Electric Field-Driven Acid Base Chemistry: Proton Transfer from Acid (HCl) to Base (NH3/H2O)

It is well-known that single H3N-HCl and H2O-HCl acid-base pairs do not react to form the ion pairs, H4N+Cl- and H3O+Cl-, in isolation. On the basis of ab initio method, we propose a physical method of external electric field (E-ext) to drive the proton transfer from acid (HCl) to base (NH3/H2O). Our results show that when E-ext along the proton-transfer direction achieves or exceeds the critical electric field (E-c), the proton transfer occurs, such as, the E-c values of proton transfer for H3N-HCl and H2O-HCl are 54 x 10(-4) and 210 x 10(-4) au, respectively. And the degree of the proton transfer can be controlled by modulating the strength of E-ext. Furthermore, we estimate the inductive strength of an excess electron (E-e) equivalent to the E-ext = 125 x 10(-4) au, which is greater than the E-c (54 x 10(-4) au) of NH3-HCl but less than the E-c (210 x 10(-4) au) of H2O-HCl This explains well the anion photoelectron spectroscopy [Eustis et al. Science 2008, 319, 936] that an excess electron can trigger the proton transfer for H3N-HCl but not for H2O-HCl. On the basis of the above estimation, we also predict that two excess electrons can induce H2O-HCl to undergo the proton transfer and form the ion pair H3O+Cl-.