Proton transfer in functionalized phosphonic acid molecules

Abstract

Ionomers with protogenic groups such as phosphonic acid have recently been proposed as suitable polymer electrolyte membranes for fuel cells operating with little humidification due to their extraordinary amphotericity. The hydrogen bonding and energetics associated with proton transfer of substituted phosphonic acid molecules and their self-condensation products (anhydride + H₂O) are examined through ab initio electronic structure calculations. The global minimum energy structures of methyl-, phenyl-, benzyl-, trifluoromethyl- and phenyldifluoro-phosphonic acids determined at the B3LYP/6-311G** level indicate that the fluorinated molecules exhibit slightly stronger binding to water than the non-fluorinated molecules. The potential energy profiles for transfer of a proton within the condensation products were obtained at the B3LYP/6-311G** level for each of the acids, and revealed that the trifluoromethyl system has the lowest endothermicity (5.3 kcal mol⁻¹) in contrast to the methyl system which has the greatest endothermicity (7.1 kcal mol⁻¹) and a barrier of 7.6 kcal mol⁻¹. When no constraints are imposed on the system, proton transfer from the anhydride to a water molecule (primary) is accompanied by a secondary proton transfer (from protonated water molecule back to the anhydride) in all cases except the trifluoromethyl anhydride where charge separation and formation of a hydronium ion occurs. It appears that the secondary proton transfer would substantially determine the emergence of transition states and the correlated endothermicities.