Sublimation of ammonium salts: A mechanism revealed by a first-principles study of the NH₄Cl system

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 H ₃N⋯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 data. This is the first time the sublimation of an ammonium salt has been successfully modeled quantum mechanically. The result supports the heretofore unexplained experimental finding that the activation energy for the sublimation process is significantly lower than the enthalpic change and that the molecular complex of NH₃ and HCl desorbs concurrently as a pair. In addition, the desorption energies of HCl, NH ₃, and H₃N⋯HCl from the neat and relaxed NH ₄Cl surfaces have been predicted.