MOLECULAR MODELING OF TRIFLUOROMETHANESULFONIC ACID FOR SOLVATION THEORY

Reported here are theoretical calculations on the trifluoromethanesulf onic (triflic) acid with and without an additional water molecule, est ablishing molecular scale information necessary to molecular modeling of the structure, thermodynamics, and ionic transport of Nafion(R) mem branes. The optimized geometry determined for the isolated triflic aci d molecule, obtained from ab initio molecular orbital calculations, ag rees with previous studies. In order to characterize side chain flexib ility and accessibility of the acid proton, potential energy and free energy surfaces for rotation about both carbon-sulfur and sulfur-oxyge n(hydroxyl) bonds are presented. A continuum dielectric solvation mode l is used to obtain free energies of electrostatic interaction with th e solvent. Electrostatic solvation is predicted to reduce the free ene rgy barrier to rotation about the F3C-SO3 bond from 3.5 kcal/mol to ab out 2.7 kcal/mol. This electrostatic effect is associated with slight additional polarization of the CF bond in the eclipsed conformation. T he 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/mo l. We carried out additional ab initio electronic structure calculatio ns with a probe water molecule interacting with the triflic acid. The minimum energy structures found here for the triflic acid molecule wit h the probe water revise results reported previously. To investigate t he reaction path for abstraction of a proton from triflic acid, we fou nd minimum energy structures and energies for isolated molecular fragm ents, and solvation free energies for: (a) a docked configuration of t riflate anion and hydronium cation and (b) a transition state for prot on interchange between triflic acid and a water molecule. Those config urations are structurally similar but energetically substantially diff erent. The activation free energy for that proton interchange is predi cted to be 4.7 kcal/mol above the reaction end-points. (C) 1998 Elsevi er Science B.V. All rights reserved.