What NMR measures

Two experiments cover almost all routine structure work. They answer different questions, and they look different because the two nuclei differ sharply in natural abundance and sensitivity.

A ¹H spectrum carries three independent readouts at once — position, area and splitting — and each answers a different question about your molecule.

Reading a ¹H spectrum

Chemical shift (δ) — the environment

Position, in parts per million, reports the electronic environment of a proton: electron-poor environments (near oxygen, or an aromatic ring) sit downfield, at higher δ. A working map:

Exchangeable protons — OH, NH, COOH — shift with solvent, concentration and temperature and can appear anywhere in the lower band; treat their positions as indicative, not fixed.

Integration — how many

Peak area is proportional to the number of protons giving that signal. Read the areas as a ratio, and the ratio is a proton count.

Coupling: the neighbours, and J

Two protons a few bonds apart sense each other's spin through the bonding electrons — this is scalar, or J, coupling. A neighbour's spin points either with the field or against it, with equal probability, and each case nudges the observed proton's frequency slightly higher or lower. So a proton with one neighbour resonates not at a single frequency but at two, split evenly about where its line would otherwise have been — a doublet. The gap between those two lines is the coupling constant, J, quoted in Hz. Two properties make it useful: J is shared exactly by both partners — the same value appears in each of their multiplets — and, unlike chemical shift, it does not change with the spectrometer, so a 7 Hz coupling is 7 Hz at 300 MHz or at 900 MHz.

Add more equivalent neighbours and the pattern builds up in a fixed way: n equivalent neighbours give n + 1 lines, with intensities following Pascal's triangle.

The size of J is diagnostic in its own right: it depends on how the two protons are connected and, for vicinal pairs, on the geometry between them.

Vicinal ³J follows the Karplus relationship — largest near 0° and 180° dihedral, near zero around 90° — which is exactly why it reads out ring stereochemistry and alkene E/Z geometry from the number alone.

One step past the n+1 rule, and the step that matters most in real spectra: a proton coupled to two different neighbours does not give a clean n+1 multiplet. Two different J values give a doublet of doublets (dd); a third gives a ddd or a doublet of triplets, and so on. Count the couplings, not just the lines — a four-line signal may be a quartet (three equivalent neighbours) or a dd (two inequivalent ones), and only the spacings tell them apart.

These rules are the first-order picture. When two coupled signals lie close in chemical shift relative to their J, their multiplets lean toward each other — inner lines grow, outer lines shrink, an effect called roofing — and exact intensities and spacings then need a full analysis. Roofing is itself useful: it points to which signals are coupled to one another.

Reading a ¹³C spectrum

A ¹³C spectrum looks sparse and clean for two reasons: at 1.1% abundance the carbons are dilute and need many scans, and routine ¹³C is proton-decoupled — so each chemically distinct carbon collapses to a single sharp line, with no coupling and normally no integration.

Getting an integral you can trust — and quantitative NMR

An integral is only as good as the conditions it was measured under, and even a careful one is semi-quantitative at best. Five things separate a trustworthy integral from a misleading one:

Even measured this way, a bare integral gives only relative proportions. To turn it into an absolute purity — the assay figure on a certificate — you add a weighed internal standard of certified purity: the mole ratio taken from the two integrals gives absolute content. That is quantitative NMR (qNMR), and it stays a demanding measurement rather than a routine one.

This is the basis of every purity figure NorrChemica reports, and of our NMR analysis service.

The peaks that aren't yours: solvent, water and contaminants

Every real spectrum shows peaks from the deuterated solvent, the water it carries, and whatever else came through with the sample. None of these belong to your compound, and reading one as an analyte signal is among the commonest mistakes. This is a lookup, so it is meant to be complete rather than short — three reference tables:

The residual-solvent line is fixed and doubles as a secondary chemical-shift reference; the water position drifts with solvent, concentration and temperature, so treat it as approximate.

The other everyday culprits are the contaminants themselves — grease from a joint, the solvent you crystallised from, a standard you added. The most common, in the two workhorse solvents:

Values from Gottlieb (1997) and Fulmer (2010); water and exchangeable-proton positions depend on concentration and temperature.

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