title: Mechanisms of proton transfer in Nafion: elementary reactions at the sulfonic acid groups.
abstract: Proton transfer reactions at the sulfonic acid groups in Nafion were theoretically studied, using complexes formed from triflic acid (CF3SO3H), H3O+ and H2O, as model systems. The investigations began with searching for potential precursors and transition states at low hydration levels, using the test-particle model (T-model), density functional theory (DFT) and ab initio calculations. They were employed as starting configurations in Born-Oppenheimer molecular dynamics (BOMD) simulations at 298 K, from which elementary reactions were analyzed and categorized. For the H3O+-H2O complexes, BOMD simulations suggested that a quasi-dynamic equilibrium could be established between the Eigen and Zundel complexes, and that was considered to be one of the most important elementary reactions in the proton transfer process. The average lifetime of H3O+ obtained from BOMD simulations is close to the lowest limit, estimated from low-frequency vibrational spectroscopy. It was demonstrated that proton transfer reactions at -SO3H are not concerted, due to the thermal energy fluctuation and the existence of various quasi-dynamic equilibria, and -SO3H could directly and indirectly mediate proton transfer reactions through the formation of proton defects, as well as the -SO3- and -SO3H2+ transition states.
authors.affiliation: School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand. kritsana@sut.ac.th
authors.forename: Kritsana
authors.initials: K
authors.lastname: Sagarik
authors.forename: Mayuree
authors.initials: M
authors.lastname: Phonyiem
authors.forename: Charoensak
authors.initials: C
authors.lastname: Lao-ngam
authors.forename: Sermsiri
authors.initials: S
authors.lastname: Chaiwongwattana
mesh_terms: D003198:Computer Simulation; D005465:Fluorocarbon Polymers / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D013451:Sulfonic Acids / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D005465:Fluorocarbon Polymers; D011522:Protons; D013451:Sulfonic Acids; D014867:Water; C040402:perfluorosulfonic acid