Solvent

For liquid-state NMR, you will need to dissolve your sample in a solvent. The solvent molecules should have all hydrogen atoms replaced with deuterium atoms (2H) for two reasons. First, if you are doing proton (XH) NMR, you do not want the solvent resonance to dominate your spectrum. Solvent molecules typically outnumber solute molecules by 1000 to 1, so you would not really see your solute spectrum at all. Second, the spectrometer needs a deuterium (2H) signal to "lock" the magnetic field strength and keep it from changing with time. Because the NMR experiment usually adds together a number of FIDs (scans), if the field changes during the experiment the frequency changes with it and the NMR peaks will not add together correctly. The deuterium NMR signal is used to monitor "drift" of the field and to correct it (more about this later). For ordinary lipophilic ("greasy") organic molecules, deuterochloroform (CDCl3) is the ideal solvent. For hydrophilic molecules (e.g., salts) the ideal solvent is D2O. For molecules that are in-between in polarity or have both polar and nonpolar parts (e.g., organic acids), there are a number of more expensive solvents to try. d6-DMSO (CD3-SO-CD3) is a very good solvent, but it is difficult to recover your sample from the solvent afterward. Fully deuterated versions of acetone, methanol, acetonitrile, benzene, and THF are available at prices that increase in that order. Of course, test your sample compound for solubility with the cheap, nondeuterated solvents first before wasting the expensive stuff! Acetone, D2O, methanol, DMSO, and acetonitrile all absorb H2O from the atmosphere when open, giving an H2O peak in the spectrum at a chemical shift that depends on the solvent.

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