Proton nuclear magnetic resonance

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.^[1] 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).

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 disulfide, 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 referencing 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. ^[2] 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.

Deuterated solvents permit the use of deuterium frequency-field lock (also known as deuterium lock or field lock) to offset the effect of the natural drift of the NMR's magnetic field ${\displaystyle B_{0}}$. In order to provide deuterium lock, the NMR constantly monitors the deuterium signal resonance frequency from the solvent and makes changes to the ${\displaystyle B_{0}}$ to keep the resonance frequency constant.^[3] Additionally, the deuterium signal may be used to accurately define 0 ppm as the resonant frequency of the lock solvent and the difference between the lock solvent and 0 ppm (TMS) are well known.

Proton NMR spectra of most organic compounds are characterized by chemical shifts in the range +14 to -4 ppm and by spin-spin coupling between protons. The integration curve for each proton reflects the abundance of the individual protons.

Simple molecules have simple spectra. The spectrum of ethyl chloride consists of a triplet at 1.5 ppm and a quartet at 3.5 ppm in a 3:2 ratio. The spectrum of benzene consists of a single peak at 7.2 ppm due to the diamagnetic ring current.

Together with carbon-13 NMR, proton NMR is a powerful tool for molecular structure characterization.