S_N2 Reactions of N₂O₅ with Ions in Water: Microscopic Mechanisms, Intermediates, and Products

Abstract

Reactions of dinitrogen pentoxide (N2O5) greatly affect the concentrations of NO3, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N2O5 with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl–, SO4 2–, ClO4 –, and RCOO– (R = H, CH3, C2H5). The following main results are obtained: (i) all the reactions take place by an SN2-type mechanism, with a transition state that involves a contact ion pair (NO2 +NO3 –) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO2 +) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO2 formation from N2O5 reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO4NO2 –) and RCOONO2, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.

My Trajectory in Molecular Reaction Dynamics and Spectroscopy

“…Experiments by the Bertram and Nathanson groups showed that the presence of sulfates and carboxylate ions in seawater suppresses the formation of ClNO 2 , which is produced by a substitution reaction of Cl − with N 2 O 5 . Model calculations by Natalia Karimova, a postdoc with me at UC Irvine, provided understanding of the experimental findings and in addition provided important insights and new predictions for the reactions of N 2 O 5 with ions in water media (137). The model that Karimova developed has N 2 O 5 initially at the surface of the hydrated ion, the latter consisting of 12 water molecules.…”

“…A prereaction complex is formed in all cases and is substantially distorted in structure compared with that of the nonhydrated ion (Figure 6). Transition state calculations and AIMD simulations were pursued (137). We expect the results and the model to also be relevant to the reactions of ions in water.…”

Insights Into the Uptake of N2O5 by Aqueous Aerosol Using Chemically Accurate Many-Body Potentials

“…5,6 Despite significant study, basic questions remain concerning the mechanism of N 2 O 5 uptake. [7][8][9][10][11][12][13] Molecular dynamics simulations can be used to obtain a molecular perspective on gaseous uptake, free of underlying rate limitation assumptions. 14 However, studying such processes theoretically imposes challenges, since uptake coefficients are exponentially sensitive to free energy differences and the simulations involve large systems and long times to model the complex dynamics.…”

“…18 Further, the branching ratios to alternative less soluble products like to ClNO 2 in the presence of NaCl have been well studied. 11,74 By quantifying the changes to both the thermodynamic and kinetic properties of N 2 O 5 in the presence of these alternative solutions, advanced molecular models such as the ones used in this work in combination with similar analysis of generalized reaction-diffusion equations incorporating alternative loss mechanics, can be exploited to provide a complete picture of reactive uptake of N 2 O 5 with the full complexity of field measurements.…”

Elucidating the mechanism of reactive uptake of N$_2$O$_5$ in aqueous aerosol

“…Ab initio molecular dynamics has been used to study hydrolysis and halide substitution reactions of N 2 O 5 in water clusters. [20][21][22][23][24] However, it is not typically feasible to study systems large enough to represent inhomogeneous systems or to evolve systems long enough to study rare events. To overcome these limitations, we have employed machine learning techniques to fit a high dimensional reactive potential to ab initio training data.…”