Ionization of N2O4in Contact with Water: Mechanism, Time Scales and Atmospheric Implications

Water-Catalyzed Formation of Reactive Oxygen Species from NO2 on a Weakly Hydrated Calcite Surface

The interfaces of weakly hydrated mineral substrates have been shown to serve as catalytic sites for chemical reactions that may not be accessible in the gas phase or under bulk conditions. Currently known mechanisms for the formation of reactive oxygen species (ROS) from nitrogen dioxide (NO2) involve NO2 dimerization. Here, we report the formation of the ROS HONO via a mechanism involving simple adsorption of a single NO2 molecule on a weakly hydrated calcite substrate. First-principles molecular dynamics simulations coupled with enhanced sampling techniques show how an adsorbed water sublayer can enhance NO2 adsorption on calcite compared to adsorption on a bare dry substrate. On the weakly hydrated calcite surface, an interfacial electric field facilitates proton extraction from water, thus allowing HONO formation from a single adsorbed NO2, i.e., without the need for the formation of a NO2 dimer precomplex. HONO formation on calcite is kinetically more favorable than that in the gas phase, with a reaction barrier of 14 kcal/mol on the weakly hydrated calcite surface compared to 27 kcal/mol in the gas phase. Further photocatalysed HONO production by visible light and HONO dissociation are hampered on calcite, unlike the process on silica. NO2 is a significant anthropogenic pollutant, and understanding its chemistry is crucial for explaining the high ROS levels and haze formation in polluted areas or prebiotic ROS generation. These findings emphasize how mineral substrates under water-restricted hydration conditions can trigger chemical pathways that are unexpected in the gas phase or under bulk conditions.

Phase State Regulates Photochemical HONO Production from NaNO3/Dicarboxylic Acid Mixtures

Field observations of daytime HONO source strengths have not been well explained by laboratory measurements and model predictions up until now. More efforts are urgently needed to fill the knowledge gaps concerning how environmental factors, especially relative humidity (RH), affect particulate nitrate photolysis. In this work, two critical attributes for atmospheric particles, i.e., phase state and bulk-phase acidity, both influenced by ambient RH, were focused to illuminate the key regulators for reactive nitrogen production from typical internally mixed systems, i.e., NaNO3 and dicarboxylic acid (DCA) mixtures. The dissolution of only few oxalic acid (OA) crystals resulted in a remarkable 50-fold increase in HONO production compared to pure nitrate photolysis at 85% RH. Furthermore, the HONO production rates (PHONO) increased by about 1 order of magnitude as RH rose from <5% to 95%, initially exhibiting an almost linear dependence on the amount of surface absorbed water and subsequently showing a substantial increase in PHONO once nitrate deliquescence occurred at approximately 75% RH. NaNO3/malonic acid (MA) and NaNO3/succinic acid (SA) mixtures exhibited similar phase state effects on the photochemical HONO production. These results offer a new perspective on how aerosol physicochemical properties influence particulate nitrate photolysis in the atmosphere.

ReaxFF-Based Molecular Dynamics Study of the Mechanism of the Reaction of N2O4 with H2O

During long-term storage of the liquid propellant N2O4, it absorbs H2O to form the N2O4(H2O)n system, and this in turn generates HNO3, HNO2, and other substances in the storage tank because of corrosion, which seriously affects the performance of weaponry. In this work, we carried out computational simulations of N2O4 with different masses of water based on ReaxFF, analyzed the reaction intermediates and products, and investigated the mechanism of the reaction of N2O4 with H2O and of N2O4(H2O)n. The results show that the reaction product ω(HNO3+HNO2) undergoes a rapid growth in the early stage of the reaction and then tends toward dynamic equilibrium; the potential energy of the system decreases with the increase of ω(H2O), the reaction rate increases, and the rate of decomposition of HNO2 to form HNO3 increases. When ω(H2O) is 0.2 or 1.0%, the intermediate products are N2O4H2O or N2O4(H2O)2, respectively, and the reaction proceeds along two paths; when ω(H2O) ≥ 2.0%, N2O4(H2O)3 appears as the intermediate product, HNO3 and HNO2 are directly produced in one step, and a stable current loop can be formed within the whole system.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., -Si-OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

Isomerization and reaction process of N2O4(H2O) n

Liquid propellant N2O4 is prone to absorb H2O to form an N2O4(H2O) n system during long-term storage, ultimately generating HNO3, HNO2, and other substances capable of corroding the storage tank, which will adversely affect the performance of weapons and equipment. In this work, the reaction process of the N2O4(H2O) n system is simulated using density functional theory, and the potential energy surface, the geometric configurations of the molecules, the charge distribution, and the bond parameters of the reaction course at n = 0-3 are analyzed. The results show that the potential energy of the system is lower and the structure is more stable when the H2O in the N2O4(H2O) n system is distributed on the same side. When n = 1 or 2, the reaction profiles are similar, and the systems are partly ionic, although still mainly covalently bonded. When n = 3, the charge on the trans-ONONO2 group and the ON-ONO2 bond length change abruptly to -0.503 a.u. and 2.57 Å, respectively, at which point the system is dominated by ionic bonds. At n = 2, a proton-transfer phenomenon occurs in the reaction course, with partial reverse charge-transfer from NO3 - to NO+, making the ON-ONO2 bond less susceptible to cleavage, further verifying that N2O4(H2O) n tends to afford the products directly in one step as H2O accumulates in the system.

Preparation and Characterization of Metastable trans-Dinitrogen Tetroxide

Under atmospheric conditions, NO₂ is in equilibrium with its dimers, N₂O₄, which can exist in the form of constitutional isomers and stereoisomers whose relative stabilities and reactivities are still being debated. Experimental limitations facing the spectroscopic characterization of the isomers of N₂O₄ prevent us from determining their relative contributions to reaction mechanisms possibly causing discrepancies in the reported reaction orders and rates. Using reflection-absorption infrared spectroscopy, molecular beam deposition, and matrix isolation techniques, it is shown that the relative abundances of NO₂ and its dimers can be controlled by heating or cooling the deposited gas. The comparison of spectra acquired from samples prepared using molecular beam deposition with those obtained using tube dosing deposition demonstrates how the N₂O₄ isomer distributions are sensitive to details of the experimental conditions and sample preparation protocols. These observations not only provide a better understanding of a possible source for the disagreements found in the literature, but also a methodology to control and quantify the chemical speciation in NO₂ vapors in terms of the relative abundances of NO₂ and of the various isomers of N₂O₄.

Metal-free catalysis for the reaction of nitrogen dioxide dimer with phenol: An unexpected favorable source of nitrate and aerosol precursors in vehicle exhaust

Atmospheric reaction mechanism and dynamics of phenol with nitrogen dioxide dimer were explored by the density functional theory and high-level quantum chemistry combined with statistical kinetic calculations within 220-800 K. The nitric acid and phenyl nitrite, the typical aerosol precursors, are the preponderant products because of the low formation free energy barrier (∼8.7 kcal/mol) and fast rate constants (∼10^-15 cm³ molecule^-1 s^-1 at 298 K). Phenyl nitrate is the minor product and it would be also formed from the transformation of phenyl nitrite in NO₂-rich environment. More importantly, kinetic effects and catalytic mechanism of a series of metal-free catalysts (H₂O, NH₃, CH₃NH₂, CH₃NHCH₃, HCOOH, and CH₃COOH) on the title reaction were investigated at the same level. The results indicate that CH₃NH₂ and CH₃NHCH₃ can not only catalyze the title reaction by lowering the free energy barrier (about 1.4-6.5 kcal/mol) but also facilitate the production of organic ammonium nitrate via acting as a donor-acceptor of hydrogen. Conversely, the other species are non-catalytic upon the title reaction. The stabilization energies and donor-acceptor interactions in alkali-catalyzed product complexes were explored, which can provide new insights to the properties of aerosol precursors. Moreover, the lifetime of phenol determined by nitrogen dioxide dimer in the presence of dimethylamine may compete with that of determined by OH radicals, indicating that nitrogen dioxide dimer is responsible for the elimination of phenol in the polluted atmosphere. This work could help us thoroughly understand the removal of nitrogen oxides and phenol as well as new aerosol precursor aggregation in vehicle exhaust.

Investigation of nonadiabatic dynamics in the photolysis of methyl nitrate (CH3ONO2) by on-the-fly surface hopping simulation

The photolysis mechanism of methyl nitrate (CH₃ONO₂) was studied using the on-the-fly surface hopping dynamics at the XMS-CASPT2 level. Several critical geometries, including electronic state minima and conical intersections, were obtained, which play essential roles in the nonadiabatic dynamics of CH₃ONO₂. The ultrafast nonadiabatic decay dynamics to the ground state were simulated, which gives a proper explanation on the broad and structureless absorption spectra of CH₃ONO₂. The photodissociation channels, including CH₃O + NO₂, CH₃O + NO + O, and others, as well as their branching ratios, were identified. When the dynamics starts from the lowest two electronic states (S₁ and S₂), the CH₃O + NO₂ channel is the dominant photolysis pathway, although we observed the minor contributions of other channels. In contrast, when the trajectories start from the third excited state S₃, both CH₃O + NO₂ and CH₃O + NO + O channels become important. Here the CH₃O-NO₂ bond dissociation takes place first, and then for some trajectories, the N-O bond of the NO₂ part breaks successively. The quasi-degeneracy of electronic states may exist in the dissociation limits of both CH₃O + NO₂ and CH₃O + NO + O channels. The current work provides valuable information in the understanding of experimental findings of the wavelength-dependent photolysis mechanism of CH₃ONO₂.

Reaction between a NO2 Dimer and Dissolved SO2: A New Mechanism for ONSO3- Formation and its Fate in Aerosol

Experimental observations indicate that sulfate formation in aerosol is sensitive to the concentrations of nitric oxide (NO₂). While it also widely exists as a dimer in the gas phase, previous studies focus on the monomer of NO₂. In this study, we employ quantum chemical calculations and ab initio molecular dynamics simulations to investigate the reaction between the NO₂ dimer (ONONO₂) and sulfite (HSO₃^-/SO₃^2-) in the gas phase and in an aerosol. Gas-phase reactions turn out to be barrierless. In an aerosol, the reaction between adsorbed ONONO₂ and HSO₃^- to form ONSO₃^- follows a stepwise mechanism with proton and electron transfer processes. The reaction between ONONO₂ and SO₃^2- is more straightforward. Nevertheless, both reactions occur at a picosecond time scale. Decomposition of ONSO₃^- can form an NO molecule and SO₃^-, which gives a complementary pathway for sulfate formation in an aerosol. Hydrolysis of ONSO₃^- to form HNO and HSO₄^- is highly impossible in an aerosol, which calls for a revisit of the atmospheric N₂O formation mechanism. The results presented in this study deepen our understanding of the interaction between NO₂ and SO₂ pollutants in the atmosphere.

My Trajectory in Molecular Reaction Dynamics and Spectroscopy

This is the story of a career in theoretical chemistry during a time of dramatic changes in the field due to phenomenal growth in the availability of computational power. It is likewise the story of the highly gifted graduate students and postdoctoral fellows that I was fortunate to mentor throughout my career. It includes reminiscences of the great mentors that I had and of the exciting collaborations with both experimentalists and theorists on which I built much of my research. This is an account of the developments of exciting scientific disciplines in which I was involved: vibrational spectroscopy, molecular reaction mechanisms and dynamics, e.g., in atmospheric chemistry, and the prediction of new, exotic molecules, in particular noble gas molecules. From my very first project to my current work, my career in science has brought me the excitement and fascination of research. What a wonderful pursuit!

Determination of the amine-catalyzed SO3 hydrolysis mechanism in the gas phase and at the air-water interface

New particle formation (NPF) involving amines in the atmosphere is considered an aggregation process, during which stable molecular clusters are formed from amines and sulfuric acid via hydrogen bond interaction. In this work, ab initio dynamics simulations of ammonium bisulfate formation from a series of amines, SO₃, and H₂O molecules were carried out in the gas phase and at the air-water interface. The results show that reactions between amines and hydrated SO₃ molecules in the gas phase are barrierless or nearly barrierless processes. The reaction rate is related to the basicity of gas-phase amines-the stronger the basicity, the faster the reaction. Furthermore, SO₃ hydrolysis catalyzed by amines occurs simultaneously with H₂SO₄-amine cluster formation. At the air-water interface, reactions between amines and SO₃ involve multiple water molecules. The reaction center's ring structure (amine-SO₃-nH₂O) promotes the transfer of protons in the water molecules. The formed ammonium cation (-RNH₃⁺) and the bisulfate anion (HSO₄^-) are present and stable by means of hydrogen bond interaction. The cluster formation mechanism provides new insights into NPF involving amines, which may play an important role in the formation of aerosols in some heavily polluted areas - e.g., those with a high amine concentration.

Fabrication of oxidized bacterial cellulose by nitrogen dioxide in chloroform/cyclohexane as a highly loaded drug carrier for sustained release of cisplatin

Carboxylated bacterial cellulose (OBC) was fabricated by oxidation with nitrogen dioxide in chloroform/cyclohexane and employed as a carrier for sustained release of antitumor substance cisplatin (CDDP). The influence of removing water method, solvent used in the synthesis, concentration of N₂O₄, and duration of the oxidation on content of carboxyl groups in reaction products was established. Due to the possibility of nitrogen dioxide to penetrate into cellulose crystallites, the carboxyl group content of the OBC reaches high values up to 4 mmol/g. In vitro degradation of OBC was determined under simulated physiological conditions. The immobilization of CDDP on OBC was studied in detail. The initial burst release of the drug from the polymer was depressed. The cytotoxicity of CDDP-loaded OBC was evaluated with HeLa cells. The unique structure and properties of OBC make it a great candidate as drug delivery carrier.

Effect of multifunctional compound monoethanolamine on Criegee intermediates reactions and its atmospheric implications

The reactions of Criegee intermediates with trace gases (such as alcohols, amines, and acids) are primarily dependent on the trace gases' functional group activity. In this study, we used density functional theory calculations and ab initio dynamics simulation methods to explore the synergistic effect of NH₂ and OH groups, in the multifunctional compound monoethanolamine (MEA), on the Criegee reaction. The results showed that among the four evaluated MEA configurations, two functional groups in the g'Gg' and tGg' configurations, -NH₂ and -OH, have the synergistic effect on the C2 stabilized Criegee intermediates (sCIs). At the gas-liquid interface, sCIs react with NH₂ groups of MEA molecules directly or are mediated by water molecules, resulting in additional product formation. The rate calculation indicated that the reaction of sCIs with NH₂ groups of MEA molecules is prior to that with OH groups. In addition, OH groups promote the reactions between sCIs and NH₂ groups of MEA, while the presence of NH₂ groups weakens the reactions of sCIs and OH groups of MEA to some extent. At 298 K, the total rate constant of anti-CH₃CHOO with NH₂ group of MEA is 4.26 × 10^-11 cm³ molecule^-1 s^-1, which is four orders of magnitude higher than that of anti-CH3CHOO hydration. Under low humidity conditions, the reactions between sCIs and MEA could contribute to the removal of sCIs.

A possible unaccounted source of atmospheric sulfate formation: amine-promoted hydrolysis and non-radical oxidation of sulfur dioxide

Numerous field and laboratory studies have shown that amines, especially dimethylamine (DMA), are crucial to atmospheric particulate nucleation. However, the molecular mechanism by which amines lead to atmospheric particulate formation is still not fully understood. Herein, we show that DMA molecules can also promote the conversion of atmospheric SO2 to sulfate. Based on ab initio simulations, we find that in the presence of DMA, the originally endothermic and kinetically unfavourable hydrolysis reaction between gaseous SO2 and water vapour can become both exothermic and kinetically favourable. The resulting product, bisulfite NH2(CH3)2 +·HSO3 -, can be readily oxidized by ozone under ambient conditions. Kinetic analysis suggests that the hydrolysis rate of SO2 and DMA with water vapour becomes highly competitive with and comparable to the rate of the reaction between SO2 and OH·, especially under the conditions of heavily polluted air and high humidity. We also find that the oxidants NO2 and N2O5 (whose role in sulfate formation is still under debate) appear to play a much less significant role than ozone in the aqueous oxidation reaction of SO2. The newly identified oxidation mechanism of SO2 promoted by both DMA and O3 provides another important new source of sulfate formation in the atmosphere.

High-resolution vacuum ultraviolet photodynamic of the nitrogen dioxide dimer (NO2)2 and the stability of its cation

Mass-selected TPES of the dimer N₂O₄ is recorded and its VUV photodynamics shows the dimer cation N₂O₄⁺ is unstable.

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

Effect of H₂O and NH₃ on the synergistic oxidation reaction of SO₂ and NO₂ is investigated by theoretical calculation using the molecule system SO₂-2NO₂-nH₂O (n = 0, 1, 2, 3) and SO₂-2NO₂-nH₂O-mNH₃ (n = 0, 1, 2; m = 1, 2). Calculated results show that SO₂ is oxidized to SO₃ by N₂O₄ intermediate. The additional H₂O in the systems can reduce the energy barrier of oxidation step. The increasing number of H₂O molecules in the systems enhances the effect and promotes the production of HONO. When the proportion of H₂O to NH₃ is 1:1, with NH₃ included in the system, the energy barrier is lower than two pure H₂O molecules in the oxidation step. The present study indicates that the H₂O and NH₃ have thermodynamic effects on promoting the oxidation reaction of SO₂ and NO₂, and NH₃ has a more significant role in stabilizing product complexes. In these hydrolysis reactions, nethermost barrier energy (0.29 kcal/mol) can be found in the system SO₂-2NO₂-H₂O. It is obvious that the production of HONO is energetically favorable. A new reaction mechanism about SO₂ oxidation in the atmosphere is proposed, which can provide guidance for the further study of aerosol surface reactions.

Impact of Flue Gas Compounds on Microalgae and Mechanisms for Carbon Assimilation and Utilization

To shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low-carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil-derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon-neutral products, such as biodiesel, the application of microalgae technology to capture CO₂ from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot-scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.

Photoinitiated Dynamics in Amorphous Solid Water via Nanoimprint Lithography

Laser pulses that act on fragile samples often alter them irreversibly, motivating single-pulse data collection. Amorphous solid water (ASW) is a good example. In addition, neither well-defined paths for molecules to travel through ASW nor sufficiently small samples to enable molecular dynamics modeling have been achieved. Combining nanoimprint lithography and photoinitiation overcomes these obstacles. An array of gold nanoparticles absorbs pulsed (10 ns) 532 nm radiation and converts it to heat, and doped ASW films grown at about 100 K are ejected from atop the irradiated nanoparticles into vacuum. The nanoparticles are spaced from one another by sufficient distance that each acts independently. Thus, a temporal profile of ejected material is the sum of about 10⁶ "nanoexperiments," yielding high single-pulse signal-to-noise ratios. The size of a single nanoparticle and its immediate surroundings is sufficiently small to enable modeling and simulation at the atomistic (molecular) level, which has not been feasible previously. An application to a chemical system is presented in which H/D scrambling is used to infer the presence of protons in films composed of D₂O and H₂O (each containing a small amount of HDO contaminant) upon which a small amount of NO₂ has been deposited. The pulsed laser heating of the nanoparticles promotes NO₂/N₂O₄ hydrolysis to nitric acid, whose protons enhance H/D scrambling dramatically.

Introductory lecture: atmospheric chemistry in the Anthropocene

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.

The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

Heterogeneous Reactions of Acetic Acid with Oxide Surfaces: Effects of Mineralogy and Relative Humidity

We have investigated the heterogeneous uptake of gaseous acetic acid on different oxides including γ-Al2O3, SiO2, and CaO under a range of relative humidity conditions. Under dry conditions, the uptake of acetic acid leads to the formation of both acetate and molecularly adsorbed acetic acid on γ-Al2O3 and CaO and only molecularly adsorbed acetic acid on SiO2. More importantly, under the conditions of this study, dimers are the major form for molecularly adsorbed acetic acid on all three particle surfaces investigated, even at low acetic acid pressures under which monomers are the dominant species in the gas phase. We have also determined saturation surface coverages for acetic acid adsorption on these three oxides under dry conditions as well as Langmuir adsorption constants in some cases. Kinetic analysis shows that the reaction rate of acetic acid increases by a factor of 3-5 for γ-Al2O3 when relative humidity increases from 0% to 15%, whereas for SiO2 particles, acetic acid and water are found to compete for surface adsorption sites.

Adsorption and transformation mechanism of NO2 on NaCl(100) surface: A density functional theory study

To understand the heterogeneous reactions between NO2 and sea salt particles in the atmosphere of coastal areas, the absorption of an NO2 molecule on the NaCl(100) surface, the dimerization of NO2 molecules and the hydrolysis of N2O4 isomers at the (100) surface of NaCl are investigated by density functional theory. Calculated results show that the most favorable adsorption geometry of isolated NO2 molecule is found to reside at the bridge site (II-1) with the adsorption energy of -14.85 kcal/mol. At the surface of NaCl(100), three closed-shell dimers can be identified as sym-O2N-NO2, cis-ONO-NO2 and trans-ONO-NO2. The reactions of H2O with sym-O2N-NO2 on the (100) surface of NaCl are difficult to occur because of the high barrier (33.79 kcal/mol), whereas, the reactions of H2O with cis-ONONO2 and trans-ONONO2 play the key role in the hydrolysis process. The product, HONO, is one of the main atmospheric sources of OH radicals which drive the chemistry of the troposphere.

Structurally Flexible C₃-Symmetric Receptors for Molecular Recognition and Their Self-Assembly Properties

The bioinspired design and synthesis of building blocks and their assemblies by the supramolecular approach has ever fascinated scientists to utilize such artificial systems for numerous purposes. Flexibility is a basic feature of natural systems. However, in artificial systems this is difficult to control, especially if there is no preorganization of the component(s) of a system. We have designed and synthesized a series of C3 -symmetric N-bridged flexible receptors and successfully utilized them to selectively entrap the notorious and toxic nitrate anion in aqueous medium. This was the first report of highest binding affinity for the nitrate anion in aqueous medium. An impressive self-sorting phenomenon of reversibly formed hydrogen-bonded capsules, which self-assembled from flexible tripodal receptors having branches of similar size and bearing the same amide functionality, has been disclosed. Encapsulated nitrate anion has been further utilized for the photochemical [2+2] cycloaddition reaction for the synthesis of strained four-membered ring structures through dynamic self-assembly. In this Personal Account, we summarize these results showing the utility of naturally inspired flexibility in artificial systems.

Computational studies of atmospherically-relevant chemical reactions in water clusters and on liquid water and ice surfaces

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.

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

Formation of atmospheric chlorine atom precursors ClNO₂ and ClNO in the reaction of HCl with oxides of nitrogen on a water film: left – formation of N–Cl bond as N–O bond breaks; right – concurrent changes in Mulliken charges.

Release of nitrous acid and nitrogen dioxide from nitrate photolysis in acidic aqueous solutions

Nitrate (NO3(-)) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3(-) leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO3¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegradation of aqueous NO3(-) under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO3(-) absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in solution. We find that NO2 is more efficiently hydrolyzed in solution when it is generated in situ during NO3(-) photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3(-) photochemistry.

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

Recent progress in “on-the-fly” trajectory simulations of molecular reactions, using different electronic structure methods is discussed, with analysis of the insights that such calculations can provide and of the strengths and limitations of the algorithms available.

NOx Reactions on Aqueous Surfaces with Gaseous HCl: Formation of a Potential Precursor to Atmospheric Cl Atoms

Chlorine atoms are highly reactive free radicals known to catalyze ozone depletion in the stratosphere and organic oxidation in the troposphere. They are readily produced photolytically upon irradiation of some stable Cl containing species, for instance, nitrosyl chloride, ClNO. We predict the formation of ClNO using ab initio molecular dynamics (AIMD) simulations of an NO2 dimer on the surface of a thin film of water upon which gaseous HCl impinges. The reactant is chloride ion formed when HCl ionizes on the water film. The same mechanism for ClNO production may occur in humid environments when ONONO2 (the asymmetric NO2 dimer examined here) comes in contact with either HCl or sea salt. The film of water serves to (1) stabilize ONONO2 on the film surface so that it is localized and physically accessible for reaction, (2) provide the medium to ionize HCl, and (3) activate ONONO2 making it more susceptible to nucleophilic attack by chloride. This substitution/elimination mechanism is new for NOx chemistry on thin water films and could not be derived from studies on small clusters.

Ground-State Intermolecular Proton Transfer of N2O4 and H2O: An Important Source of Atmospheric Hydroxyl Radical?

To evaluate the significance of the generation of atmospheric hydroxyl radical from reaction of N2O4 with H2O, CASPT2//CASSCF as well as CASPT2//CASSCF/Amber QM/MM approaches were employed to map the minimum-energy profiles of sequential reactions, NO2 dimerization and ground-state intermolecular proton transfer of trans-ONONO2 as well as the photolysis of HONO. A highly efficient ground-state intermolecular proton transfer of trans-ONONO2 is found to dominate the generation of hydroxyl radical under atmospheric conditions. Although proton transfer occurs with high efficiency, the precursor reaction of dimerization producing trans-ONONO2 has to overcome a 17.1 kcal/mol barrier and cannot compete with the barrierless channel of symmetric O2N-NO2 formation from isolated NO2 monomers. Our computations reveal that the photolysis of HONO without a barrier definitely makes significant contributions to the concentration of the atmospheric hydroxyl radical, but its importance is influenced by the lack of trans-ONONO2 isomer in the atmospheric environment.