To get around this problem, we use special NMR solvents in which all protons have been replaced by deuterium. If we use a common laboratory solvent (diethyl ether, acetone, dichloromethane, ethanol, water, etc.) to dissolve our NMR sample, however, we run into a problem – there many more solvent protons in solution than there are sample protons, so the signals from the sample protons will be overwhelmed. In most cases, a sample being analyzed by NMR is in solution. The NMR instrument records which frequencies were absorbed, as well as the intensity of each absorbance. In doing so, the protons absorb radiation at the two resonance frequencies. the resonance frequencies) cause those H a and H b protons which are aligned with B 0 to 'flip' so that they are now aligned against B 0. The two specific frequencies which match ω a and ω b(i.e. Then, the sample is hit with electromagnetic radiation in the radio frequency range. At first, the magnetic moments of (slightly more than) half of the protons are aligned with B 0, and half are aligned against B 0. First, a sample compound (we'll use methyl acetate) is placed inside a very strong applied magnetic field (B 0).Īll of the protons begin to precess: the H a protons at precessional frequency ω a, the H b protons at ω b. A full explanation of how a modern NMR instrument functions is beyond the scope of this text, but in very simple terms, here is what happens. Given that chemically nonequivalent protons have different resonance frequencies in the same applied magnetic field, we can see how NMR spectroscopy can provide us with useful information about the structure of an organic molecule.
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