| Photochemical Smog is one of the most harmful pollutions to urban atmosphere. The essence of photochemical smog is ozone pollution which is formed by atmospheric nitrogen oxides and hydrocarbons under ultraviolet ray radiation. In particular there has been a steady increase of the concentration of nitrogen oxides, principally referring to nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O). These compounds are actively involved in both the tropospheric and stratospheric chemistry and contribute to environmental problems such as photochemical smog, acid rain, global warning and particularly, stratospheric ozone depletion. NOx (NOx=NO+NO2) are generated at the earth's surface by combustion and biological processes. Therefore, human activities have a direct impact on the annual emissions of NOx. An important source for NOx emission during combustion is the oxidation of nitrogen-containing compounds. Among them, the chemistry of organic-nitro compounds, especially peroxyacetyl nitrate, play the significant role in propellent ignition, combustion and air pollution.Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is formed in atmosphere in the oxidative degradation of many organic compounds of both anthropogenic and biogenic origin. It was first identified by Stephens et al. during smog episodes in the Los Angeles basin in the 1950s. It is an important oxidant component of summer smog and can cause eye irritation and plant damage. Moreover, PAN plays an important role in the transport and recycling of NOx in the troposphere. And also in the clean troposphere it is one of the most abundant reactive nitrogen-containing species PAN forms in urban areas. Because of the central role in the chemistry of the atmosphere, the research on understanding the properties and structures of the PAN molecule, as well as further investigation to its decomposition, have attracted great interest to date in both experimental and theoretical aspects. Information on their dissociation mechanism and kinetics is critical for understanding their extremely complex reactions in the atmosphere.Presently, the generation and global distribution of PAN in the atmosphere is well understood. Also its spectroscopic and photochemical properties have been well characterized and several papers have been published about its thermal decomposition。Although there are some reports on the unimolecular dissociation pathways of PAN, to the best of our knowledge, however, few people has studied the mechanism for the small molecular clusters-mediated decomposition of peroxyacetyl nitrate (PAN). This work mainly investigated the mechanism of the decomposition of Peroxyacetyl Nitrate (CH3C(O)OONO2) in small molecular clusters containing one to three water, hydrogen fluoride and hydrogen chloride molecules at the B3LYP/6-311++G(d,p) and B3LYP/6-311+G(3df,3pd) levels. We report the transition state, energetics and minimum-energy pathways (MEP) for PAN with multiple molecules in the gas phase. Based on our calculated results, the possible decomposition mechanisms of PAN influenced by small molecular clusters are proposed.1. DFT and ab initio calculations have been used to study the mechanism of the hydrolysis of Peroxyacetyl Nitrate at the O-N bond (PAN, CH3C(O)OONO2) in neutral water clusters containing one to three solvating water molecules. We report the geometries, energetics and vibrational frequencies of the reactants, intermediates and transition states for PAN with multiple waters in the gas phase. We also examine relative energies for key stationary points on the potential energy surface for the addition of multiple waters to PAN. When the number of water molecules that make up the chain is increased, the energy barrier decreases. As the size of the water cluster is increased, PAN shows increasing ionization along the O-N bond, consistent with the proposed predissociation in which the electrophilicity of the nitrogen atom is enhanced. This reaction is found to proceed through an attack of a water oxygen to the PAN nitrogen in concert with a proton transfer to a PAN oxygen atom. Based on the trends in calculated activation energies, it can be concluded that the neutral hydrolysis of PAN is likely to proceed via a cooperative and concerted mechanism involving active participation of three additional water molecules in the gas phase. For comparison with previous studies, the energy barrier of PAN hydrolysis at C-O bond and the bond dissociation energy at O-N bond are also calculated at the same level. Our results suggest that in addition to the above two decomposition pathways which have been reported by the literature, the last dissociation pathway via hydrolysis at the N–O bond fission is feasible.2. Density functional theory has been used to study the mechanism of the decomposition of Peroxyacetyl Nitrate (CH3C(O)OONO2) in hydrogen fluoride clusters containing one to three hydrogen fluoride molecules at the B3LYP/6-311++G(d,p) and B3LYP/6-311+G(3df,3pd) levels. The calculations clarify some of the uncertainties in the mechanism of PAN decomposition in the gas phase. The energy barrier decreases from 30.5 kcal mol-1 (single hydrogen fluoride) to essentially 18.5 kcal mol-1 when catalyzed by three hydrogen fluoride molecules. As the size of the hydrogen fluoride cluster is increased, PAN shows increasing ionization along the O–N bond, consistent with the proposed predissociation in which the electrophilicity of the nitrogen atom is enhanced. This reaction is found to proceed through an attack of a fluorin to the PAN nitrogen in concert with a proton transfer to a PAN oxygen. Based on the trends in calculated activation energies, it can be concluded that the decomposition of PAN is likely to proceed via a cooperative and concerted mechanism involving active participation of three additional hydrogen fluoride molecules in the gas phase. Comparing with the thermal decomposition of PAN occurring via the nitrate O–N bond cleavage, our results suggest that in addition to the thermal decomposition pathways which have been reported by the literature, the pathway that PAN decomposition occurs via two or three hydrogen fluoride molecules is feasible.3. Density functional theory has been used to study the mechanism of the decomposition of Peroxyacetyl Nitrate (CH3C(O)OONO2) in hydrogen chloride clusters containing one to three hydrogen chloride molecules at the B3LYP/6-311++G(d,p) level. Decomposition of PAN at the N–O bond in the clusters containing one-, two-, and three- hydrogen chloride molecules via barriers of 24.3, 23.4 and 26.3 kcal mol-1, respectively. The energy barriers first decrease then become to high. This reaction is found to proceed through an attack of a chloride to the PAN nitrogen in concert with a proton transfer to a PAN oxygen. Based on the trends in calculated activation energies, it can be concluded that the decomposition of PAN is likely to proceed via a cooperative and concerted mechanism. Comparing with the thermal decomposition of PAN occurring via the nitrate O–N bond cleavage, our results suggest that the gas-phase decomposition of peroxyacetyl nitrate mediated by the hydrogen chloride at the O-N bond would not be competitive with the gas-phase homolysis of PAN. |
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