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Nuclear magnetic resonance (NMR) spectroscopy can measure radio-frequency Zeeman transitions of proton spins in a magnetic field. It is more convenient to sweep the magnetic field through the resonances at a fixed frequency, typically 60 MHz. The resonances are sensitive to the chemical environment of nonequivalent protons, an effect known as the chemical shift. A classic example is the ethanol molecule C ... C ... OH, which shows three chemically-distinct hydrogen atom sites, thus three NMR peaks with intensity ratios 3:2:1. The relevant parameter is ... , representing the fractional deviation of the chemical shift measured in parts per million (ppm) from that of tetramethylsilane (TMS), a convenient standard assigned the reference value ... . A small amount of TMS is often added to the sample being measured to calibrate the ... -scale. At higher resolution, it is possible to identify further splitting of the chemically-shifted peaks due to spin-spin interactions with neighboring groups of protons. Thus the C ... resonance is split into a 1:2:1 triplet by interactions with the two C ... protons. Correspondingly, the C ... resonance is split into a 1:3:3:1 quartet by interactions with the three C ... protons. The OH resonance is not usually split because of the rapid exchange of these protons via hydrogen bonding. The spectroscopic procedures described in this Demonstration are intended as a simplified introduction to the principles of NMR. Modern NMR spectroscopy makes use of Fourier-transform techniques, which produce the entire spectrum simultaneously.
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