Proton transfer reaction of hydrogen chloride with ammonia: is it possible in the gas phase?

The mechanism of proton transfer between HCl and NH3 and the effects of individual water molecules on the reaction are investigated by calculating the structures and energetics of a series of molecule clusters HCl–NH3–(H2O)n (n = 0, 1, 2, 3) using high level ab initio theory. Without water, the system exists as a simple hydrogen-bonded complex and no proton transfer occurs from HCl to NH3. The first water molecule was found to induce a nearly flat potential energy pathway for proton transfer, but at least two water molecules must be involved for complete proton transfer from HCl to NH3. The study supports the likelihood of gas phase proton transfer in HCl–NH3 in the presence of water vapor.