The asymmetric dimerization of nitrogen dioxide

Phys. Chem. Chem. Phys.

Gerber

2015

Nitryl chloride (ClNO2) and nitrosyl chloride (ClNO) are potential sources of highly reactive atmospheric chlorine atoms, hence of much interest, but their formation pathways are unknown. This work predicts production of these nitrogen oxychlorides from ab initio molecular dynamics (AIMD) simulations of N2O5 or an NO2 dimer on the surface of a thin film of water which is struck by gaseous HCl. Both of these heterogeneous reactions proceed at the liquid/vapor interface by an SN2 mechanism where the nucleophile is chloride ion formed from the ionization of HCl on the aqueous surface. The film of water enhances the otherwise very slow gas phase reaction to occur by (1) stabilizing and localizing the adsorbed N2O5 or NO2 dimer so it is physically accessible for reaction, (2) ionizing the impinging HCl, and (3) activating the adsorbed oxide for nucleophilic attack by chloride. Though both nitrogen oxychloride products are produced by SN2 reactions, the N2O5 mechanism is unusual in that the electrophilic N atom to be attacked oscillates between the two normally equivalent NO2 groups. Chloride ion is found to react with N2O5 less efficiently than with N2O4. The simulations provide an explanation for this. These substitution/elimination mechanisms are new for NOx/y chemistry on thin water films and cannot be derived from small cluster models.

Section: Models and Methodologymentioning

confidence: 99%

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

Hammerich

Phys. Chem. Chem. Phys.

Gerber

2015

“…[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.…”

mentioning

confidence: 99%

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

Varner

Phys. Chem. Chem. Phys.

Gerber

2014

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...

“…160,196,236 As described above, theory predicts the formation and chemistry of intermediates such as the asymmetric N 2 O 4 dimer and NO + NO 3 À . [164][165][166][167][168][169] It is likely that similar intermediates are involved at ice interfaces.…”

mentioning

confidence: 99%

“…[164][165][166][167][168][169] In the troposphere there are widely varying amounts of water vapor, which typically form different structures on surfaces, ranging from islands at low relative humidity (RH) to multi-layer lms at high RH. 170 It is likely that the details of the water structure affect its interactions with gases such as NO 2 , or the dimer N 2 O 4 , as well as the nature of the surface-bound species.…”

mentioning

confidence: 99%

Introductory lecture: atmospheric chemistry in the Anthropocene

2017

Faraday Discuss.

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.