Resolving the amine-promoted hydrolysis mechanism of N₂O₅ under tropospheric conditions.

Abstract

Hydrolysis of N₂O₅ under tropospheric conditions plays a critical role in assessing the fate of O₃, OH, and NO_x in the atmosphere. However, its removal mechanism has not been fully understood, and little is known about the role of entropy. Herein, we propose a removal path of N₂O₅ on the water clusters/droplet with the existence of amine, which entails a low free-energy barrier of 4.46 and 3.76 kcal/mol on a water trimer and droplet, respectively, at room temperature. The free-energy barrier exhibits strong temperature dependence; a barrierless hydrolysis process of N₂O₅ at low temperature (≤150 K) is observed. By coupling constrained ab initio molecular dynamics (constrained AIMD) simulations with thermodynamic integration methods, we quantitively evaluated the entropic contributions to the free energy and compared NH₃-, methylamine (MA)-, and dimethylamine (DMA)-promoted hydrolysis of N₂O₅ on water clusters and droplet. Our results demonstrate that methylation of NH₃ stabilizes the product state and promotes hydrolysis of N₂O₅ by reducing the free-energy barriers. Furthermore, a quantitative analysis of the internal coordinate distribution of the reaction center and the relative position of surrounding species reveals that the significant entropic contribution primarily results from the ensemble effect of configurations observed in the AIMD simulations. Such an ensemble effect becomes more significant with more water molecules included. Lowering the temperature effectively minimizes the entropic contribution, making the hydrolysis more exothermic and barrierless. This study sheds light on the importance of the promoting effect of amines and the entropic effect on gas-phase hydrolysis reactions, which may have far-reaching implications in atmospheric chemistry.