The mechanism for the gas-phase production of particulate ammonium halides is studied by density functional theory and ab initio calculations on the molecular clusters of ammonia-hydrogen halide, ͑NH 3 -HX) n , nϭ1, 2, 4; XϭF, Cl, Br. The equilibrium structures of the clusters demonstrate an abrupt transition from hydrogen-bonding in the single unit of NH 3 -HX to the NH 4 ϩ¯XϪ ion-pair interactions in ͑NH 3 -HX) 4 resulting from spontaneous proton transfer from hydrogen halide to ammonia. The

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... nset of the ion-pair form is at nϭ2 for HCl and HBr and at nϭ4 for HF. The ion-pair structure is the stable form of the larger clusters to maximize favorable electrostatic interactions and to closely resemble the crystal structures of ammonium halides. The calculated vibrational frequencies of the clusters provide additional evidence in support of the transition from hydrogen-bonding to ion-pair interactions as cluster size increases. The study shows that it is possible to form particulate ammonium halides directly from pure NH 3 and HX vapors in the homogeneous gas phase.