Quantum Chemical Prediction of pK_a Values of Cationic Ion-Exchange Groups in Polymer Electrolyte Membranes

The durability of anion-exchange membranes (AEMs) in alkaline fuel cells is largely determined by the stability of anion-exchange groups. Despite continuous research efforts, the commonly employed cations still have limited stability against hydroxide that can act as a strong base and nucleophile. This work is concerned with base-catalyzed degradation of organic cations initiated by proton abstraction to form reactive ylides or carbenes. We report on the performance of 24 methods combining density functional theory and electronic structure methods with implicit solvation calculations for predicting pK_a values of organic cations in water and DMSO. The most accurate computational protocols are obtained using a combination of M06-2X/6-311++G∗ with the SMD solvation model for water (the mean absolute error of 0.4 pK_a units) and B3LYP/aug-cc-pVTZ with the IEFPCM solvation model for DMSO (the mean absolute error of 1.4 pK_a units). The aqueous pK_a calculation protocol is cross-validated against the experimental C-H acidity constants outside the conventional range of 0-14 pK_a values. This study rationalizes alkaline degradation of imidazolium cations with C2-alkyl substituents and provides a theoretical scale of C-H acidity for potential anion-exchange groups in AEMs.