) according to the previously reported and here improved procedure (see Supporting Information (SI)). Hydrogen/deuterium (H/D) exchange was achieved by mixing the PILs with D2O and removing water several times until nearly 100% deuteration was reached as proven by 1 H NMR. The resulting PILs ([TEA-d1][OMs]) and ([TEA-d16][OMs]) have been dried under vacuum (at 3·10 - 3 mbar) for several days and the final water concentration (< 15 ppm) has been checked by Karl-Fischer titration. The PILs were then loaded into 5 mm glass tubes, properly degassed by bubbling through nitrogen, sealed off and finally transferred into an NMR probe for analysis with 2 H NMR spectroscopy.