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 H 4 N + X - and H 3 O + 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 1.131×10 7 V/cm for H3N-HBr, 1.378×10 8 V/cm for H 3 N-HF, 9.358×10 7 V/cm for H 2 O-HBr and 2.304×10 8 V/cm for H 2 O-HF, respectively. Furthermore, one or three excess electrons can trigger the proton transfer from H 3 N-HBr and H 3 N-HF to H 4 N + Br - and H 4 N + F - , while two and four excess electrons can induce the proton transfer from H 2 O-HBr and H 2 O-HF to H 3 O + Br - and H 3 O + F - , respectively. Compared with that of the analogous NH 3 /H 2 O-HCl systems, the strength of E c of proton transfer increases from HBr to HCl and HF for either H 3 N-HX or H 2 O-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 NH 3 with a stronger basicity is generally smaller than that of H 2 O systems for the same conjugated acid.