Electric Field-driven Acid-base Transformation: Proton Transfer from Acid(HBr/HF) to Base(NH3/H2O)

The proton-transfer between ammonia/water and HF/HBr without and with the stimulus of external electric fields(E_ext) was investigated with the ab initio calculations. When external electric field is applied, the proton transfer occurs, resulting in ion-paired H₄N⁺X^- and H₃O⁺X^-(X=Br and F) from hydrogen-bonded complexes in view of the great changes of geometrical structures, dipole moments, frontier molecular orbitals and potential energy surfaces in the critical external electric fields(E_c) of 1.131×107 V/cm for H₃N-HBr, 1.378×108 V/cm for H₃N-HF, 9.358×107 V/cm for H₂O-HBr and 2.304×108 V/cm for H₂O-HF, respectively. Furthermore, one or three excess electrons can trigger the proton transfer from H₃N-HBr and H₃N-HF to H₄N⁺Br^- and H₄N⁺F^-, while two and four excess electrons can induce the proton transfer from H₂O-HBr and H₂O-HF to H₃O⁺Br^- and H₃O⁺F^-, respectively. Compared with that of the analogous NH₃/H₂O-HCl systems, the strength of E_c of proton transfer increases from HBr to HCl and HF for either H₃N-HX or H₂O-HX series, which is understandable by the fact that the acidity sequence is HBr>HCl>HF. And the larger of acidity of conjugated acid, the smaller of needed E_c. On the other hand, the E_c for the systems of NH₃ with a stronger basicity is generally smaller than that of H₂O systems for the same conjugated acid.