Dynamics of proton exchange in a model phosphonic acid-functionalized polymer

Abstract

The dynamics and mechanism of proton exchange in phosphonic acid-functionalized polymers were studied using poly(vinyl-phosphonic acid) (PVPA) as a model system along with quantum chemical calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations at the B3LYP/TZVP level as model calculations. This theoretical study began with searching for the smallest, most active polymer segments and their intermediate conformations which could be involved in the local proton-exchange process. The B3LYP/TZVP results confirmed that a low local dielectric environment and excess proton conditions are required to generate the intermediate conformations, and the shapes of the potential energy curves of the proton exchange between the two phosphonic acid functional groups are sensitive to the local conformational changes. In contrast, a high local dielectric environment increases the energy barriers, thereby preventing the proton from returning to the original functional group. Based on the static results, a mechanism for the proton exchange between the two functional groups involving fluctuations in the local dielectric environment and a local conformational change was proposed. The BOMD results confirmed the proposed mechanism by showing that the activation energies for the proton exchange in the hydrogen bond between two immobilized phosphonic acid moieties, obtained from the exponential relaxation behaviors of the envelopes of the velocity autocorrelation functions and the 1H Nuclear Magnetic Resonance (NMR) line-shape analyses, are too low to be the rate-determining process. Instead, coupled librational motion in the backbone which leads to the interconversion between the two intermediate conformations possesses higher activation energy, and therefore represents one of the most important rate-determining processes. These findings suggested that the rate of the proton exchange in the model phosphonic acid-functionalized polymer is determined by the polymer mobility which, in this case, is the large-amplitude librational motion of the vinyl backbone. © 2015 Wiley Periodicals, Inc.