The chemical shift scale runs to the left from zero (where TMS resonates), i.e. backwards from the usual style. Chemical shift values around zero are obviously small but are confusingly called ‘high field’ because this is the high magnetic field end of the scale. We suggest you say ‘large’ or ‘small’ chemical shift and ‘large’ or ‘small’ δ, but ‘high’ or ‘low’ field to avoid confusion. Alternatively, use ‘upfi eld’ for high fi eld (small δ) and ‘downfi eld’ for low field (large δ). One helpful description we have already used is shielding. Each carbon nucleus is sur rounded by electrons that shield the nucleus from the applied field. Simple saturated carbon nuclei are the most shielded: they have small chemical shifts (0–50 ppm) and resonate at high field. One electronegative oxygen atom moves the chemical shift downfi eld into the 50–100 ppm region. The nucleus has become deshielded. Unsaturated carbon atoms experi ence even less shielding (100–150 ppm) because of the way in which electrons are distributed around the nucleus. If they are also bonded to oxygen (the most common unsaturated car bons bonded to oxygen are those of carbonyl groups), then the nucleus is even more deshielded and moves to the largest chemical shifts around 200 ppm. The next diagram summarizes these different ways of talking about NMR spectra.

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Some examples of 1H NMR spectra

A guided tour of the 13C NMR spectra of some simple molecules

Regions of the 13C NMR spectrum

The reference sample—tetramethyl silane, TMS

The chemical shift scale

Why do chemically distinct nuclei absorb energy at different frequencies?

NMR also uses radio waves

NMR uses a strong magnetic field

Nuclear magnetic resonance What does it d

Atomic composition can be determined by high-resolution mass spectrometry

Mass spectrometry detects isotopes

Mass spectrometry by chemical ionization, electrospray, or other methods