Extra Electron Key To Forming Ammonium Salt

Students in freshman chemistry learn that NH₃ and HCl react in solution to form NH₄⁺Cl^-. Those two reactants alone in the gas phase, however, merely form a hydrogen-bonded complex, NH₃···HCl. Exploring conditions necessary to form NH₄⁺Cl^-, chemistry professors Kit H. Bowen of Johns Hopkins University and Maciej Gutowski of Heriot-Watt University, in Scotland, used experiment and theory to demonstrate that an extra electron is key to producing the ionic salt (Science 2008, 319, 936). Using anion photoelectron spectroscopy to study [(NH₃)(HCl)]^- in the gas phase, the researchers observed spectral features indicating proton shuttling within the complex. They hypothesize that electron attachment to NH₃···HCl induces intermolecular proton transfer to yield (NH₄⁺Cl^-)^-. Computational analysis suggests that the additional electron localizes to the NH₃ end of the NH₃···HCl complex, providing an extra negative charge that facilitates proton transfer from HCl to NH₃. The process may be a prototype of electron-induced proton transfer in other systems, the authors say.