NMR spectroscopy
/ EN-em-AR /
If mass spectrometry weighs a molecule and infrared names its groups, nuclear magnetic resonance maps the molecule from the inside — it is the single most powerful tool for figuring out how the atoms are actually connected. NMR is also the science behind the MRI scanner in a hospital; the same physics that images your knee can, on a tabletop, reveal the architecture of a molecule. It is the centerpiece of structure elucidation.
The physical basis is subtle but worth picturing. Certain atomic nuclei, including the ordinary hydrogen nucleus (a single proton) and carbon-13, behave like tiny spinning magnets. Place the sample in a strong magnetic field and these nuclear magnets line up with the field, like compass needles. Hit them with a pulse of radio waves and you flip some of them against the field; as they relax back, they emit a faint radio signal whose precise frequency depends on the exact magnetic environment each nucleus feels. Nuclei in different surroundings resonate at slightly different frequencies, and the instrument records each one as a separate signal.
The proton (1H) spectrum is the workhorse, and it carries four kinds of information at once: chemical shift tells the electronic environment of each set of hydrogens (where on the chart the signal sits); integration tells how many hydrogens give that signal (the area under it); spin-spin splitting tells how many hydrogens sit on neighboring carbons (how the signal is divided into sub-peaks); and equivalence tells which hydrogens are identical by symmetry. Read together, these four clues let a chemist reconstruct the carbon-hydrogen framework piece by piece, which is why NMR is the tool that most often closes the case.
Ethanol (CH3CH2OH) gives three proton signals: a 3H triplet for the CH3, a 2H quartet for the CH2, and a 1H signal for the OH. Their positions, areas, and splitting together pin down the structure.
1H NMR carries four clues at once: shift, integration, splitting, and equivalence.
NMR is non-destructive and reads the carbon-hydrogen framework, but it needs more sample than mass spectrometry and only sees nuclei that have spin (1H and 13C, not the common carbon-12 or oxygen-16). It is powerful but not infinitely sensitive.