First-principles study of water effect on the sublimation of ammonium perchlorate

The kinetics and mechanism for the sublimation/decomposition of ammonium perchlorate (AP) with and without water present on the decomposing surface have been investigated by first-principles calculations, using a generalized gradient approximation with the plane-wave density functional theory. Calculated results show that H₂O can enhance the sublimation of AP; the sublimation energies with (H₂O)_n (n = 0, 1, 2, 3) from the surface were predicted to be 28.1, 21.4, 18.6, and 14.2 kcal/mol, respectively. Notably, H₂O was found not to affect the proton transfer between the NH₄ ⁺/ClO₄ ^- ion pair, but was found to significantly enhance the sublimation of AP by co-desorbing with the ion pair. The rate constants for the dominant sublimation processes, the desorption of the molecular complex, H₃N· · ·HOClO₃, and the codesorption of H₂O with AP, (H₂O)_n· · ·NH₄ClO₄ (n = 1, 2), predicted by canonical variational transition state theory can be presented by k_des = 6.53 × 10¹² exp (-28.8 kcal/mol/RT) (n = 0) s^-1, 1.69 × 10¹⁰ exp (-20.3 kcal/mol/RT) (n = 1) s^-1, and 1.08 × 10¹¹ exp (-17.7 kcal/mol/RT) (n = 2) s^-1, respectively, with a significant enhancement by the H₂O molecules. Interestingly, the structures of AP in the water complexes are more ionic than those without H₂O in the gas phase. In addition, the energy changes for proton transfer on the surface have been compared with those in solution. The calculated proton transfer energies on the crystalline AP surface with (H₂O)_n (n = 0, 1, 2), 31.1, 29.7, and 32.5 kcal/mol, were found to be close to the corresponding values of 30.8, 30.5, and 30.4 kcal/mol calculated by the (H₂O)_n-NH₄ClO₄ complexes in solution using the polarizable continuum model. These calculated proton transfer energies on the surface and in solution are close to the experimental values inside AP solid, 26-31 kcal/mol.