Some bodily processes actually change tissues in ways that are noticeable on an MRI. So if a chemist looks at the NMR spectrum of her unknown sample and sees a huge peak near 906 MHz, then she knows that her sample probably has at least one hydrocarbon chain somewhere on it. This means that NMR may be used to generate “spectra” corresponding to the amount of resonance at various frequencies, which in turn reveals details of the structure of molecules. A proton all by itself may absorb and reemit 900 MHz photons, but when it gets near other charges (such as in a large hydrocarbon chain), the magnetic field around it is gets twisted and distorted and so its resonant frequency may shift to something like 906 MHz.
NMR works because the characteristic frequency of the re-emitted photons varies very slightly based on the structure of the molecule. In the technique (and just as in MRI), an unknown sample is placed in a static magnetic field, briefly excited with radio-frequency photons (light), and then allowed to re-emit those photons.
NMR spectroscopy was originally developed to help chemists who had created strange compounds that they couldn’t identify.
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