N₂O₅at water surfaces: binding forces, charge separation, energy accommodation and atmospheric implications

Abstract

Interactions of N2O5 with water media are of great importance in atmospheric chemistry and have been the topic of extensive research for over two decades. Nevertheless, many physical and chemical properties of N2O5 at the surface or in bulk water are unknown or not microscopically understood. This study presents extensive new results on the physical properties of N2O5 in water and at the surface of water, with a focus on their microscopic basis. The main results are obtained using ab initio molecular dynamics and calculations of a potential of mean force. These include: (1) collisions of N2O5 with water at 300 K lead to trapping at the surface for at least 20 ps and with 95% probability. (2) During that time, there is no N2O5 hydrolysis, evaporation, or entry into the bulk. (3) Charge separation between the NO2 and NO3 groups of N2O5, fluctuates significantly with time. (4) Energy accommodation of the colliding N2O5 at the surface takes place within picoseconds. (5) The binding energy of N2O5 to a nanosize amorphous ice particle at 0 K is on the order of 15 kcal mol-1 for the main surface site. N2O5 binding to the cluster is due to one weak hydrogen bond and to interactions between partial charges on the N2O5 and on water. (6) The free-energy profile was calculated for transporting N2O5 from the gas phase through the interface and into bulk water. The corresponding concentration profile exhibits a propensity for N2O5 at the aqueous surface. The free energy barrier for entry from the surface into the bulk was determined to be 1.8 kcal mol-1. These findings are used to interpret recent experiments. We conclude with implications of this study for atmospheric chemistry.

Highly Accurate Many-Body Potentials for Simulations of N2O5 in Water: Benchmarks, Development, and Validation

“…92 For the geometry optimizations, a total of 400 dimers and 400 trimers were extracted from ab initio MD simulations of N2O5 in bulk water and N2O5 at a water liquid/vapor interface performed in Ref. 5 . Only the unique minima, as determined by a root-mean-square distance with a tolerance of 0.1 Å, were considered and submitted to frequency calculations.…”

“…100 For the benchmarks illustrated in Figure 1, 10 monomers, 10 dimers, and 10 trimers have been selected from the ab initio MD simulations performed in Ref. 5 . A limited number of structures was considered due to the substantial computational cost associated with the FNO-DF-CCSD(T) calculations using the larger basis sets.…”

“…1 Because of its importance, N2O5 has been extensively studied over the past few decades, both theoretically and experimentally. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Notwithstanding all these efforts, the mechanism of reactive uptake of N2O5 to aqueous aerosol is still not fully understood at a molecular level. The initial step of this process is the physical accommodation of N2O5 in aqueous aerosol.…”

“…Classical force fields, which are widely used in computer simulations because of their computational efficiency, have been employed in molecular dynamics (MD) simulations to study the hydration properties of N2O5. 5,23 However, due to their inherent approximations and limited accuracy, classical force fields might not be relied upon for quantitative predictions. In ab initio MD (AIMD), the configurations of a system are explored at a given thermodynamic condition with energies and forces computed on-the-fly from first-principles electronic structure calculations 24,25 .…”

“…AIMD simulations, accessing limited time scales, have been employed to explore the physical accommodation of N2O5 to bulk water. 5 While AIMD can be directly used without the need for any prior parametrizations, condensed phase simulations are typically restricted to density functional theory (DFT), which has limited accuracy and displays difficulties in correctly describing the structure of liquid water. 26 In addition, the high computational cost associated with AIMD precludes sampling over long time scales, especially for systems that contain a large number of atoms.…”