37.3 Carbon-13 NMR Spectroscopy

2026 Syllabus Objectives

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

  1. Analyse and interpret a carbon-13 NMR spectrum of a simple molecule to deduce:
    • (a) the different environments of the carbon atoms present
    • (b) the possible structures for the molecule
  2. Predict or explain the number of peaks in a carbon-13 NMR spectrum for a given molecule

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What is Carbon-13 NMR Spectroscopy?

NMR stands for Nuclear Magnetic Resonance. It is a technique used by chemists to work out the structure of unknown molecules. Think of it like a fingerprint scanner — instead of fingerprints, it reads the carbon atoms inside a molecule.

Carbon-13 NMR (often written as ¹³C NMR) focuses specifically on carbon atoms in a molecule. It gives us information about how many different types of carbon atoms exist in a molecule and what chemical environment each carbon is in.

Important note: Carbon exists as different isotopes. The most common is carbon-12, but carbon-13 (which makes up about 1% of natural carbon) is the one that is detected in this type of NMR. Carbon-12 is not detected by NMR.

What is a "Chemical Environment"?

This is the most important concept in Carbon-13 NMR, so let's break it down carefully.

Every carbon atom in a molecule is attached to different neighbouring atoms or groups. These neighbouring atoms affect the carbon's surroundings — we call this its chemical environment.

  • If two carbon atoms are in identical surroundings (bonded to exactly the same types of atoms/groups in the same arrangement), they are said to be in the same chemical environment.
  • If two carbon atoms have different surroundings, they are in different chemical environments.

Why does this matter? Carbon atoms in different environments produce different peaks (signals) on a Carbon-13 NMR spectrum. Carbon atoms in the same environment produce only one peak together.

What Does a Carbon-13 NMR Spectrum Look Like?

A Carbon-13 NMR spectrum is a graph. Here is what you need to know about reading it:

  • The horizontal axis is called the chemical shift, measured in units called ppm (parts per million). This tells you what type of chemical environment a carbon is in — different environments appear at different positions along this axis.
  • Each peak (a vertical line or signal on the graph) represents a carbon atom in a unique chemical environment.
  • The number of peaks tells you how many different carbon environments are present in the molecule.

Key point: Carbon-13 NMR does not tell you how many carbon atoms there are in total — it tells you how many different types (environments) of carbon atoms there are. If three carbons are in identical environments, they still only show one peak between them.

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