Peaks and Valleys: The Inside Story on NMR
A complex quantum phenomenon, NMR works because the nuclei of many atoms -- those with an odd number of protons or neutrons -- spin like tops, creating a tiny magnetic field. When you put these atoms inside another magnetic field, the spinning nuclei line up in the same direction, as opposed to their normal random orientations, and they wobble or "precess," like a top slowing down. Furthermore -- and here's the key -- each different kind of atom, carbon versus hydrogen, for instance, has its own distinctive wobble, its own resonance frequency, which makes it possible for skilled researchers like Alan Benesi to read the peaks and valleys of the signals they give off.
"With a solid," explains Benesi, "we put the sample into a little rotor, and we put that down into the sweet spot, the highest magnetic field part of the superconducting magnet, and we spin the rotor and excite it with radio frequency pulses." After the sample is zapped, sometimes in complicated ways that reveal subtle frequency transitions, it gives off signals that can be monitored for the tell-tale clues of a particular atom, including differences that occur according to an atom's surrounding chemical environment. "In a carbon sample, for example," says Benesi, "we can tell the type of carbon, whether it's in a methyl (CH3-) or carboxyl (-COOH) group, from its frequency. And that's very useful chemical information."
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