Reaction of a charge-separated ONONO2 species with water in the formation of HONO: an MP2 Molecular Dynamics study

Abstract

À) with water is modelled in ONONO 2 Á (H 2 O) 4 clusters. Molecular Dynamics simulations using second-order Møller-Plesset perturbation (MP2) theory support the feasibility of the reaction of a charge-separated species to produce HONO and nitric acid.It is well known that nitrous acid (HONO) plays a major role in the chemistry of the atmosphere, e.g. as a source of hydroxyl (OH) radicals. 1,2 While heterogeneous reactions of NO 2 with water on tropospheric surfaces are believed to be important in generating HONO, the actual mechanisms involved are not known with certainty. On a chemical basis, the involvement of the NO 2 dimer seems reasonable, 3 although other mechanisms have been proposed for production of HONO both in the dark and under irradiation. [4][5][6][7][8][9][10][11] Initial steps of one proposed mechanism 3 include NO 2 dimerization to form N 2 O 4 and autoionization of asymmetric N 2 O 4 , i.e. ONONO 2 , to form (NO + )(NO 3 À ).ONONO 2 -(NO + )(NO 3 À )Reaction of the (NO + )(NO 3 À ) ion pair with water to form HONO and nitric acid (HONO 2 ) is the final step. A number of studies have addressed the formation of NO 2 dimers in both the symmetric (with a central NN bond) and asymmetric (ONONO 2 ) forms, and the interconversion between the two. [12][13][14][15][16][17][18] While the symmetric form of the dimer is favoured, there are pathways that can generate the asymmetric trans-ONONO 2 form that is the most likely precursor to the ion pair and subsequently HONO. 12,13,15,16,18 de Jesus Medeiros and Pimentel, in a study of the dimerization of NO 2 and the symmetric to asymmetric isomerization of the dimer in water clusters, predicted direct HONO formation from the NO 2 Á(H 2 O) n + NO 2 reaction and from the transition state for symmetric to asymmetric isomerization. 15 These modifications of the proposed mechanism described above bypass formation of (NO + )(NO 3 À ) through autoionization or, in the case of the NO 2 Á(H 2 O) n + NO 2 reaction, formation of the NO 2 dimer. Further exploration of the (NO + )(NO 3 À ) path will allow for comparison between potential routes and provide additional insight into the detailed mechanism that converts NO 2 in the presence of water to HONO. This is particularly useful for understanding other important reactions of NO 2 on surfaces in the presence of water, such as the reaction with HCl to form ClNO, since such chemistry likely shares common intermediates with NO 2 hydrolysis. [19][20][21] In ab initio Molecular Dynamics [AIMD], the reaction trajectory is propagated classically on the ab initio potential surface providing a means for studying reactive processes and the influence of the immediate environment. 22 The system wave function or electron density is calculated at each time step, i.e. on-the-fly, for determination of the energy, the interatomic forces and other properties. Partial charges are an obvious metric to investigate charge separation in the reaction of an ion pair as charge characteristics can indicate a shift in the nature of the interactio...

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

“…61 Clusters of ONONO 2 , HCl, and 0-2 waters in DRP MP2/cc-pVDZ simulations yielded a mechanism where hydrogen bonded water forms a proton wire for H + transfer from HCl to water to the NO 3 moiety forming HNO 3 and nitrosyl chloride. A few studies have addressed the effect of water by examining the dinitrogen oxide in a small water cluster 58,59,61,[84][85][86][87][88] or by trying to incorporate the effect of bulk water. 63 Many electronic structure studies have been performed on N 2 O 5 and N 2 O 4 64-83 providing information on equilibrium geometries, electronic and thermodynamic parameters, and spectroscopic properties.…”

Synergistic Reaction of SO2 with NO2 in Presence of H2O and NH3: A Potential Source of Sulfate Aerosol

“…For NO 2 dimer, three isomers, symmetric sys-O 2 N-NO 2 , and asymmetric t-ONONO 2 , c-ONONO 2 , can be found. The sys-O 2 N-NO 2 is very stable due to its symmetry, while asymmetric ONO-NO 2 can be rapidly converted to NO + and NO 3 − in the presence of water [31,32,33,34]. The ion pairs (NO + NO 3 − ) can also oxidize many organic and inorganic compounds [35].…”

Ab initio and semi-empirical Molecular Dynamics simulations of chemical reactions in isolated molecules and in clusters

“…However, additional calculations were carried out to guide selection of an appropriate method for simulations described below, it was found that a double-zeta basis set with diffuse functions, as well as MP2 with scaling of the opposite-spin component (SOS-MP2), were required for a qualitatively correct and more accurate description of the relevant portion of the ONONO 2 Á(H 2 O) 4 potential surface. 45 to examine the system in a state with sufficient energy to overcome the reaction barrier, but without large excess kinetic energy. The velocities were propagated with a 20 au timestep using the Leapfrog Verlet algorithm as implemented in the Turbomole program package.…”