Carbon-13 NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is probably the single most powerful analytical tool in modern organic chemistry. It tells you not just the molecular formula (like MS) or the functional groups (like IR), but the detailed connectivity of every atom — which carbon is bonded to which, how many hydrogens are on each, and what their neighbours look like. For OCR H432 you study two NMR techniques: ¹³C NMR (this lesson, looking at carbon environments) and ¹H NMR (Lesson 14, looking at hydrogen environments).

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.3.2 (a): principles of ¹³C NMR and interpretation of ¹³C spectra.

1. The Idea Behind NMR

Some nuclei — including ¹H and ¹³C — have a property called spin, which makes them behave like tiny magnets. When you place them in a strong external magnetic field, they align either with the field (low energy) or against it (high energy). If you then hit them with a radio-frequency pulse of just the right frequency, you can flip them from aligned-with to aligned-against. The energy absorbed at that flipping frequency is what the NMR instrument detects.

The clever part: the exact frequency needed for each nucleus depends on the local chemical environment — the number and types of electrons around it, and what atoms are nearby. Different environments give different frequencies, and each frequency appears as a distinct peak in the spectrum.

Key Definition — Chemical environment: A nucleus and the specific arrangement of atoms and bonds attached to it. Two nuclei are in the same environment if they are chemically equivalent by symmetry.

For ¹³C specifically: only ~1% of natural carbon is the ¹³C isotope (the other 99% is ¹²C which has zero spin and is invisible to NMR). The signal is therefore weak and needs many scans to average out noise, but modern instruments handle this routinely.

2. What a ¹³C Spectrum Looks Like

A ¹³C NMR spectrum is a plot of signal intensity (y-axis) against chemical shift δ in parts per million (ppm, x-axis). Some key conventions:

The x-axis runs backwards — higher ppm values are on the left.
The reference peak (at δ = 0) is tetramethylsilane (TMS): (CH₃)₄Si. It gives a sharp singlet at 0 ppm.
¹³C chemical shifts span a wide range: 0 to 220 ppm (much wider than ¹H, which is 0–12 ppm).
Each peak corresponds to one unique carbon environment in the molecule.

2.1 Why TMS?

TMS is the universal reference because:

Its 12 equivalent H atoms (and 4 equivalent C atoms) give a single sharp peak, making it easy to identify.
Silicon is less electronegative than carbon, so the C nuclei in TMS are very shielded and appear at a chemical shift lower than almost any other organic carbon. TMS therefore marks the "right edge" of the spectrum at δ = 0 ppm.
It is inert, soluble in most deuterated NMR solvents, and volatile enough to be removed at the end of the experiment.

3. Counting Carbon Environments

The first step in interpreting a ¹³C spectrum is to work out how many different carbon environments the molecule has. Each unique environment gives one peak.

3.1 How to count environments

Two carbons are in the same environment if you can swap them by some symmetry operation (rotation or reflection) and the molecule looks identical. Otherwise they are in different environments.

Example 1: Ethanol, CH₃CH₂OH

C1 of CH₃ is bonded to 3H and 1 C(OH) → one environment.
C2 of CH₂OH is bonded to 2H, 1 C, and 1 O → a different environment.
Total: 2 environments → 2 peaks.

Example 2: Propan-2-ol, (CH₃)₂CHOH

The two CH₃ groups are equivalent by the plane of symmetry through C–OH → one environment.
The CH carbon (C2) is bonded to 2 CH₃, 1 H, and 1 OH → a different environment.
Total: 2 environments → 2 peaks.

Example 3: Butan-2-one, CH₃COCH₂CH₃

C1 of CH₃ (left) is bonded to 3H and the C=O.
C2 is the carbonyl C (C=O).
C3 of CH₂ is bonded to 2H, the C=O, and the terminal CH₃.
C4 of CH₃ (right) is bonded to 3H and CH₂.
All four are different → 4 environments → 4 peaks.

Example 4: Benzene, C₆H₆

All 6 carbons are equivalent by rotation around the C6 axis.
Total: 1 environment → 1 peak.

Example 5: Methylbenzene (toluene), CH₃–C₆H₅

The CH₃ → 1 environment.
The aromatic ring has four unique environments: the ipso C (attached to CH₃), ortho Cs (two, equivalent), meta Cs (two, equivalent), para C (one).
Total: 1 + 4 = 5 environments → 5 peaks.

graph TD
  A[Count carbon environments] --> B[Draw molecule]
  B --> C[Look for planes of symmetry]
  C --> D[Group equivalent C together]
  D --> E[Each group = 1 peak]

Lesson 13: Carbon-13 NMR Spectroscopy