An ab initio study of the primary hydration and proton transfer of CF3SO3H and CF3O(CF2)2SO3H: Effects of the hybrid functional and inclusion of diffuse functions

Electronic structures calculations were undertaken on two perfluorosulfonic acids (CF3SO3H and CF3O(CF2)2SO3H) with varying number of hydrating water molecules to assess the effects of the enhanced electron withdrawing afforded by an additional CF2 group: the side chain of the short-side-chain (SSC) PFSA membrane. Both Beckeʹs 3-parameter hybrid functional (B3LYP) and the extended hybrid functional (X3LYP) employing the 6-311G** split valence Gaussian basis set with and without the inclusion of diffuse functions were performed to determine the impact on proton dissociation and separation as a function of the degree of the hydration of the acids. The calculations underscore the increased acidity of CF3O(CF2)2SO3H due to the enhanced electron withdrawing of the β –CF2– but did not reveal any differences in the strength of the binding of the water molecules over that computed for CF3SO3H. The calculated structural parameters obtained from optimized geometries of both acids with from 1 to 4 water molecules revealed that the inclusion of diffuse functions has negligible effects. Furthermore, the results obtained with the X3LYP functional were nearly identical to those obtained with the B3LYP functional.