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Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or 1 H 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]
[5] In this common practice, users adjust residual solvent signals of 1 H or 13 C NMR spectra with calibrated spectral tables. [6] [7] If substances other than the solvent itself are used for internal referencing, the sample has to be combined with the reference compound, which may affect the chemical shifts.
Gutmann, a chemist renowned for his work on non-aqueous solvents, described an acceptor-number scale for solvent Lewis acidity [4] with two reference points relating to the 31 P NMR chemical shift of Et 3 PO in the weakly Lewis acidic solvent hexane (δ = 41.0 ppm, AN 0) and in the strongly Lewis acidic solvent SbCl 5 (δ = 86.1 ppm, AN 100).
19 F NMR chemical shifts in the literature vary strongly, commonly by over 1 ppm, even within the same solvent. [5] Although the reference compound for 19 F NMR spectroscopy, neat CFCl 3 (0 ppm), [6] has been used since the 1950s, [7] clear instructions on how to measure and deploy it in routine measurements were not present until recently. [5]
Because of the magnitude and severity of the problems with chemical shift referencing in biomolecular NMR, a number of computer programs have been developed to help mitigate the problem (see Table 1 for a summary). The first program to comprehensively tackle chemical shift mis-referencing in biomolecular NMR was SHIFTCOR. [2] Table 1.
Directly bound nuclei have hyperfine shifts of thousands of ppm but are usually not oberservable due to extremely fast relaxation and line broadening. [5] 1 H NMR spectrum of 1,1'-dimethylnickelocene, illustrating the dramatic chemical shifts observed in some paramagnetic compounds. The sharp signals near 0 ppm are from solvent.
With a gyromagnetic ratio 40.5% of that for 1 H, 31 P-NMR signals are observed near 202 MHz on an 11.7-Tesla magnet (used for 500 MHz 1 H-NMR measurements). Chemical shifts are typically referenced to 85% phosphoric acid, which is assigned the chemical shift of 0, and appear at positive values (downfield of the standard). [2]
The vast majority of molecules in a solution are solvent molecules, and most regular solvents are hydrocarbons and so contain NMR-active hydrogen-1 nuclei. In order to avoid having the signals from solvent hydrogen atoms overwhelm the experiment and interfere in analysis of the dissolved analyte, deuterated solvents are used where >99% of the ...