Selenate-An internal chemical shift standard for aqueous ⁷⁷ Se NMR spectroscopy

The ⁷⁷ Se NMR spectra of selenate were studied under various circumstances, such as concentration, pH, temperature, ionic strength, and D₂ O:H₂ O ratio, in order to examine its potential as a water-soluble internal chemical shift standard. The performance of selenate as a chemical shift reference and that of other attempted ones from the literature (dimethyl selenide, tetramethylsilane/TMS, and 3-(trimethylsilyl)propane-1-sulfonate/DSS) was also explored. The uncertainty in the resulting chemical shift relative to the effective spectral width is comparable to that of DSS. Compared to the currently prevalent water-soluble external chemical shift reference, selenic acid solution, the properties of internal selenate are much more favorable in terms of ease of use. We have also demonstrated that selenate can be used in reducing media, which is inevitable for the analysis of selenol compounds. Thus, it can be stated that sodium selenate is a robust internal chemical shift reference in aqueous media for ⁷⁷ Se NMR measurements; the chemical shift of this reference in a solution containing 5 V/V% D₂ O at 25°C and 0.15 mol·dm^-3 ionic strength is 1048.65 ppm relative to 60 V/V% dimethyl selenide in CDCl₃ and 1046.40 ppm relative to the ¹ H signal of 0.03 V/V% TMS in CDCl₃ . In summary, a water-soluble, selenium-containing internal chemical shift reference compound was introduced for ⁷⁷ Se NMR measurements for the first time in the literature, and with the aforementioned results all previous ⁷⁷ Se measurements can be converted to a unified scale defined by the International Union of Pure and Applied Chemistry.