A Model Assessment of the Occurrence and Reactivity of the Nitrating/Nitrosating Agent Nitrogen Dioxide (•NO2) in Sunlit Natural Waters

Nitrogen dioxide (^•NO₂) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with ^•OH, and upon the oxidation of nitrite by ^•OH itself. ^•NO₂ is mainly scavenged by dissolved organic matter, and here, it is shown that ^•NO₂ levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (N₂O₄) is also formed in the pathway of ^•NO₂ hydrolysis, but with a very low concentration, i.e., several orders of magnitude below ^•NO₂, and even below ^•OH. Therefore, at most, N₂O₄ would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with ^•NO₂. Although it is known that nitrite oxidation by CO₃^•− in high-alkalinity surface waters gives a minor-to-negligible contribution to ^•NO₂ formation, it is shown here that NO₂⁻ oxidation by Br₂^•− can be a significant source of ^•NO₂ in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of ^•OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by ^•NO₂ preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by ^•OH, which is scavenged by bromide in saltwater. The enhancement of ^•NO₂ reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.     

Modelling photochemical transformation of emerging organic pollutants in surface waters: effect of water level fluctuations following outflow or evaporation,relevant to arid and semi-arid environments

Water bodies located in arid and semi-arid environments undergo important fluctuations of the water level, and water loss in the dry season may occur through phenomena such as outflow and evaporation. Water scarcity often exacerbates the impact of pollution, because harmful compounds can reach very high concentration values in the presence of relatively low water volumes. Among self-cleansing (depollution) processes of water bodies, photochemical reactions are expected to play an important role in pollutant attenuation, although sometimes they yield harmful intermediates. In this work, the effects of water-level fluctuations on photochemical reactions were studied by means of a modelling approach. The model applies to well-mixed water bodies, such as the shallow ones that are more likely to undergo fluctuations of the water level. We investigated how water outflow and evaporation (as well as a mixed scenario) might modify the steady-state concentrations of the most important photoreactive transients (^?OH, CO₃^–?, ¹O₂ and triplet states of chromophoric dissolved organic matter, ³CDOM*). Under the same conditions, the possible phototransformation of emerging pollutants such as carbamazepine, ibuprofen, 2,4-dichloro-6-nitrophenol and 2,4-dinitrophenol was also assessed. It is shown that water loss would generally enhance photochemical reactions, but to a variable extent depending on the actual pathway. Outflow would favour all photoprocesses, and particularly those involving ^?OH and CO₃^–?. Conversely, evaporation would enhance reaction with ¹O₂ and ³CDOM* while having practically no effect on ^?OH, CO₃^–? and direct photolysis. Enhancement of photochemical self-cleansing of water bodies would partially reduce the pollution impact, but in some cases harmful compounds may be generated.     

Photolysis of the Insensitive Explosive 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB)

The explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is of particular interest due to its extreme insensitivity to impact, shock and heat, while providing a good detonation velocity. To determine its fate under environmental conditions, TATB powder was irradiated with simulated sunlight and, in water, under UV light at 254 nm. The hydrolysis of particles submerged in neutral and alkaline solutions was also examined. We found that, by changing experimental conditions (e.g., light source, and mass and physical state of TATB), the intermediates and final products were slightly different. Mono-benzofurazan was the major transformation product in both irradiation systems. Two minor transformation products, the aci-nitro form of TATB and 3,5-diamino-2,4,6-trinitrophenol, were detected under solar light, while 1,3,5-triamino-2-nitroso-4,6-dinitrobenzene, 1,3,5-triamino-2,4-dinitrobenzene and mono-benzofuroxan were produced under UV light. The product identified as 3,5-diamino-2,4,6-trinitrophenol was identical to the one formed in the dark under alkaline conditions (pH 13) and in water incubated at either 50 °C or aged at ambient conditions. Interestingly, when only a few milligrams of TATB were irradiated with simulated sunlight, the aci-isomer and mono-benzofurazan derivative were detected; however, the hydrolysis product 3,5-diamino-2,4,6-trinitrophenol formed only much later in the absence of light. This suggests that the water released from TATB to form mono-benzofurazan was trapped in the interstitial space between the TATB layers and slowly hydrolyzed the relatively stable aci-nitro intermediate to 3,5-diamino-2,4,6-trinitrophenol. This environmentally relevant discovery provides data on the fate of TATB in surface environments exposed to sunlight, which can transform the insoluble substrate into more soluble and corrosive derivatives, such as 3,5-diamino-2,4,6-trinitrophenol, and that some hydrolytic transformation can continue even without light.     

New Insights into the Redox Properties of Pyridinium Appended 1,2-Dithienylcyclopentenes

The optical and redox properties of a methyl pyridinium appended 1,2-dithienylethene photochromic derivative have been thoroughly investigated. A complex multi-step photo/redox mechanism is proposed for the closed isomer on the ground of spectro-electrochemical and theoretical data. The generated compounds are not stable over the time because of chemical reactions associated to the redox processes and a new dithienylethene derivative incorporating a seven-membered ring has been isolated and characterized.     

Insights into the Thermal and Photochemical Reaction Mechanisms of Azidoacetonitrile. Spectroscopic and MS-CASPT2 Calculations

This work studies the photochemical and thermal decompositions of azidoacetonitrile (N₃CH₂CN) from both the experimental and theoretical points of view. The data of the photochemical experiments are taken from the literature, while the thermal decomposition have been carried out by us. In addition, we have performed ab initio calculations of the multiconfigurational type [complete active space self-consistent field (CASSCF) and the multistate multireference perturbation theory (MS-CASPT2)]. It is found that the first step of both type of decompositions is N₂ elimination and formation of closed shell singlet nitrene. Afterwards, the nitrene tends to rapidly rearrange into formimidoyl cyanide (HNCHCN). As both reactions progress, the imine isomerizes into formimidoyl isocyanide (HNCHNC). The photoisomerization of the imine takes places thorough a conical intersection, while the same reaction on the ground electronic state occurs via a conventional transition state. The last step of the global reaction is decomposition of the imines into HCN and CNH. In photochemical conditions, the conjunction of the imines and its dissociation products (HCN and CNH) yields adenine     

Organophotocatalysis: Insights into the Mechanistic Aspects of Thiourea‐Mediated Intermolecular [2+2] Photocycloadditions

Mechanistic investigations of the intermolecular [2+2] photocycloaddition of coumarin with tetramethylethylene mediated by thiourea catalysts reveal that the reaction is enabled by a combination of minimized aggregation, enhanced intersystem crossing, and altered excited‐state lifetime(s). These results clarify how the excited‐state reactivity can be manipulated through catalyst–substrate interactions and reveal a third mechanistic pathway for thiourea‐mediated organo‐photocatalysis.     

Photo‐Hydrogen‐Evolving Molecular Catalysts Consisting of Polypyridyl Ruthenium(II) Photosensitizers and Platinum(II) Catalysts: Insights into the Reaction Mechanism

The mechanism of photoinduced hydrogen evolution from water driven by the first photo‐hydrogen‐evolving molecular catalyst ( 1 ), given by a coupling of [Ru(bpy)₂(5‐amino‐phen)]²⁺ and [PtCl₂(4,4′‐dicarboxy‐bpy)] (bpy=2,2′‐bipyridine, phen=1,10‐phenanthroline), was investigated in detail. The H₂ evolution rate was found to obey Michaelis–Menten enzymatic kinetics with regard to the concentration of EDTA (ethylenediamine tetra‐acetic acid disodium salt, sacrificial electron donor), which indicates that an ion‐pair formation between the dicationic 1 and the dianionic form of EDTA (pH 5) is a key step leading to H₂ formation. A 2:1 coupling product of 1 and ethylenediamine (i.e., a {Ru^II₂Pt^II₂} complex 2 ) was found to show significantly higher photo‐hydrogen‐evolving (PHE) activity than 1 , which revealed the validity of the bimolecular activation proposed in our previous study. The PHE activity of 2 was also observed to be linear to the concentration of 2 , which indicates that H₂ formation through the intermolecular path competes with the intramolecular path. Molecular orbital diagrams, conformational features, and Pt???H(water or acetic acid) hydrogen bonds were characterized by DFT calculations.     

Insights into the Crystallization and Structural Evolution of Glycine Dihydrate by In Situ Solid‐State NMR Spectroscopy

In situ solid‐state NMR spectroscopy is exploited to monitor the structural evolution of a glycine/water glass phase formed on flash cooling an aqueous solution of glycine, with a range of modern solid‐state NMR methods applied to elucidate structural properties of the solid phases present. The glycine/water glass is shown to crystallize into an intermediate phase, which then transforms to the β polymorph of glycine. Our in situ NMR results fully corroborate the identity of the intermediate crystalline phase as glycine dihydrate, which was first proposed only very recently.     

Pesticides in water and the performance of the liquid-phase microextraction based techniques. A review

The control of pesticides in surface, drinking and groundwater is nowadays a real necessity. In the European Community, their concentration must comply with the established parametric and environmental quality standards (EQSs). Regarding the new legislation, this article updates the information concerning the monitoring of pesticides and the technical specifications for their measurement in water samples where ultra-sensitive analytical methods are required. For some compounds, like pesticides, there is still a need to improve the performance of the existing methods. High sensitive techniques like gas chromatography tandem mass spectrometry (GC–MS/MS) and liquid chromatography coupled with mass spectrometry (LC–MS) have been developed. However, for most of the substances present at trace and ultra-trace levels the extraction and preconcentration steps are so far essential for their detection. Advances at a micro scale have been made and different types of microextractions are being developed. Liquid-phase microextraction (LPME) is an example. The study of this technique has increased in the last years and some innovations have been recently reported for pesticides water analysis. This article reviews the new developed LPME-based techniques and compares its performance with the analytical specifications established for pesticides water monitoring. The results show that LPME-based techniques can be a promising tool to improve the nowadays performance of methods used in pesticides water control.