About the choice of protogenic group for polymer electrolyte membranes: Alkyl or aryl phosphonic acid?

The bond rotational potential profiles for various phosphonic acids and phosphonate anions are calculated using the density functional theory at B3LYP/6-311++G(d, p) level of theory. And the bond rotational barriers are related to the apparent activation energy for proton motion evaluated from the line broadenings of the NMR signals. The alkyl phosphonic acids, the alkylene diphosphonic acids, and the benzyl phosphonic acids have similar bond rotational potential profiles, however, that for the phenyl phosphonic acids have quite different bond rotational potential profiles. The bond rotational barriers for the alkyl phosphonic acids are about 0.114 eV, slightly lower than that for the alkylene diphosphonic acids. A spacer about three carbon atoms in length could shield the interaction from the neighboring phosphonic acid groups. For the aromatic phosphonic acids or p- or m-substituted aromatic phosphonic acids, the rotational barriers are almost negligible. Therefore, it is concluded that aromatic phosphonic acids are more preferable than alkyl phosphonic acids for protogenic groups.

Research Highlights
► We calculate bond rotational barriers of phosphonic acids and phosphonate anions.
► We relate bond rotational barriers to the apparent activation energy of proton.
► We calculate the acidities of gas phase and solvated phosphonic acids.
► An ethylene spacer could shield interaction between two neighboring acid groups.
► Aromatic phosphonic acids are more preferable than alkyl phosphonic acids.