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1. Mass spectroscopy study of the intermediate magic-size cluster species during cooperative cation exchange

   https://doi.org/10.1063/5.0151904  

   Yao, Yuan; Lynch, Reilly; Robinson, Richard D. (July 2023, The Journal of Chemical Physics)

   Cation exchange is a versatile post-synthetic method to explore a wide range of nanoparticle compositions, phases, and morphologies. Recently, several studies have expanded the scope of cation exchange to magic-size clusters (MSCs). Mechanistic studies indicated that MSC cation exchange undergoes a two-stage reaction pathway instead of the continuous diffusion-controlled mechanism found in nanoparticle cation exchange reactions. The cation exchange intermediate, however, has not been well-identified despite it being the key to understanding the reaction mechanism. Only indirect evidence, such as exciton peak shifts and powder x-ray diffraction, has been used to indicate the formation of the cation exchange intermediate. In this paper, we investigate the unusual nature of cation exchange in nanoclusters using our previously reported CdS MSC. High-resolution mass spectra reveal two cation exchanged reaction intermediates [Ag2Cd32S33(L) and AgCd33S33(L), L: oleic acid] as well as the fully exchanged Ag2S cluster. Crystal and electronic structure characterizations also confirm the two-stage reaction mechanism. Additionally, we investigate the Cu/CdS MSC cation exchange reaction and find a similar two-stage reaction mechanism. Our study shows that the formation of dilutely exchanged intermediate clusters can be generally found in the first stage of the MSC cation exchange reaction. By exchanging different cations, these intermediate clusters can access varying properties compared to their unexchanged counterparts. 

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2. Brine formation in cold desert, shallow groundwater systems: Antarctic Ca-Cl brine chemistry controlled by cation exchange, microclimate, and organic matter

   https://doi.org/10.1130/B37251.1  

   King, I.C.; Johnson, J.T.E.; Kuang, L.; Naylor, S.; Subak, T.; Koleszar, A.M.; Levy, J.S. (February 2024, Geological Society of America Bulletin)

   Groundwater in the McMurdo Dry Valleys of Antarctica is commonly enriched in calcium and chloride, in contrast to surface and groundwater in temperate regions, where calcium chemistry is largely controlled by the dissolution of carbonates and sulfates. These Antarctic Ca-Cl brines have extremely low freezing points, which leads to moist soil conditions that persist unfrozen and resist evaporation, even in cold, arid conditions. Several hypotheses exist to explain these unusual excess-calcium solutions, including salt deliquescence and differential salt mobility and cation exchange. Although the cation exchange mechanism was shown to explain the chemistry of pore waters in permafrost cores from several meters depth, it has not been evaluated for near-surface groundwater and wetland features (water tracks) in which excess-calcium pore-water solutions are common. Here, we use soluble salt and exchangeable cation concentrations to determine whether excess calcium is present in water-track brines and if cation exchange could be responsible for calcium enrichment in these cold desert groundwaters. We show that calcium enrichment by cation exchange is not occurring universally across the McMurdo Dry Valleys. Instead, evidence of the present-day formation of Ca-Cl−rich brines by cation exchange is focused in a geographically specific location in Taylor Valley, with hydrological position, microclimate, soil depth, and organic matter influencing the spatial extent of cation exchange reactions. Up-valley sites may be too cold and dry for widespread exchange, and warm and wet coastal sites are interpreted to host sediments whose exchange reactions have already gone to completion. We argue that exchangeable cation ratios can be used as a signature of past freeze-concentration of brines and exchange reactions, and thus could be considered a geochemical proxy for past groundwater presence in planetary permafrost settings. Correlations between water-track organic matter, fine sediment concentration, and cation exchange capacity suggest that water tracks may be sites of enhanced biogeochemical cycling in cold desert soils and serve as a model for predicting how active layers in the Antarctic will participate in biogeochemical cycling during periods of future thaw. 

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3. Ipso Substitution of Aromatic Bromine in Chlorinated Waters: Impacts on Trihalomethane Formation

   https://doi.org/10.1021/acs.est.3c00852  

   Psoras, Andrew W.; McCoy, Seth W.; Reber, Keith P.; McCurry, Daniel L.; Sivey, John D. (January 2023, Environmental Science & Technology)

   Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C–Br bonds into C–Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br–) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters. 

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4. Catalytic Behavior of Mono‐ N ‐Protected Amino‐Acid Ligands in Ligand‐Accelerated C−H Activation by Palladium(II)

   https://doi.org/10.1002/anie.202002484  

   Salazar, Chase A.; Gair, Joseph J.; Flesch, Kaylin N.; Guzei, Ilia A.; Lewis, Jared C.; Stahl, Shannon S. (April 2020, Angewandte Chemie International Edition)

   Abstract Mono‐N‐protected amino acids (MPAAs) are increasingly common ligands in Pd‐catalyzed C−H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C−H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand‐accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand‐to‐metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C−H activation and catalytic C−H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50–100‐fold increase in rate when only 0.05 equivalents of MPAA are present relative to Pd^II. These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to Pd^IIenables a single MPAA to support C−H activation at multiple Pd^IIcenters. 

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5. Cation exchange route to a Eu(II)-containing tantalum oxide

   https://doi.org/10.1016/j.jssc.2023.124338  

   O'Donnell, Shaun; Gabilondo, Eric; Jana, Subhendu; Koldemir, Aylin; Block, Theresa; Whangbo, Myung-Hwan; Kremer, Reinhard; Pöttgen, Rainer; Maggard, Paul A. (December 2023, Journal of Solid State Chemistry)

   Traditional synthetic efforts to prepare Eu(II)-containing oxides have principally involved the use of high temperature reactions starting from EuO or a controlled, highly-reducing, atmosphere. Conversely, chimie douce approaches that are more amenable to the targeted syntheses of new, and potentially metastable, Eu(II)-oxides have yet to be explored. Herein, a cation-exchange route to new Eu(II)-containing oxides, e.g., EuTa4-xO11 (x = 0.04), has been discovered and its structure determined by powder X-ray diffraction (Space group P6322 (#182), a = 6.2539(2) Å; c = 12.3417(2) Å). The compound derives from the cation exchange of Na2Ta4O11, via a reaction with EuBr2 at 1173 K, and replacement by half the number of divalent Eu cations. Rietveld refinements show preferential ordering of the Eu cations over one of the two possible cation sites, i.e., Wyckoff site 2d (~94%; Eu1) versus 2b (~6%; Eu2). Total energy calculations confirm an energetic preference of the Eu cation in the 2d site. Tantalum vacancies of ~1% occur within the layer of Eu cations and TaO6 octahedra, and ~20% partial oxidation of Eu(II) to Eu(III) cations from charge balance considerations. 151Eu M¨ossbauer spectroscopy measured at 78 K found a Eu(II):Eu(III) ratio of 69:31, with a relatively broad line width of the former signal of Γ = 7.6(2) mm s–1. Also, the temperature-dependent magnetic susceptibility could be fitted to a Curie Weiss expression, giving a μeff = 6.2 μB and θCW = 10 K and confirming a mixture of Eu(II)/Eu(III) cations. The optical bandgap of EuTa4-xO11 was found to be ~1.5 eV (indirect), significantly redshifted as compared to ~4.1 eV for Na2Ta4O11. Spin-polarized electronic structure calculations show that this redshift stems from the addition of Eu 4f7 states as a higher-energy valence band. Thus, these results demonstrate a new cation-exchange approach that represents a useful synthetic pathway to new Eu(II)-containing ox- ides for tunable magnetic and optical properties. 

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