17O NMR spectroscopy: Referencing in diamagnetic and paramagnetic solutions

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.