Issue Date: February 18, 2008
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.
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