There is both a scarcity of data as well as ambiguity in the quality of experimental data (for instance, although the relative uncertainties of ∆G o acid, gas (AH) are quoted as just 1 kJ mol -1 , 47 values from different authors in the literature sometimes differ by several kcal mol -1 ). In some cases, the gas-phase acidities were measured at temperatures significantly higher than 298 K, but our model was trained with calculations assuming 298 K. Such potential issues may contribute to the relatively high errors seen herein.