Proton Transport Pathways in an Acid–Base Complex Consisting of a Phosphonic Acid Group and a 1,2,3-Triazolyl Group

Abstract

The proton transport pathways in an acid-base complex consisting of a phosphonic acid group and a 1,2,3-triazolyl group were studied using density functional theory (DFT) calculations in terms of stable configurations and transition states of the molecular or ionic dimers and trimers and verified by proof-of-concept experiments including experimental measurements of overall conductivity and (1)H NMR and FTIR spectroscopy of the methylphosphonic acid (MPA) and 1,2,3-triazole (Tri) complex as well as overall proton conductivity of polymeric blend of poly(vinylphosphonic acid) (PVPA) and poly(4-vinyl-1H-1,2,3-triazole) (PVTri). From the DFT calculations of dimers and trimers composed of ethylphosphonic acid (EPA), Tri, and their deprotonated counterparts, it was concluded that the intermolecular hydrogen bonds of the transition states corresponding to proton transport are much shorter than those of stable configurations, but the O-H and N-H bonds are much longer than those of stable configurations. The tautomerization activation energy decreases from 0.927-1.176 eV in Tri-Tri dimers to 0.336-0.444 eV in the EPA-Tri dimers. From the proof-of-concept experiments, about a 50 fold increase in overall conductivity was observed in the MPA-Tri complex consisting of 10% (molar ratio) MPA compared to pure Tri, and the calculated activation energy is consistent with the experimental activation energy evaluated from temperature dependence of proton conductivity of pure Tri and the MPA-Tri complex. In addition, the fast proton exchange between MPA and Tri, consistent with the DFT calculations, was verified by (1)H NMR and FTIR spectroscopy. Finally, a polymeric blend of PVPA and PVTri was prepared, and its proton conductivity at about 2.1 mS·cm(-1) in anhydrous state at 100 °C was observed to be significantly higher than that of PVPA or of poly(VPA-co-1-vinyl-1,2,4-triazole). The proton conductivity of the polymeric PVPA and PVTri blend in humidity state is in the same range as that of NAFION 117.