Electron-Driven Acid-Base Chemistry: Proton Transfer from Hydrogen Chloride to Ammonia

In contrast to widely familiar acid-base behavior in solution, single molecules of NH₃ and HCl do not react to form the ionic salt, NH₄⁺Cl^-, in isolation. We applied anion photoelectron spectroscopy and ab initio theory to investigate the interaction of an excess electron with the hydrogen-bonded complex NH₃^…HCl. Our results show that an excess electron induces this complex to form the ionic salt. We propose a mechanism that proceeds through a dipole-bound state to form the negative ion of ionic ammonium chloride, a species that can also be characterized as a deformed Rydberg radical, NH₄, polarized by a chloride anion, Cl^-.