Molecular modeling of trifluoromethanesulfonic acid for solvation theory

Abstract

Reported here are theoretical calculations on the trifluoromethanesulfonic (triflic) acid with and without an additional water molecule, establishing molecular scale information necessary to molecular modeling of the structure, thermodynamics, and ionic transport of Nafion® membranes. The optimized geometry determined for the isolated triflic acid molecule, obtained from ab initio molecular orbital calculations, agrees with previous studies. In order to characterize side chain flexibility and accessibility of the acid proton, potential energy and free energy surfaces for rotation about both carbon–sulfur and sulfur–oxygen(hydroxyl) bonds are presented. A continuum dielectric solvation model is used to obtain free energies of electrostatic interaction with the solvent. Electrostatic solvation is predicted to reduce the free energy barrier to rotation about the F 3C–SO 3 bond from 3.5 kcal/mol to about 2.7 kcal/mol. This electrostatic effect is associated with slight additional polarization of the CF bond in the eclipsed conformation. The energetic barrier to rotation of the acid hydroxyl group away from the sulfonic acid oxygen plane, out into the solvent is substantially flattened by electrostatic solvation effects. The maximum free energy for those solvent accessible proton conformations is about 1.0 kcal/mol. We carried out additional ab initio electronic structure calculations with a probe water molecule interacting with the triflic acid. The minimum energy structures found here for the triflic acid molecule with the probe water revise results reported previously. To investigate the reaction path for abstraction of a proton from triflic acid, we found minimum energy structures and energies for isolated molecular fragments, and solvation free energies for: (a) a docked configuration of triflate anion and hydronium cation and (b) a transition state for proton interchange between triflic acid and a water molecule. Those configurations are structurally similar but energetically substantially different. The activation free energy for that proton interchange is predicted to be 4.7 kcal/mol above the reaction end-points.