NMR spectroscopy
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NMR spectroscopy treats certain atomic nuclei like tiny compass needles. Place a molecule in a strong magnetic field and nuclei such as hydrogen line up with the field, like compasses pointing north. Give them a precise pulse of radio waves and they tip over and wobble; as they settle back, they sing out radio signals that reveal where each atom sits in the molecule.
More precisely, each nucleus resonates at a frequency that depends on its magnetic surroundings. Electrons nearby shield it slightly, shifting its frequency — the chemical shift — so atoms in different chemical environments appear at different positions in the spectrum. Neighboring nuclei also split each other's signals through spin-spin coupling, mapping out which atoms are bonded near which.
It matters because no other single technique reveals a molecule's full skeleton so directly: NMR can tell you not just which functional groups are present but exactly how the atoms are connected. Its honest cost is that it needs relatively large, pure samples and expensive superconducting magnets, making it more a laboratory workhorse than a field tool.
The proton NMR of ethanol shows three groups of peaks in a 3 : 2 : 1 ratio. The pattern, plus the way neighboring peaks split one another, lets a chemist read off CH₃–CH₂–OH directly.
Peak positions, areas, and splitting together reconstruct the molecular skeleton.
The same physics powers MRI scanners in hospitals; medical imaging just drops the word "nuclear" to avoid alarming patients. Chemical NMR focuses on identifying molecules rather than imaging tissue.