Optimizing D2O/H2O Ratio for NMR pH Measurements

Solvents with 2H isotopes (deuterium) are frequently used in NMR spectroscopy to avoid large solvent signals and to lock the external magnetic field of the spectrometer to the deuterium resonance frequency. In this research, deuterated water (D2O) was used in investigations of the NMR chemical-shift dependency on the pH of the solution. It was found that 2-fluoro-3-hydroxymethylpyridine shows a large 19F chemical-shift dependency on pH; however, in subsequent studies, we realized that the chemical shift of 2-fluoro-3-hydroxymethylpyridine also depends on the 2H/1H composition of the solvent water. Several D2O/H2O compositions were tested, and their influence on the relationship between 19F chemical-shift and pH was evaluated. While the chemical shift dependency is relatively small for aqueous solutions with high amounts of D2O (e.g., 90%), it increased substantially if the D2O portion in the solvent is reduced. The highest chemical-shift dependency was found for an about equal mixture of D2O and H2O