Stability and hydration structure of model perfluorosulfonic acid compound systems, CF₃SO₃H(H₂O)_n (n=1-4), and its isotopomer by the direct treatment of H/D nuclear quantum effects

We theoretically analyzed the stability and hydration structures of perfluorosulfonic acid (PFSA) for polymer electrolyte fuel cell (PEFC) by using model compound systems, CF₃SO₃H and its D isotopomer with one to four water molecules (H₂O/D₂O). We applied to the systems the multi-component molecular orbital method that takes into account the quantum effects of H and D nuclei directly. By comparing the results of H/D isotopomers, we clearly demonstrated the geometrical difference in hydrogen bonds due to the difference of H/D quantum behavior. The interaction energy of deuterated hydrogen bonds became small due to the weakness of hydrogen bond by the deuterium substitution. When four water molecules are hydrated, only the proton transfer structure for H compound was obtained due to the drastic change of the potential. We also analyzed the details of geometry and electronic charge of H/D isotopic oxonium ions, H₃O⁺, H₂DO⁺, HD₂O⁺, and D₃O⁺, in proton transfer structures.