17O NMR spectroscopy: Referencing in diamagnetic and paramagnetic solutions

Abstract

Several referencing techniques for measuring ¹⁷O NMR chemical shifts in diamagnetic and paramagnetic solutions have been examined. 1,4-Dioxane is recommended as an external reference standard for diamagnetic solutions because the ¹⁷O resonance position is independent of temperature between 27 and 90°C. At 40°C the ¹⁷O chemical shift, within experimental error, corresponds to that of doubly distilled water. Nitromethane cannot be considered as a suitable internal reference because of large solvent effects on the ¹⁷O chemical shift. Further, in organic solutions its lanthanide-induced chemical shifts are not negligible. For the study of paramagnetic solutions the combined use of a cylindrical and a spherical cell is proposed: the ¹⁷O resonance of either the magnetically active solvent or a convenient compound added to the solution is recorded in both cells; the chemical shift difference between the two measurements is the bulk magnetic susceptibility shift.