Search

Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change (1), stratospheric ozone loss (2), acidification of soils and water (3), [...]

The Pan-Eurasian Experiment (PEEX) is a multi-disciplinary, multi-scale and multicomponent research, research infrastructure and capacity building program. PEEX has originated from a bottom-up approach by the science communities, and is aiming at resolving the major uncertainties in Earth System Science and global sustainability issues concerning the Arctic and boreal Pan-Eurasian regions, as well as China. The vision of PEEX is to solve interlinked global grand challenges influencing human wellbeing and societies in northern Eurasia and China. Such challenges include climate change, air quality, biodiversity loss, urbanization, chemicalization, food and fresh water availability, energy production and use of natural resources by mining, industry, energy production and transport sectors. Our approach is integrative and supra-disciplinary, recognizing the important role of the Arctic and boreal ecosystems in the Earth system. The PEEX vision includes establishing and maintaining long-term, coherent and coordinated research activities as well as continuous, comprehensive research and educational infrastructures and related capacity building across the PEEX domain. In this paper we present the PEEX structure, summarize its motivation, objectives and future outlook. [ABSTRACT FROM AUTHOR]

It has recently been demonstrated that the O2SO3-1ion forms in the atmosphere as a natural consequence of ionizing radiation. Here, we present a density functional theory-based study of the reactions of O2SO3-1 with O3. The most important reactions are (a) oxidation of O2SO3-1 to O2SO3-1 and (b) cluster decomposition into SO3, O2 and O2. The former reaction is highly exothermic and the nascent O2SO3-1 will rapidly decompose into SO2 and O2. If the origin of O2SO3-1 is SO2 oxidation by O2, the latter reaction closes a catalytic cycle wherein SO2 is oxidized to SO3. The relative rates between the two major sinks for O2SO3-1 is assessed, thereby providing a measure of the maximum turnover number of ion catalysed SO2 oxidation, i.e. how many SO2 can be oxidized per free electron. The rate ratio between reactions (a) and (b) is significantly altered by the presence or absence of a single water molecule, but reaction (b) is in general much more probable. Although we are unable to assess the overall importance of this cycle in the real atmosphere due to the unknown influence of CO2 and NOx, we roughly estimate that ion induced catalysis may contribute with several percent of H2SO4 levels in typical CO2 free and low NOx reaction chambers, e.g. the CLOUD chamber at CERN. [ABSTRACT FROM AUTHOR]

Bicyclic guanidines are widely used in organic chemistry as strong basic reagents and catalysts. They are also utilized in CO 2 chemistry, ring opening polymerization reactions and in cellulose dissolution or processing as ionic liquids. Despite their wide applicability, they are usually sensitive to water, especially under alkaline conditions. The salts of guanidines usually exhibit slower degradation rates but are still somewhat prone to hydrolysis. We studied the hydrolysis rates of a series of different bicyclic guanidines and their carboxylate salts under different aqueous conditions. It appears that bicyclic guanidines and their salts are in principle stable in water but undergo hydrolysis in the presence of hydroxide ions (OH - ). Therefore, it is feasible to design bicyclic guanidine bases that are highly tolerant to water, but can still simultaneously possess different desirable chemical characteristics, like hydrophobicity to allow compatibility under varying conditions., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Quantum chemical calculations were employed to construct Jablonski diagrams for a series of phenolic carbonyls, including vanillin, iso-vanillin, 4-hydroxybenzaldehyde, syringaldehyde, and coniferyl aldehyde. These molecules can enter the Earth's atmosphere from forest fire emissions and participate in photochemical reactions within the atmospheric condensed phase, including cloud and fog droplets and aqueous aerosol particles. This photochemistry alters the composition of light-absorbing organic content, or brown carbon, in droplets and particles through the formation and destruction of key chromophores. This study demonstrates that following photon absorption, phenolic carbonyls efficiently transition to triplet states via intersystem crossings (ISC), with rate coefficients ranging from 10 9 to 10 10 s -1 . Despite the presence of multiple potential ISC pathways due to several lower-lying triplet states, a single channel is found to dominate for each system. We further investigated the dependence of the ISC rate constant ( k ISC ) on the vibrational excitation energy of the first accessible (ππ*) singlet excited state (S 1 or S 2 , depending on the molecule), and compared it with the measured wavelength dependence of the photochemical quantum yield ( Φ loss ). Although our model only accounts for intramolecular nonradiative electronic transitions, it successfully captures the overall trends. All studied molecules, except coniferyl aldehyde, exhibit saturation in the dependence of both k ISC and Φ loss on the wavelength (or vibrational excitation energy). In contrast, coniferyl aldehyde displays a single maximum, followed by a monotonic decrease as the excitation energy increases (wavelength decreases). This distinct behavior in coniferyl aldehyde may be attributed to the presence of a double-bonded substituent, which enhances π-electron conjugation, and reduces the exchange energy and thus the adiabatic energy gap between the S 1 (ππ*) state and the target triplet state. For small energy gaps, the classical acceptor modes of the ISC process are less effective, leading to a low effective density of final states. Larger gaps enhance the effective density of states, making the wavelength dependence of the ISC more pronounced. Our calculations show that while all the studied phenolic carbonyls have similar acceptor modes, coniferyl aldehyde has a substantially smaller adiabatic gap (1700 cm -1 ) than the other molecules. The magnitude of the adiabatic energy gap is identified as the primary factor determining the energy/wavelength dependence of the ISC rate and thus Φ loss .

In the last few decades, atmospheric formation of secondary organic aerosols (SOA) has gained increasing attention due to their impact on air quality and climate. However, methods to predict their abundance are mainly empirical and may fail under real atmospheric conditions. In this work, a close-to-mechanistic approach allowing SOA quantification is presented, with a focus on a chain-like chemical reaction called "autoxidation". A novel framework is employed to (a) describe the gas-phase chemistry, (b) predict the products' molecular structures and (c) explore the contribution of autoxidation chemistry on SOA formation under various conditions. As a proof of concept, the method is applied to benzene, an important anthropogenic SOA precursor. Our results suggest autoxidation to explain up to 100% of the benzene-SOA formed under low-NO x laboratory conditions. Under atmospheric-like day-time conditions, the calculated benzene-aerosol mass continuously forms, as expected based on prior work. Additionally, a prompt increase, driven by the NO 3 radical, is predicted by the model at dawn. This increase has not yet been explored experimentally and stresses the potential for atmospheric SOA formation via secondary oxidation of benzene by O 3 and NO 3 ., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Bicyclic guanidines are utilized in organic synthesis as base catalysts or reagents. They also offer a platform for coordination chemistry, for example in CO 2 activation, and their carboxylate salts offer an efficient media for cellulose dissolution. We have studied a series of bicyclic guanidines with varying ring sizes and with varying methyl substituents with a specific aim to find hydrolytically stable acetate salts for dissolution and processing of cellulose. Different superbase synthesis pathways were tested, followed by hydrolytic stability and cellulose dissolution capacity tests. The synthesis pathways were designed to enable the scale up of the production of the superbases considering the availability of the starting molecules and the feasibility of the synthesis. As a result, we found several hydrolytically stable bicyclic guanidine structures, which can overcome many of the reoccurring problems as carboxylate salts or free bases., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The effect of localized surface plasmon resonance (LSPR) of a system consisting of a highly dipolar merocyanine dye and a silver nanoparticle (NP) was studied experimentally and theoretically. A theoretical model for estimating the fluorescence quantum yield ( φ fl ) using quantum chemical calculations of intramolecular and intermolecular electronic transition rate constants was developed. Calculations show that the main deactivation channels of the lowest excited singlet state of the studied merocyanines are internal conversion ( k IC (S 1 → S 0 )) and fluorescence ( k r (S 1 → S 0 )). The intersystem-crossing transition has a low probability due to the large energy difference between the singlet and triplet levels. In the presence of plasmonic NPs, the fluorescence quantum yield is increased by a factor of two according to both experiment and computations. The calculated values of φ fl , when considering changes in k r (S 1 → S 0 ) and the energy-transfer rate constant ( k transfer ) from the dye to the NP was also twice as large at distances of 6-8 nm between the NP and the dye molecule. We also found that the LSPR effect can be increased or decreased depending on the value of the dielectric constant ( ε m ) of the environment.

Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O 3 surface concentrations. Although iodic acid (HIO 3 ) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO 3 via reactions (R1) IOIO + O 3  → IOIO 4 and (R2) IOIO 4  + H 2 O → HIO 3  + HOI +  (1) O 2 . The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO 3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation., (© 2022. The Author(s).)

Highly oxygenated organic molecules (HOMs) are important sources of atmospheric aerosols. Resolving the molecular-level formation mechanisms of these HOMs from freshly emitted hydrocarbons improves the understanding of aerosol properties and their influence on the climate. In this study, we measure the electrical mobility and mass-to-charge ratio of α-pinene oxidation products using a secondary electrospray-differential mobility analyzer-mass spectrometer (SESI-DMA-MS). The mass-mobility spectrum of the oxidation products is measured with seven different reagent ions generated by the electrospray. We analyzed the mobility-mass spectra of the oxidation products C 9-10 H 14-18 O 2-6 . Our results show that acetate and chloride yield the highest charging efficiencies. Analysis of the mobility spectra suggests that the clusters have 1-5 isomeric structures (i.e., ion-molecule cluster structures with distinct mobilities), and the number is affected by the reagent ion. Most of the isomers are likely cluster isomers originating from binding of the reagent ion to different sites of the molecule. By comparing the number of observed isomers and measured mobilities and collision cross sections between standard pinanediol and pinonic acid to the values observed for C 10 H 18 O 2 and C 10 H 16 O 3 produced from oxidation of α-pinene, we confirm that pinanediol and pinonic acid are the only isomers for these elemental compositions in our experimental conditions. Our study shows that the SESI-DMA-MS produces new information from the first steps of oxidation of α-pinene.

The magnetically induced current-density susceptibility tensor (CDT) of the lowest singlet and triplet states of the metallocenothiaporphyrins, where the metal is V, Cr, Mn, Fe, Co, Ni, Mo, Tc, Ru, or Rh, have been studied with the gauge-including magnetically induced currents (GIMIC) method. The compounds containing V, Mn, Co, Tc or Rh were studied as cations because the neutral molecules have an odd number of electrons. The calculations show that the aromatic nature of most of the studied molecules follows the Hückel and Baird rules of aromaticity. CDT calculations on the high-spin states of the neutral metallocenothiaporphyrins with V, Mn, Co, Tc or Rh also shows that these molecules follow a unified extended Hückel and Baird aromaticity orbital-count rule stating that molecules with an odd number of occupied conjugated valence orbitals are aromatic, whereas molecules with an even number of occupied conjugated orbitals are antiaromatic.

The recently synthesized cyclo[18]carbon molecule has been characterized in a number of studies by calculating electronic, spectroscopic, and mechanical properties. However, cyclo[18]carbon is only one member of the class of cyclo[ n ]carbons-standalone carbon allotrope representatives. Many of the larger members of this class of molecules have not been thoroughly investigated. In this work, we calculate the magnetically induced current density of cyclo[ n ]carbons in order to elucidate how electron delocalization and aromatic properties change with the size of the molecular ring ( n ), where n is an even number between 6 and 100. We find that the Hückel rules for aromaticity (4 k + 2) and antiaromaticity (4 k ) become degenerate for large C n rings ( n > 50), which can be understood as a transition from a delocalized electronic structure to a nonaromatic structure with localized current density fluxes in the triple bonds. Actually, the calculations suggest that cyclo[ n ]carbons with n > 50 are nonaromatic cyclic polyalkynes. The influence of the amount of nonlocal exchange and the asymptotic behavior of the exchange-correlation potential of the employed density functionals on the strength of the magnetically induced ring current and the aromatic character of the large cyclo[ n ]carbons is also discussed.

Peroxyl radicals (RO O . ) are key intermediates in atmospheric chemistry, with relatively long lifetimes compared to most other radical species. In this study, we use multireference quantum chemical methods to investigate whether photolysis can compete with well-established RO O . sink reactions. We assume that the photolysis channel is always RO O . + h ν => RO + O( 3 P). Our results show that the maximal value of the cross-section for this channel is σ = 1.3 × 10 -18 cm 2 at 240 nm for five atmospherically representative peroxyl radicals: CH 3 O O . , C(O)HCH 2 O O . , CH 3 CH 2 O O . , HC(O)O O . and CH 3 C(O)O O . . These values agree with experiments to within a factor of 2. The rate constant of photolysis in the troposphere is around 10 -5 s -1 for all five RO O . . As the lifetime of peroxyl radicals in the troposphere is typically less than 100 s, photolysis is thus not a competitive process. Furthermore, we investigate whether or not electronic excitation to the first excited state (D 1 ) by infrared radiation can facilitate various H-shift reactions, leading, for example, in the case of CH 3 O O . to formation of O . H and CH 2 O or HO O . and CH 2 products. While the activation barriers for H-shifts in the D 1 state may be lower than in the ground state (D 0 ), we find that H-shifts are unlikely to be competitive with decay back to the D 0 state through internal conversion, as this has a rate of the order of 10 13 s -1 for all studied systems., Competing Interests: We declare we have no competing interests., (© 2020 The Authors.)

Plasmon-assisted transformations of organic compounds represent a novel opportunity for conversion of light to chemical energy at room temperature. However, the mechanistic insights of interaction between plasmon energy and organic molecules is still under debate. Herein, we proposed a comprehensive study of the plasmon-assisted reaction mechanism using unsymmetric iodonium salts (ISs) as an organic probe. The experimental and theoretical analysis allow us to exclude the possible thermal effect or hot electron transfer. We found that plasmon interaction with unsymmetrical ISs led to the intramolecular excitation of electron followed by the regioselective cleavage of C-I bond with the formation of electron-rich radical species, which cannot be explained by the hot electron excitation or thermal effects. The high regioselectivity is explained by the direct excitation of electron to LUMO with the formation of a dissociative excited state according to quantum-chemical modeling, which provides novel opportunities for the fine control of reactivity using plasmon energy.

Effective and fast algorithms for calculating rate constants for internal conversion (IC) and intersystem crossing (ISC) in the Franck-Condon and Herzberg-Teller approximations have been developed and implemented. The methods have been employed for calculating IC and ISC rate constants for the pyrromethene-567 dye (PM567), hetero[8]circulene (4B) and free-base porphyrin (H2P). The fluorescence quantum yields obtained by comparing calculated rate constants for the radiative and non-radiative processes are in good agreement with experimental data.

High molecular weight "ROOR'" dimers, likely formed in the gas phase through self- and cross-reactions of complex peroxy radicals (RO 2 ), have been suggested to play a key role in forming ultrafine aerosol particles in the atmosphere. However, the molecular-level reaction mechanism producing these dimers remains unknown. Using multireference quantum chemical methods, we explore one potentially competitive pathway for ROOR' production, involving the initial formation of triplet alkoxy radical (RO) pairs, followed by extremely rapid intersystem crossings (ISC) to the singlet surface, permitting subsequent recombination to ROOR'. Using CH 3 OO + CH 3 OO as a model system, we show that the initial steps of this reaction mechanism are likely to be very fast, as the transition states for both the formation and the decomposition of the CH 3 O 4 CH 3 tetroxide intermediate are far below the reactants in energy. Next, we compute ISC rates for seven different atmospherically relevant 3 (RO···R'O) complexes. The ISC rates vary significantly depending on the conformation of the complex and also exhibit strong stereoselectivity. Furthermore, the fastest ISC process is usually not between the lowest-energy triplet and singlet states but between the triplet ground state and an exited singlet state. For each studied (RO···R'O) system, at least one low-energy conformer with an ISC rate above 10 8 s -1 can be found. This demonstrates that gas-phase dimer formation in the atmosphere very likely involves ISCs originating in relativistic quantum mechanics.

Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere. The nucleation of sulfuric acid and organic vapours is thought to be responsible for the formation of new particles over continents, whereas iodine oxide vapours have been implicated in particle formation over coastal regions. The molecular clustering pathways that are involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems, but direct molecular-level observations of nucleation in atmospheric field conditions that involve sulfuric acid, organic or iodine oxide vapours have yet to be reported. Here we present field data from Mace Head, Ireland, and supporting data from northern Greenland and Queen Maud Land, Antarctica, that enable us to identify the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours, with average oxygen-to-iodine ratios of 2.4 found in the clusters. On the basis of this high ratio, together with the high concentrations of iodic acid (HIO 3 ) observed, we suggest that cluster formation primarily proceeds by sequential addition of HIO 3 , followed by intracluster restructuring to I 2 O 5 and recycling of water either in the atmosphere or on dehydration. Our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine-containing species in the formation of new aerosol particles, and identifies the key nucleating compound., Competing Interests: The authors declare no competing financial interests.

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.

The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer. In low-temperature experiments (T = 201-298 K), association kinetics were observed, and the measured time-resolved C(3)H(5) radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {p = 0.4-36 Torr, [He] = (1.7-118.0) × 10(16) cm(-3)}. The bimolecular rate coefficients obtained vary in the range (0.88-11.6) × 10(-13) cm(3) s(-1). In higher-temperature experiments (T = 320-420 K), the C(3)H(5) radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical second- and third-law procedures, plotting the resulting ln K(p) values versus 1/T in a modified van't Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of ΔH(298)° = -78.3 ± 1.1 kJ mol(-1) and ΔS(298)° = -129.9 ± 3.1 J mol(-1) K(-1), with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as ΔH(298)° = -75.6 ± 2.3 kJ mol(-1).

The structure and stabilities of NH(3) adsorbed on different sites of a Ni(111) surface are compared based on density functional, plane-waves calculations within a periodic framework. The surface has been modeled by 4- and 5-layer slabs with 2 x 2 and 3 x 3 unit cells. Calculated results are in good agreement with available experimental data, confirming the atop adsorption site to be the most favorable, with no preferred azimuthal orientation for the H atoms. For NH(3) adsorbed at the atop site, the one-dimensional potential energy profiles along the N-H and N-Ni bonds and the coupling between adjacent N-H bond oscillators have been calculated and fitted to an analytical expression using an accurate anharmonic potential model. Variational calculations have been performed to obtain frequencies for the N-H and N-Ni stretching vibrations and N-H stretching line widths. The model for calculating line widths has also been tested with CO adsorbed at the hcp hollow of the Ni(111) surface.