Electric field-driven acid-base transformation: proton transfer from acid(HBr/HF) to base(NH3/H2O)

Abstract

The proton-transfer between ammonia/water and HF/HBr without and with the stimulus of external electric fields(Eext) 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(Ec) 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 Ec 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 Ec. On the other hand, the Ec for the systems of NH3 with a stronger basicity is generally smaller than that of H2O systems for the same conjugated acid.