¹H NMR: Principles

Nuclear magnetic resonance (NMR) spectroscopy is arguably the most powerful analytical technique available for determining the structure of organic molecules. While mass spectrometry tells you the molecular mass and IR reveals functional groups, NMR tells you about the carbon and hydrogen framework — how many distinct environments exist, how many atoms are in each, and how they are connected.

This lesson focuses on the principles of proton (¹H) NMR, the most commonly used form of NMR spectroscopy.

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The Physical Basis of NMR

Certain atomic nuclei possess a property called nuclear spin. Nuclei with an odd number of protons, an odd number of neutrons, or both, have non-zero spin and are described as "NMR-active." The nuclei most relevant to organic chemistry are:

• ¹H (proton) — spin = ½
• ¹³C (carbon-13) — spin = ½
• ²H (deuterium) — spin = 1

The common isotope ¹²C has zero spin and is NMR-inactive. This is why ¹³C NMR relies on the small natural abundance (1.1%) of the ¹³C isotope.

Behaviour in a Magnetic Field

When placed in a strong external magnetic field (B₀), nuclei with spin = ½ can adopt one of two orientations: