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.
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 to the ice surface. SEMD simulations support this, with the important finding that the photochemistry takes place mainly on the ground state. In conclusion, dynamics simulations using quantum chemical potentials are a useful tool in atmospheric chemistry of water media, capable of comparison with experiment.
