Influence of Water Content on Basicities in Acetonitrile

The influence of water content at impurity level (5–10,000 ppm) in acetonitrile, on the changes in relative basicity differences (ΔpK _a values) of 13 pairs of bases, was studied both experimentally and computationally (COSMO-RS). The ΔpK _a values involving smaller bases with localized charge in the cationic form were found to be more affected. A computational parameter, weighted average negative sigma (WANS), was proposed to quantify the charge delocalization in cations and succeeded in describing the observed changes of ΔpK _a. The results validate the previously published basicity scale in acetonitrile with respect to solvent dryness and give guidelines for better experimental planning.