We compare the role of neighboring group substitutions on proton dissociation of hydrated acidic moieties suitable for proton exchange membranes through electronic structure calculations. Three pairs of ionomers containing similar electron withdrawing groups within the pair were chosen for the study: two fully fluorinated sulfonyl imides (CF 3SO 2NHSO 2CF 3 and CF 3CF 2SO 2NHSO 2CF 3), two partially fluorinated sulfonyl imides (CH 3SO 2NHSO 2CF 3 and C 6H 5SO 2NHSO 2CF 2CF 3), and two aromatic sulfonic acid based materials (CH 3C 6H 4SO 3H and CH 3OC 6H 3OCH 3C 6H 4SO 3H). Fully optimized counterpoise (CP) corrected geometries were obtained for each ionomer fragment with the inclusion of water molecules at the B3LYP/6-311G* level of density functional theory. Spontaneous proton dissociation was observed upon addition of three water molecules in each system, and the transition to a solvent-separated ion pair occurred when four water molecules were introduced. No considerable quantitative or qualitative differences in proton dissociation, hydrogen bond networks formed, or water binding energies were found between systems containing similar electron withdrawing groups. Each of the sulfonyl imide ionomers exhibited qualitatively similar results regarding proton dissociation and separation. The fully fluorinated sulfonyl imides, however, showed a greater propensity to exist in dissociated and ion-pair separated states at low degrees of hydration than the partially fluorinated sulfonyl imides. This effect is due to the additional electron withdrawing groups providing charge stabilization as the dissociated proton migrates away from the imide anion.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry