TY - JOUR
T1 - Acid/base equilibria in clusters and their role in proton exchange membranes
T2 - Computational insight
AU - Glezakou, Vassiliki Alexandra
AU - Dupuis, Michel
AU - Mundy, Christopher J.
PY - 2007
Y1 - 2007
N2 - We describe molecular orbital theory and ab initio molecular dynamics studies of acid/base equilibria of clusters AH : (H2O)n ⇔ A- : H+(H2O)n in low hydration regime (n = 1-4), where AH is a model of perfluorinated sulfonic acids, RSO3H (R = CF3CF2), encountered in polymer electrolyte membranes for fuel cells. Free energy calculations on the neutral and ion pair structures for n = 3 indicate that the two configurations are close in energy and are accessible in the fluctuation dynamics of proton transport. For n = 1, 2 the only relevant configuration is the neutral form. This was verified through ab initio metadynamics simulations. These findings suggest that bases are directly involved in the proton transport at low, n = 3, hydration level, but not at lower hydration levels, n ≤ 2. In addition, the gas phase proton affinity of the model sulfonic acid H was found to be comparable to the proton affinity of water. The free energy profile for proton exchange between a protonated acid-water cluster configuration and a neutral acid-hydronium ion cluster configuration showed that such configurations are nearly isoenergetic. Thus, protonated acids can also play a role in proton transport under low hydration conditions and under high concentration of protons.
AB - We describe molecular orbital theory and ab initio molecular dynamics studies of acid/base equilibria of clusters AH : (H2O)n ⇔ A- : H+(H2O)n in low hydration regime (n = 1-4), where AH is a model of perfluorinated sulfonic acids, RSO3H (R = CF3CF2), encountered in polymer electrolyte membranes for fuel cells. Free energy calculations on the neutral and ion pair structures for n = 3 indicate that the two configurations are close in energy and are accessible in the fluctuation dynamics of proton transport. For n = 1, 2 the only relevant configuration is the neutral form. This was verified through ab initio metadynamics simulations. These findings suggest that bases are directly involved in the proton transport at low, n = 3, hydration level, but not at lower hydration levels, n ≤ 2. In addition, the gas phase proton affinity of the model sulfonic acid H was found to be comparable to the proton affinity of water. The free energy profile for proton exchange between a protonated acid-water cluster configuration and a neutral acid-hydronium ion cluster configuration showed that such configurations are nearly isoenergetic. Thus, protonated acids can also play a role in proton transport under low hydration conditions and under high concentration of protons.
UR - http://www.scopus.com/inward/record.url?scp=35748978501&partnerID=8YFLogxK
U2 - 10.1039/b709752b
DO - 10.1039/b709752b
M3 - Article
AN - SCOPUS:35748978501
SN - 1463-9076
VL - 9
SP - 5752
EP - 5760
JO - Physical Chemistry Chemical Physics
JF - Physical Chemistry Chemical Physics
IS - 43
ER -