Direct formation of solid ammonium chloride particles from HCl and NH3 vapors

The possibility of direct formation of particulate ammonium chloride (NH4Cl) from ammonia (NH3) and hydrogen chloride (HCl) gases was investigated by density functional theory and ab initio methods. Equilibrium structures were determined for the clusters constructed, respectively, from one, two, and four units of NH3–HCl in the close-packed form. While a single NH3–HCl unit is a hydrogen-bonded complex, clusters of two and four NH3–HCl units closely resemble the crystal structure of NH4Cl and contain the discrete NH4+ and Cl− ions. The ionic form results from a prompt and complete proton transfer from HCl to NH3 that takes place in the larger clusters where favorable electrostatic interactions are maximized. The implications for nucleation in the pure NH3 and HCl system are discussed.