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

R. S. Zhu, J. H. Wang, Ming-Chang Lin*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

41 Scopus citations


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.

Original languageEnglish
Pages (from-to)13831-13838
Number of pages8
JournalJournal of Physical Chemistry C
Issue number37
StatePublished - 20 Sep 2007


Dive into the research topics of 'Sublimation of ammonium salts: A mechanism revealed by a first-principles study of the NH4Cl system'. Together they form a unique fingerprint.

Cite this