Sublimation of Ammonium Salts: A Mechanism Revealed by a First-Principles Study of the NH4Cl System

Abstract

The mechanism for the sublimation of ammonium salts in general is not well-elucidated; the relationship between sublimation energies and activation energies of sublimation has not been understood. We have studied the kinetics and mechanism for the sublimation of NH4Cl, a prototype ammonium salt, by first-principles calculations using generalized gradient approximation in the plane-wave density functional theory. Supercells containing 8 to 64 NH4Cl units were used, and the predicted sublimation energy, 41.0 kcal/mol for NH4Cl(c) → NH3(g) + HCl(g), is in excellent agreement with the experimental value, 42.2 kcal/mol. The result of statistical-theory calculations indicates that the desorption of the H3N···HCl molecular complex with a 15.5 ± 1.0 kcal/mol variational barrier, instead of the individual NH3 and HCl molecules, from the relaxed crystal surface is the rate-controlling step. The desorption rate is predicted to be 25.0 exp(−13.2 kcal/mol/RT) cm/s, which is in close agreement with the experimental dat...