Nuclear magnetic resonance — Chemistry, 14–17
Nuclear magnetic resonance, or NMR, reveals the chemical surroundings of certain atomic nuclei. Chemists use the pattern of signals to work out how atoms are connected in a molecule.
A molecular fingerprint
Some nuclei behave like tiny spinning magnets. In a strong magnetic field, radio waves can make them absorb energy, but only at particular frequencies. Their signals depend on the nearby atoms, so an NMR spectrum acts like a map of different chemical environments inside the molecule.
Why NMR was needed
A formula tells you how many atoms a substance contains, but not always how those atoms are arranged. Chemists needed a non-destructive way to distinguish molecules with the same formula or confirm a newly made compound. NMR solves this by turning invisible local environments into separate, measurable signals.
Reading ethanol’s signals
Ethanol has the structure CH₃CH₂OH. In a simple proton NMR spectrum, the three CH₃ hydrogens share one environment, the two CH₂ hydrogens share another, and the OH hydrogen gives a third signal. So you expect three main signals, with relative areas 3:2:1. The pattern supports the proposed structure.
One signal is not one atom
It is tempting to count peaks and say, “This molecule has four hydrogen atoms because I see four marks.” The spectrum does not count atoms one by one; it groups nuclei that have equivalent chemical surroundings. One broad-looking signal may represent several atoms, while tiny splits can come from neighbouring nuclei.
Where NMR is used
NMR helps identify medicines, natural products and substances in biological samples. In hospitals, a related technique creates MRI images by detecting signals from hydrogen in water and fat. The chemistry instrument and the medical scanner use the same physical idea, but their purposes and data are different.
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