Computational Studies of Atmospherically-Relevant Chemical Reactions in Water Clusters and on Liquid Water and Ice Surfaces

Abstract

CONSPECTUS: Reactions on water and ice surfaces and in other aqueous media are ubiquitous in the atmosphere, but the microscopic mechanisms of most of these processes are as yet unknown. This Account examines recent progress in atomistic simulations of such reactions and the insights provided into mechanisms and interpretation of experiments. Illustrative examples are discussed. The main computational approaches employed are classical trajectory simulations using interaction potentials derived from quantum chemical methods. This comprises both ab initio molecular dynamics (AIMD) and semiempirical molecular dynamics (SEMD), the latter referring to semiempirical quantum chemical methods. Presented examples are as follows: (i) Reaction of the (NO(+))(NO3(-)) ion pair with a water cluster to produce the atmospherically important HONO and HNO3. The simulations show that a cluster with four water molecules describes the reaction. This provides a hydrogen-bonding network supporting the transition state. The reaction is triggered by thermal structural fluctuations, and ultrafast changes in atomic partial charges play a key role. This is an example where a reaction in a small cluster can provide a model for a corresponding bulk process. The results support the proposed mechanism for production of HONO by hydrolysis of NO2 (N2O4). (ii) The reactions of gaseous HCl with N2O4 and N2O5 on liquid water surfaces. Ionization of HCl at the water/air interface is followed by nucleophilic attack of Cl(-) on N2O4 or N2O5. Both reactions proceed by an SN2 mechanism. The products are ClNO and ClNO2, precursors of atmospheric atomic chlorine. Because this mechanism cannot result from a cluster too small for HCl ionization, an extended water film model was simulated. The results explain ClNO formation experiments. Predicted ClNO2 formation is less efficient. (iii) Ionization of acids at ice surfaces. No ionization is found on ideal crystalline surfaces, but the process is efficient on isolated defects where it involves formation of H3O(+)-acid anion contact ion pairs. This behavior is found in simulations of a model of the ice quasi-liquid layer corresponding to large defect concentrations in crystalline ice. The results are in accord with experiments. (iv) Ionization of acids on wet quartz. A monolayer of water on hydroxylated silica is ordered even at room temperature, but the surface lattice constant differs significantly from that of crystalline ice. The ionization processes of HCl and H2SO4 are of high yield and occur in a few picoseconds. The results are in accord with experimental spectroscopy. (v) Photochemical reactions on water and ice. These simulations require excited state quantum chemical methods. The electronic absorption spectrum of methyl hydroperoxide adsorbed on a large ice cluster is strongly blue-shifted relative to the isolated molecule. The measured and calculated adsorption band low-frequency tails are in agreement. A simple model of photodynamics assumes prompt electronic relaxation of the excited peroxide due t...

Mechanism for formation of atmospheric Cl atom precursors in the reaction of dinitrogen oxides with HCl/Cl−on aqueous films

“…[50][51][52][53][54][55][56] For AIMD simulations modeling a macroscopic 58 When examined with a higher level of theory with optimized geometries, an MP2/SOS-RIMP2/aSVP AIMD study with four waters exhibited a maximum charge separation, obtained via natural population analysis, between the NO and NO 3 moieties of 1.5 a.u. 60 A dynamic B3LYP/6-31G study based on gradients and Hessians indicated that clusters of symmetric N 2 O 4 with 0-7 H 2 O isomerized to the asymmetric ONONO 2 isomer in water assisted as well as non-water assisted pathways and suggested the participation of NO + NO 3 À . expected for an ion pair.…”

“…Solvent mitigated femtosecond transfer processes reduce the charge to BÀ0.25 a.u. 60 But in contrast to dinitrogen pentoxide, the charge on each fragment is rather constant (except for the vibrational motions) until reaction at 8.7 ps. As the hydronium ion migrates away via proton hopping and the aqueous solvation shell establishes itself, the charge slowly decreases further until solvated Cl À reaches a charge of À0.72 a.u.…”

Dark air–liquid interfacial chemistry of glyoxal and hydrogen peroxide

“…58 Water plays a vital role in the molecular arrangement at the aqueous surface to advance interfacial reactions. 59,60 Additional 2D PCA results are depicted in Supplementary Fig. 17.…”

Resolving the HONO formation mechanism in the ionosphere via ab initio molecular dynamic simulations

“…The D layer is the innermost layer of the ionosphere, ranging from 60 to 90 km in altitude, where Lyman series-α hydrogen radiation from the Sun gives rise to abundant nitrosonium ions (NO + ). In addition to the ionospheric reaction between NO + and water, explorations of the chemical reactivity of NO + and water clusters (2-4) have implications for understanding the mechanisms of atmospherically relevant reactions in water clusters (5)(6)(7)(8)(9).…”