields the nucleus from the external field, reducing the effective field it experiences.
The position of an NMR signal is reported as the chemical shift (δ) in parts per million (ppm) relative to a reference compound, tetramethylsilane (TMS), which is defined as δ = 0.00 ppm.
Key Definition: Chemical shift (δ) is the position of an NMR absorption relative to TMS, measured in ppm. It reflects the electronic environment of the nucleus.
TMS, Si(CH₃)₄, is used as the reference for several reasons:
The following table gives the approximate ¹H chemical shift ranges for common hydrogen environments. These values will typically be provided in a data booklet in exams.
| Hydrogen Environment | Chemical Shift δ (ppm) | Example |
|---|---|---|
| R–CH₃ (alkyl, primary) | 0.7–1.2 | CH₃ in ethane |
| R–CH₂–R (alkyl, secondary) | 1.2–1.4 | CH₂ in propane |
| R₃CH (alkyl, tertiary) | 1.4–1.6 | CH in 2-methylpropane |
| R–C(=O)–CH₃ (next to C=O) | 2.0–2.5 | CH₃ in propanone |
| R–O–CH₃ (next to oxygen) | 3.3–3.9 | OCH₃ in methanol |
| R–CHO (aldehyde H) | 9.4–9.9 | H in ethanal |
| Ar–H (aromatic H) | 6.5–8.0 | H in benzene |
| R–OH (alcohol OH) | 1.0–5.5 (variable) | OH in ethanol |
| R–COOH (carboxylic acid OH) | 10.0–12.0 | OH in ethanoic acid |
| R–NH₂ (amine NH) | 1.0–4.5 (variable) | NH₂ in ethylamine |
Exam Tip: The O–H and N–H chemical shifts are highly variable because they depend on concentration, solvent, and hydrogen bonding. They are the only protons removed by a D₂O shake.
The chemical shift depends on the electron density around the proton:
Electronegativity effects: The more electronegative the nearby atom, the more the proton is deshielded:
The effect diminishes with distance: in CH₃CH₂Cl, the CH₂ protons (δ ≈ 3.5) are more deshielded than the CH₃ protons (δ ≈ 1.3).
Protons in the same chemical environment give the same NMR signal. To determine the number of environments:
Examples:
The area under each peak (the integral) is proportional to the number of protons in that environment. NMR software or spectrometers display integral traces or numerical ratios.
For example, in ethanol (CH₃CH₂OH):
Exam Tip: In exam questions, you may be given integration ratios. Always simplify them and compare with the molecular formula. If the molecular formula has 10 hydrogen atoms and the ratio is 3:2:2:3, that accounts for all 10 hydrogens (3+2+2+3 = 10), confirming 4 different environments.
Protons on adjacent (neighbouring) carbons interact magnetically, causing NMR signals to split into multiple peaks. This is called spin–spin coupling or splitting.
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