37.4 Proton (¹H) NMR Spectroscopy

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Analyse and interpret a ¹H NMR spectrum to deduce:
    • (a) The different environments of protons using chemical shift values
    • (b) The relative numbers of each type of proton from relative peak areas
    • (c) The number of equivalent protons on the adjacent carbon from the splitting pattern, using the n + 1 rule (singlet, doublet, triplet, quartet, multiplet)
    • (d) The possible structures for the molecule
  2. Predict the chemical shifts and splitting patterns of protons in a given molecule
  3. Describe the use of tetramethylsilane (TMS) as the standard for chemical shift measurements
  4. State the need for deuterated solvents (e.g. CDCl₃) when obtaining a proton NMR spectrum
  5. Describe the identification of O–H and N–H protons by proton exchange using D₂O

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What is Proton ¹H NMR Spectroscopy?

NMR spectroscopy stands for Nuclear Magnetic Resonance spectroscopy. It is a powerful analytical technique — a method used by chemists to work out the structure of an unknown molecule.

Proton NMR (written as ¹H NMR) focuses specifically on hydrogen atoms (protons) in a molecule. The technique works because hydrogen nuclei behave like tiny magnets. When placed in a strong magnetic field and exposed to radio waves, they absorb energy and produce signals. The position and shape of these signals tell us a great deal about the molecule's structure.

Think of it like this: every hydrogen atom in a molecule "feels" its surroundings. Depending on what atoms or groups are nearby, each hydrogen atom is in a slightly different chemical environment. NMR detects these differences.

1. The Chemical Shift (δ) and Proton Environments

What is a Chemical Shift?

The chemical shift (symbol: δ, pronounced "delta") is the position of a peak on the NMR spectrum. It is measured in units called ppm (parts per million). The chemical shift tells you what type of chemical environment a proton is in.

The scale runs from 0 ppm on the right to about 12 ppm (or higher) on the left. Peaks appearing further to the left (higher δ value) are said to be downfield; peaks to the right (lower δ value) are upfield.

Why Do Different Protons Give Different Chemical Shifts?

Different groups of atoms around a proton affect how much it is shielded from the external magnetic field.

  • Shielding means that electrons around the proton "protect" it from the full magnetic field. A heavily shielded proton resonates at a lower δ value (upfield).
  • Deshielding means that electron-withdrawing groups nearby pull electron density away from the proton, so it experiences more of the magnetic field. A deshielded proton resonates at a higher δ value (downfield).

For example, a proton attached to a carbon next to an electronegative atom like oxygen or nitrogen is deshielded, and so it appears at a higher chemical shift value.

Common Chemical Shift Values (Reference Table)

You need to learn approximate ranges for common proton environments:

Type of ProtonChemical EnvironmentApproximate δ (ppm)
R–CH₃, R–CH₂–R, R–CH–RAlkyl (C–H) protons0.5 – 2.0
R–CO–CH₃Protons next to a C=O group2.0 – 2.5
R–O–CH₃ or R–O–CH₂–Protons next to an oxygen (ether/ester)3.3 – 4.3
–CH=CH–Alkene (C=C) protons4.5 – 6.0
Aromatic ring (benzene ring) protonsAr–H6.5 – 8.0
R–CO–HAldehyde protons9.5 – 10.0
R–CO–OHCarboxylic acid O–H10.0 – 12.0
R–O–HAlcohol O–H0.5 – 5.0 (variable, broad)
R–NH₂ or R–NHAmine N–H0.5 – 5.0 (variable, broad)

Exam Tip: You will usually be given a chemical shift table in the exam. You must be able to use it to identify what type of environment each peak corresponds to.

What are Proton Environments?

Two protons are in the same environment if they are in identical chemical surroundings — surrounded by the same atoms and bonds in the same arrangement. These are called equivalent protons. They give one single peak (or group of peaks) at the same chemical shift.

For example, in ethanol (CH₃CH₂OH):

  • The 3 protons on CH₃ are all equivalent to each other → one signal
  • The 2 protons on CH₂ are equivalent to each other → one signal
  • The 1 proton on OH → one signal

So ethanol gives three signals in total in its ¹H NMR spectrum.

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