Proton dissociation and transfer in hydrated phosphoric acid clusters

Abstract

The dynamics and mechanisms of proton dissociation and transfer in hydrated phosphoric acid (H3PO4) clusters under excess proton conditions were studied based on the concept of presolvation using the H3PO4–H3O+–nH2O complexes (n = 1–3) as the model systems and ab initio calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations at the RIMP2/TZVP level as model calculations. The static results showed that the smallest, most stable intermediate complex for proton dissociation (n = 1) is formed in a low local‐dielectric constant environment (e.g., ε = 1), whereas proton transfer from the first to the second hydration shell is driven by fluctuations in the number of water molecules in a high local‐dielectric constant environment (e.g., ε = 78) through the Zundel complex in a linear H‐bond chain (n = 3). The two‐dimensional potential energy surfaces (2D‐PES) of the intermediate complex (n = 1) suggested three characteristic vibrational and 1H NMR frequencies associated with a proton moving on the oscillatory shuttling and structural diffusion paths, which can be used to monitor the dynamics of proton dissociation in the H‐bond clusters. The BOMD simulations over the temperature range of 298–430 K validated the proposed proton dissociation and transfer mechanisms by showing that good agreement between the theoretical and experimental data can be achieved with the proposed rate‐determining processes. The theoretical results suggest the roles played by the polar solvent and iterate that insights into the dynamics and mechanisms of proton transfer in the protonated H‐bond clusters can be obtained from intermediate complexes provided that an appropriate presolvation model is selected and that all of the important rate‐determining processes are included in the model calculations. © 2015 Wiley Periodicals, Inc.

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

“…Due to their ability to approximate the effects of electron correlations with reasonable computational resources, DFT methods have been widely used in computational chemistry research. However, because the accuracy of DFT depends on the types of interactions in the chemical systems, the applicability and performance of B3LYP/TZVP calculations were systematically examined in our previous work by benchmarking with the second‐order Møller–Plesset perturbation theory (MP2) method with the resolution of the identity (RI) approximation and the TZVP basis set (abbreviated RIMP2/TZVP). Because the key properties, for example, the H‐bond structures, interaction energies, and 1 H NMR and vibrational frequencies of the transferring protons obtained from both methods are comparable, B3LYP/TZVP calculations were applied in this study.…”

“…Because this theoretical study focused on the dynamics of the structural‐diffusion process in the H‐bonds between the phosphonic acid‐functional groups, and our previous results showed that the intermediate complex that determines the rate of the proton‐exchange process is preferentially formed in excess proton conditions, both neutral and protonated PVPA (PV(PA) n and H + PV(PA) n , respectively, n = number of H 2 PO 3 functional groups) were tentatively chosen as model systems. The likelihood of proton exchange was anticipated primarily from the asymmetric stretching coordinate Δ d DA = | R OH – R O.H |, where R OH and R O.H are the OH and O.H distances in the OH.O Hbond, respectively.…”

“…Hence, the shared‐proton H‐bond with Δ d DA close to zero is considered as an “active” intermediate complex in the proton‐exchange process. For example, the smallest, most active intermediate complex for proton exchange in aqueous solutions is the Zundel complex, which possesses the shared‐proton (O.H + .O) structure with Δ d DA ≤ 0.1 Å; in contrast, the H‐bond in the close‐contact (OH + .O) structure is considered to be “inactive” because Δ d DA ≥ 0.4 Å …”

“…In our previous work, the dynamics and mechanisms of proton dissociation and transfer in hydrated H 3 PO 4 clusters under excess proton conditions were studied using H 3 PO 4 H 3 O + nH 2 O complexes ( n = 1–3) as model systems and ab initio calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations at the RIMP2/TZVP level as model calculations. The static results revealed that the smallest, most stable intermediate complex for proton dissociation is formed in a low local dielectric environment, and proton transfer from the first to the second hydration shell is driven by fluctuations in the number of water molecules in a high local dielectric environment.…”

“…Based on experience obtained in the previous work, and the fact that previous theoretical and experimental investigations were conducted mostly under neutral conditions, we proceed to study the complex mechanisms of proton exchange in polymeric materials under excess proton conditions. To investigate connections between the mobility of polymer structures and the efficiency of proton exchange between functional groups, the simplest proton‐conducting polymer with phosphonic acid‐functional groups, PVPA, was selected as a model system.…”