Proton NMR

Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or ¹H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen (H) is used, practically all the hydrogen consists of the isotope ¹H (hydrogen-1; i.e. having a proton for a nucleus). A full ¹H atom is called protium.

Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere. Deuterated (deuterium = ²H, often symbolized as D) solvents especially for use in NMR are preferred, e.g. deuterated water, D₂O, deuterated acetone, (CD₃)₂CO, deuterated methanol, CD₃OD, deuterated dimethyl sulfoxide, (CD₃)₂SO, and deuterated chloroform, CDCl₃. However, a solvent without hydrogen, such as carbon tetrachloride, CCl₄ or carbon disulphide, CS₂, may also be used.

Historically, deuterated solvents were supplied with a small amount (typically 0.1%) of tetramethylsilane (TMS) as an internal standard for calibrating the chemical shifts of each analyte proton. TMS is a tetrahedral molecule, with all protons being chemically equivalent, giving one single signal, used to define a chemical shift = 0 ppm. It is volatile, making sample recovery easy as well. Modern spectrometers are able to reference spectra based on the residual proton in the solvent (e.g. the CHCl₃, 0.01% in 99.99% CDCl₃). Deuterated solvents are now commonly supplied without TMS.

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