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Generation of sulfamoyl radicals via visible-light mediated fixation of sulfur dioxide as novel approach for the synthesis of sulfonamides
No full textHerein, we report a so far unprecedented approach for the light-mediated generation of sulfamoyl radicals using sulfur dioxide a key building block and the direct application of these radicals in the synthesis of sulfonamides. In the presence of different photoredox catalysts, sulfamoyl radicals can be generated directly from SO2 or the SO2 surrogate DABSO (1,4-diazabicyclo[2.2.2]octane·bis (sulfur dioxide) adduct) and N-aminopyridinium salts as nitrogen radical precursors. Trapping of the in situ generated sulfamoyl radicals with selected electron-rich olefins affords different sulfonamides in moderate to good yields in a three-component procedure. This transformation provides an attractive and conceptually novel approach for the in situ generation of sulfamoyl radicals as synthetic intermediates for the assembly of the sulfonamide functionality, a privileged motif in active pharmaceutical ingredients
A Trapped Dinuclear-Cu Intermediate of the CuAAC “Click” Reaction that Behaves as a Mechanical Crypt for Copper
No full textSince the metal templated synthesis of catenanes by Sauvage, many mechanically chelating ligands, in which the donor atoms are provided by more than one interlocked covalent component, have been reported. However, despite the importance of controlling metal ion properties in a range of applications from catalysis to medicine, and the unusual properties of metal ions bound in the cavity of a catenane being reported as early as 1985, very few studies focus on the effect of the mechanical bond on the metal complex; typically, it is simply an intermediate en route to a metal-free synthetic target or used to control mechanical motion in prototypical molecular machines. Here we report perhaps the starkest demonstration of the ability of the mechanical bond to control metal ion properties, the unexpected isolation of an interlocked di-nuclear Cu complex in which these typically kinetically labile metal ions are sequestered such that it takes days to remove them from the ligand even under relatively harsh conditions. DFT modelling suggests that the complex is formed as a trapped intermediate in the Cu-mediated alkyne-azide cycloaddition reaction which represents the first unambiguous observation of such a species in this important “click” reaction
Visible light-mediated photocatalytic coupling between tetrazoles and carboxylic acids for protein and cell labelling
No full textWe present a photocatalytic reaction of tetrazoles to form nitrile imines, which can be coupled with carboxylic acids in aqueous environments. This reaction is applied for photocatalyst-dependent labelling of proteins and cells
Temperature-Dependent Water Oxidation Kinetics: Implications and Insights
No full textAs a vital process for solar fuel synthesis, water oxidation remains a challenging reaction to perform using durable and cost-effective systems. Despite decades of intense research, the understanding of the detailed processes involved is still limited, particularly under photochemical conditions. Recent research has shown that the overall kinetics of water oxidation by a molecular dyad depends on the coordination between the photocharge generation and the subsequent chemical steps. This work explores similar effects by heterogeneous solar water oxidation systems. By varying a key variable, the reaction temperature, we discovered distinctly different behaviors on two model systems, TiO2 and Fe2O3. TiO2 exhibited a monotonically increasing water oxidation performance with rising temperatures across the entire applied potential range, between 0.1 V and 1.5 V vs. the reversible hydrogen electrode (RHE). In contrast, Fe2O3 showed increased performance with temperature at high applied potentials (>1.2 V vs. RHE) but decreased performance at low applied potentials (<1.2 V vs. RHE). This decrease in performance with temperature on Fe2O3 was attributed to increased electron-hole recombination, as confirmed by intensity modulated photocurrent spectroscopy (IMPS). The origin of the differing temperature dependences on TiO2 and Fe2O3 was further ascribed to their different surface chemical kinetics. These results highlight the chemical nature of charge recombination in photoelectrochemical (PEC) systems, where surface electrons recombine with holes stored in surface chemical species. It also indicates that PEC kinetics are not constrained by a single rate determining chemical step, highlighting the importance of an integrated approach to studying the system. Moreover, the results suggest that for practical solar water splitting devices, higher temperatures are not always beneficial for reaction rates, especially under low driving force conditions
Selection or removal of compounds from complex samples by peak isolation using an enhanced GCxGC configuration
No full textWe developed a novel approach to isolate and recollect or remove nearly any compound from complex GCxGC chromatograms. This was achieved by modifying a thermal desorption-GCxGC-Q-TOF system with a Deans switch, a passive splitter, and careful optimization of flows and capillary dimensions. The setup was evaluated with 106 standard chemicals covering a wide range of volatility (boiling points: 56 – 343 ⁰C) and polarity (log P: -0.2 – 9.4). We found that the recovery from tube to tube can become highly efficient if a custom-made adapter is attached directly on the flame ionization detector port (average recovery rate of 76 ± 7%). Furthermore, we were able to isolate peaks separated by a minimum distance of 50 milliseconds from each other at the base throughout the whole chromatographic run. In addition, the setup was designed for easy adaptation by repurposing existing instrument control software to define the isolation windows for the compounds of interest (first and second column retention time windows). We expect this novel development to allow several new applications, e.g., the isolation and selective enrichment or removal of molecules in food and flavour analysis; the investigation of suspect chemicals (incl. unknowns) for effect directed analysis (e.g., bioassays), and the isolation of compounds in the high-ng range (upon multiple isolation iterations) to be subjected to further chemical analyses (e.g. chemical ionization tandem mass spectrometry)
Modeling the ionization efficiency of small molecules in positive electrospray ionization
No full textTechnological advancements in liquid chromatography (LC) electrospray ionization (ESI) high resolution mass spectrometry (HRMS) have made it an increasingly popular analytical technique in non-targeted analysis (NTA) of environmental and biological samples. One critical limitation of current methods in NTA is the lack of available analytical standards for many of the compounds detected in biological and environmental samples. Computational approaches can provide estimates of concentrations by modeling the ionization efficiency of a compound expressed as the relative response factor (RRF). In this paper, we explore the application of molecular dynamics (MD) in the development of a predictive model for RRF. We obtained measurements of RRF for 48 compounds with LC - quadrupole time-of-flight (QTOF) MS and calculated their RRF by dividing the observed peak areas by their concentrations. We used the CGenFF force field to generate the topologies and GROMACS to conduct the (MD) simulations (t = 1 ns). We calculated the Lennard-Jones and Coulomb interactions between the analytes and all other molecules in the ESI droplet, which were then used to construct a multilinear regression model for predicting RRF. The best performing model showed a coefficient of determination (R2) of 0.82 and a mean absolute error (MAE) of 0.13 log units. This performance is comparable to other predictive models including machine learning models. While there is a need for further evaluation of diverse chemical structures, our approach showed great promise in predictions of RRF
New states of water and the emerging field of constrained physics
No full textHave new states of water been discovered? Utilizing molecular dynamics and first-principles analyses across thousands of models, this paper introduces a dual-limit temperature regulation method under mass and spatial constraints, yielding water molecular chains and 2D water films, uncovering the key to unlocking a new world of water states. This has led to the emergence of dazzling and magical water structures, accompanied by the continuous emergence and summarization of corresponding electrical, optical, thermal, and mechanical laws as new structures appear. Concepts such as "new states of water", "water molecular chains", "spiral ionic networks", "universal dual-limit temperature control method" and "confined physics" are proposed. These insights delve deep into fundamental science, enhancing our understanding of water and exploring its unprecedented states. Furthermore, they extend to charged ions and polar molecules, pioneering new avenues in materials science, nanotechnology, atomic engineering, and energy conversion, with the potential to propel multiple disciplines and shape society
Stereoselective Synthesis of Hexa-Aryl Borazines: Leaving Flatlands towards sp²-Based 3D Molecular Architectures
No full textBorazine and its derivatives can be considered critical doping units for engineering hybrid C(sp2)-based molecules with tailored optoelectronic properties. Herein, we report the
first synthesis of hexaarylborazines that, bearing ortho-substituted aryl moieties, extend threedimensionally. Using a one-pot protocol, we first form an electrophilic chloroborazole and then react it with an aryl lithium (ArLi). By selecting the appropriate ortho-substituent, we can guide
the ArLi to add to the BN-core in a specific way, ultimately controlling the stereochemical outcome of the three-substitution reaction. Rationalization of the stereochemical model through computational analysis allowed us to show that when aryl lithium nucleophiles bearing rigid long-range ortho-substituents are used, i.e., stiff substituents. The ortho-substituent shields its side of the electrophilic B3N3 core, biasing the incoming ArLi to add anti at each addition step, forming the final tri-aryl borazine exclusively as cc-isomer. Leveraging this stereoselective approach, prototypical multichromophoric borazine derivatives were prepared, and we showcased how the stereochemical arrangement of these chromophores distinctly influences their redox behavior. This methodology paves the way for previously inaccessible borazines to serve as privileged precursors to transcend the conventional bidimensionality associated with graphenoid systems and pioneer the construction of new forms of three-dimensional C(sp2)-based architectures
Toluene Hydrogenation Catalyzed by Pt Nanoparticles: Kinetically Relevant Steps, Binding Ensembles, and Temperature Effects on Turnover Rates.
No full textMetal surfaces mediate the hydrogenation of aromatic rings in arenes, but the diversity of hydrocarbon intermediates formed has hindered conclusive identification of the predominant surface species and kinetically relevant steps. Here, kinetic data and density functional theory calculations are combined to elucidate the mechanism of toluene hydrogenation on Pt nanoparticles at temperatures where turnover rates decrease with increasing temperature (> 490 K), a behavior typical of arene hydrogenations. Concurrent measurements of site-time-yields to methylcyclohexane (MCH), the final product, and the pressures of methylcyclohexene isomers (MCHE), intermediates, reveal that partial toluene hydrogenation to MCHE isomers reaches equilibrium, with the subsequent hydrogenation of MCHE limiting MCH formation rates. MCHE concentrations decrease with increasing temperature due to their exothermic formation, resulting in a concomitant decrease in hydrogenation rates. The transitions states that mediate MCHE hydrogenation are found to form exothermically from gaseous MCHE and H2 reactants (-20 ± 20 kJ mol-1), leading to lower hydrogenation rates as temperatures increase. Kinetic barriers exist, despite the negative enthalpic barrier, due to the entropic penalties for binding and reacting MCHE isomers (-210 ± 40 J mol-1 K-1). These hydrogenation processes occur on surfaces saturated by H-adatoms and by bound methylenebenzene, which forms by abstracting benzylic H from toluene. These species bind to different Pt atom ensembles (1 and 6 Pt atoms for H and methylenebenzene, respectively), which differ from those which mediate hydrogenation (5.3 ± 0.4 Pt atoms). Multi-site kinetic models, developed using lattice statistics to rigorously account for these site requirements, are essential for interpreting reactant pressure and temperature effects. These findings underscore the need for multi-site descriptions of metal-catalyzed reactions, particularly when surface species and transition states differ in sizes, orientations, and numbers of surface contacts
Predicting Liquid-Liquid Phase Separation of Submicron Proxies for Atmospheric Secondary Aerosol
No full textLiquid–liquid phase separation (LLPS) of atmospheric aerosols can significantly impact climate, air quality, and human health. However, their complex composition, small size, and history-dependent properties result in great uncertainty in the modeling of aerosol phase state and atmospheric processes. Herein, using cryogenic transmission electron microscopy (cryo-TEM), we examined model submicron aerosols composed of organic compounds and ammonium sulfate, and established a parameterization for the separation relative humidity (SRH) that accounts for chemical composition, particle size, and equilibration time. We evaluated different variables that describe chemical composition: O/C ratio, partition coefficient, solubility, molar mass, and polarizability. The O/C ratio fits the SRH of micrometer droplets best, and by using a scaling factor to translate the micrometer SRH parameterization to submicron aerosols, we incorporate the effects of size and equilibration time. The measured scaling factor for the submicron mean SRH (30nm – 1m, 20 min equilibration times) is 0.80, the factor becomes 1 with equilibration time over 1 hour, and is equal to 0, meaning that SRH is absent, when the aerosol dry diameter is smaller than 30 nm. Our parameterization will aid in universal SRH modeling, potentially leading to more accurate predictions of aerosol mass, optical properties, hygroscopicity, and heterogeneous chemistry