Taming Molecular Folding: Anion-Templated Foldamers with Tunable Quaternary Structures.

Higher-order foldamers represent a unique class of supramolecules at the forefront of molecular design. Herein we control quaternary folding using a novel approach that combines halogen bonding (XBing) and hydrogen bonding (HBing). We present the first anion-templated double helices induced by halogen bonds (XBs) and stabilized by "hydrogen bond enhanced halogen bonds" (HBeXBs). Our findings demonstrate that the number and orientation of hydrogen bond (HB) and XB donors significantly affect the quaternary structure and guest selectivity of two similar oligomers. This research offers new design elements to engineer foldamers and tailor their quaternary structure for specific guest binding.

Adsorption, Orientation, and Speciation of Amino Acids at Air-Aqueous Interfaces for the Direct Air Capture of CO2.

Amino acids make up a promising family of molecules capable of direct air capture (DAC) of CO2 from the atmosphere. Under alkaline conditions, CO2 reacts with the anionic form of an amino acid to produce carbamates and deactivated zwitterionic amino acids. The presence of the various species of amino acids and reactive intermediates can have a significant effect on DAC chemistry, the role of which is poorly understood. In this study, all-atom molecular dynamics (MD) based computational simulations and vibrational sum frequency generation (vSFG) spectroscopy studies were conducted to understand the role of competitive interactions at the air-aqueous interface in the context of DAC. We find that the presence of potassium bicarbonate ions, in combination with the anionic and zwitterionic forms of amino acids, induces concentration and charge gradients at the interface, generating a layered molecular arrangement that changes under pre- and post-DAC conditions. In parallel, an enhancement in the surface activity of both anionic and zwitterionic forms of amino acids is observed, which is attributed to enhanced interfacial stability and favorable intermolecular interactions between the adsorbed amino acids in their anionic and zwitterionic forms. The collective influence of these competitive interactions, along with the resulting interfacial heterogeneity, may in turn affect subsequent capture reactions and associated rates. These effects underscore the need to consider dynamic changes in interfacial chemical makeup to enhance DAC efficiency and to develop successful negative emission and storage technologies.

Molecular Design of Functional Polymers for Silica Scale Inhibition.

Silica polymerization, which involves the condensation reaction of silicic acid, is a fundamental process with wide-ranging implications in biological systems, material synthesis, and scale formation. The formation of a silica-based scale poses significant technological challenges to energy-efficient operations in various industrial processes, including heat exchangers and water treatment membranes. Despite the common strategy of applying functional polymers for inhibiting silica polymerization, the underlying mechanisms of inhibition remain elusive. In this study, we synthesized a series of nitrogen-containing polymers as silica inhibitors and elucidated the role of their molecular structures in stabilizing silicic acids. Polymers with both charged amine and uncharged amide groups in their backbones exhibit superior inhibition performance, retaining up to 430 ppm of reactive silica intact for 8 h under neutral pH conditions. In contrast, monomers of these amine/amide-containing polymers as well as polymers containing only amine or amide functionalities present insignificant inhibition. Molecular dynamics simulations reveal strong binding between the deprotonated silicic acid and a polymer when the amine groups in the polymer are protonated. Notably, an extended chain conformation of the polymer is crucial to prevent proximity between the interacting monomeric silica species, thereby facilitating effective silica inhibition. Furthermore, the hydrophobic nature of alkyl segments in polymer chains disrupts the hydration shell around the polymer, resulting in enhanced binding with ionized silicic acid precursors compared to monomers. Our findings provide novel mechanistic insights into the stabilization of silicic acids with functional polymers, highlighting the molecular design principles of effective inhibitors for silica polymerization.

Unravelling photoisomerization dynamics in a metastable-state photoacid.

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Exploring Cr and molten salt interfacial interactions for molten salt applications.

Molten salts play an important role in various energy-related applications such as high-temperature heat transfer fluids and reaction media. However, the extreme molten salt environment causes the degradation of materials, raising safety and sustainability challenges. A fundamental understanding of material-molten salt interfacial evolution is needed. This work studies the transformation of metallic Cr in molten 50/50 mol% KCl-MgCl2via multi-modal in situ synchrotron X-ray nano-tomography, diffraction and spectroscopy combined with density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Notably, in addition to the dissolution of Cr in the molten salt to form porous structures, a δ-A15 Cr phase was found to gradually form as a result of the metal-salt interaction. This phase change of Cr is associated with a change in the coordination environment of Cr at the interface. DFT and AIMD simulations provide a basis for understanding the enhanced stability of δ-A15 Cr vs. bcc Cr, by revealing their competitive phase thermodynamics at elevated temperatures and probing the interfacial behavior of the molten salt at relevant facets. This study provides critical insights into the morphological and chemical evolution of metal-molten salt interfaces. The combination of multimodal synchrotron analysis and atomic simulation also offers an opportunity to explore a broader range of systems critical to energy applications.

Direct Extraction of Sodium Hydroxide by Calix[4]pyrrole-Based Ion-Pair Receptors.

Described in this work are calix[4]pyrrole-based ion-pair receptors, cis/trans-1 and cis/trans-2, designed for the extraction of sodium hydroxide. An X-ray diffraction analysis of a single crystal of the cis-1·NaOH isomer isolated from a mixture of cis/trans-1 revealed a unique dimeric supramolecular structure. An average dimer in toluene-d8 solution was inferred on the basis of diffusion-ordered spectroscopy (DOSY). Support for the proposed stoichiometry came from density functional theory (DFT) calculations. The structural stability of the dimeric cis-1·NaOH complex in toluene solution was further confirmed by ab initio molecular dynamics (AIMD) simulation with explicit representation of solvent. Under conditions of liquid-liquid extraction (LLE), purified receptors cis- and trans-2 were both found to remove NaOH from a pH 11.01 aqueous source phase into toluene with extraction efficiencies (E%) of 50-60% when used equimolar to NaOH. However, in all cases, precipitation was observed. Complexities associated with precipitation could be avoided by immobilization of the receptors onto a chemically inert poly(styrene) resin by means of solvent impregnation. The use of solvent-impregnated resins (SIRs) eliminated precipitation in solution while retaining the extraction efficiency toward NaOH. This allowed both the pH and salinity of the alkaline source phase to be lowered.

Photochemically-Driven CO2 Release Using a Metastable-State Photoacid for Energy Efficient Direct Air Capture.

One of the grand challenges underlying current direct air capture (DAC) technologies relates to the intensive energy cost for sorbent regeneration and CO2 release, making the massive scale (GtCO2 /year) deployment required to have a positive impact on climate change economically unfeasible. This challenge underscores the critical need to develop new DAC processes with substantially reduced regeneration energies. Here, we report a photochemically-driven approach for CO2 release by exploiting the unique properties of an indazole metastable-state photoacid (mPAH). Our measurements on simulated and amino acid-based DAC systems revealed the potential of mPAH to be used for CO2 release cycles by regulating pH changes and associated isomers driven by light. Upon irradiating with moderate intensity light, a ≈55 % and ≈68 % to ≈78 % conversion of total inorganic carbon to CO2 was found for the simulated and amino acid-based DAC systems, respectively. Our results confirm the feasibility of on-demand CO2 release under ambient conditions using light instead of heat, thereby providing an energy efficient pathway for the regeneration of DAC sorbents.

Complete Description of the LaCl3-NaCl Melt Structure and the Concept of a Spacer Salt That Causes Structural Heterogeneity.

Lanthanides are important fission products in molten salt reactors, and understanding their structure and that of their mixtures is relevant to many scientific and technological problems including the recovery and separation of rare earth elements using molten salt electrolysis. The literature on molten salts and specifically on LaCl3 and LaCl3-NaCl mixtures is often fragmented, with different experiments and simulations coinciding in their explanation for certain structural results but contradicting or questioning for others. Given the very practical importance that actinide and lanthanide salts have for energy applications, it is imperative to arrive at a clear unified picture of their local and intermediate-range structure in the neat molten state and when mixed with other salts. This article aims to unequivocally answer a set of specific questions: is it correct to think of long-lived octahedral coordination structures for La3+? What is the nature as a function of temperature of networks and intermediate-range order particularly upon dilution of the trivalent ion salt? Is the so-called scattering first sharp diffraction peak (FSDP) for neat LaCl3 truly indicative of intermediate-range order? If so, why is there a new lower-q peak when mixed with NaCl? Are X-ray scattering and Raman spectroscopy results fully consistent and easily described by simulation results? We will show that answers to these questions require that we abandon the idea of a most prominent coordination state for M3+ ions and instead think of multiple competing coordination states in exchange due to significant thermal energy in the molten state.

A Photoresponsive Receptor with a 105 Magnitude of Reversible Anion-Binding Switching.

Invited for the cover of this issue are Jeffrey Einkauf, Vyacheslav Bryantsev, Bruce Moyer, and Radu Custelcean from Oak Ridge National Laboratory. The image depicts an anion receptor functionalized with a new photoswitchable chromophore, the diiminoguanidinium group, with exceptionally strong sulfate-binding affinity that can be turned off by photoirradiation with UV light. Read the full text of the article at 10.1002/chem.202200719.

A Photoresponsive Receptor with a 105 Magnitude of Reversible Anion-Binding Switching.

In a leap toward anion separation that uses only energy input for binding and release cycles, we report herein a new class of photoswitchable anion receptors featuring a diiminoguanidinium functionality that displays a change of more than five orders of magnitude in switched-off binding strength towards sulfate, a representative oxyanion, upon photoirradiation with UV light. The (E,E)-2-pyridyl-diiminoguanidinium cation, synthesized as the triflate salt, binds sulfate with extraordinary strength in [D6 ]DMSO owing to its bidentate guanidinium hydrogen bonding, which can chelate the O-S-O edge of sulfate. Upon photoisomerization to the Z,Z isomer, the anion-binding site is essentially shut off by intramolecular hydrogen bonds to the 2-pyridyl substituents, as shown by anion-binding titrations, theoretical calculations, and X-ray structural analysis. This approach will allow the development of advanced anion-separation cycles that use only energy input and generate no chemical waste, and thus address challenging chemical separation problems in a more sustainable way.

Improving Rare-Earth Mineral Separation with Insights from Molecular Recognition: Functionalized Hydroxamic Acid Adsorption onto Bastnäsite and Calcite.

Enhancing the separation of rare-earth elements (REEs) from gangue materials in mined ores requires an understanding of the fundamental interactions driving the adsorption of collector ligands onto mineral interfaces. In this work, we examine five functionalized hydroxamic acid ligands as potential collectors for the REE-containing bastnäsite mineral in froth flotation using density functional theory calculations and a suite of surface-sensitive analytical spectroscopies. These include vibrational sum frequency generation, attenuated total reflectance Fourier transform infrared, Raman, and X-ray photoelectron spectroscopies. Differences in the chemical makeup of these ligands on well-defined bastnäsite and calcite surfaces allow for a systematic relationship connecting the structure to adsorption activity to be framed in the context of interfacial molecular recognition. We show how the intramolecular hydrogen bonding of adsorbed ligands requires the inclusion of explicit water solvent molecules to correctly map energetic and structural trends measured by experiments. We anticipate that the results and insights from this work will motivate and inform the design of improved flotation collectors for REE ores.

Two Ligands of Interest in Recovering Uranium from the Oceans: The Correct Formation Constants of the Uranyl(VI) Cation with 2,2'-Bipyridyl-6,6'-dicarboxylic Acid and 1,10-Phenanthroline-2,9-dicarboxylic Acid.

The ligands BDA (2,2'-bipyridyl-6,6'-dicarboxylic acid) and PDA (1,10-phenanthroline-2,9-dicarboxylic acid) are of interest as functional group types for ion-exchange materials for extracting uranium from the oceans, reported in a previous paper for PDA Lashley, M. A. ( Inorg. Chem. 2016 55 10818 10829). Yang, Y. ( Inorg. Chem. 2019, 58, 6064 6074) have published what they claim to be a more accurate result for the formation of the UO22+/PDA complex of log K1 = 22.84 compared with our reported value of log K1 = 16.5, as well as log K1 = 21.52 for the BDA complex. The determination of log K1 for the PDA and BDA complexes with the UO22+ cation was carried out by Yang et al. using a competition reaction between DTPA (diethylenetriamine pentaacetic acid) and BDA or PDA, monitoring the absorbance due to the BDA and PDA ligands. This competition method using absorbance versus pH titrations was developed for determining the formation constants of the complexes of several polypyridyl ligands plus PDA complexes of metal ions, which were too stable for log K determination by competition with protons. A key feature of such titrations is that in the competition reaction, the displacement of the pyridyl donor ligand (e.g., PDA) by the competing ligand (e.g., DTPA), the absorbance spectrum of the displaced pyridyl donor ligand should be observed. Competing ligands used to date have been EDTA (ethylenediamine tetraacetic acid), DTPA, or the hydroxide ion. In the study of Yang et al., no such displaced PDA or BDA was apparent in the absorbance spectra in their titrations so that their reported log K1 values have no validity. Their log K1 values are so much higher than log K1 for the uranyl DTPA complex (∼13.6) that DTPA could not possibly displace BDA or PDA from the uranyl cation, and a competition reaction could not possibly occur. We report the correct value of log K1 = 15.4 (ionic strength = zero) for the uranyl BDA complex, to illustrate the correct determination of such a constant by a competition reaction between BDA and hydroxide, showing how the characteristic absorbance spectrum for a BDA complex, here the UO22+ complex, disappears, and the distinctive absorbance spectrum of the free nonprotonated BDA ligand appears as the pH is increased, and BDA is displaced by the hydroxide ion.

A Holistic Approach for Elucidating Local Structure, Dynamics, and Speciation in Molten Salts with High Structural Disorder.

To examine ion solvation, exchange, and speciation for minority components in molten salts (MS) typically found as corrosion products, we propose a multimodal approach combining extended X-ray absorption fine structure (EXAFS) spectroscopy, optical spectroscopy, ab initio molecular dynamics (AIMD) simulations, and rate theory of ion exchange. Going beyond conventional EXAFS analysis, our method can accurately quantify populations of different coordination states of ions with highly disordered coordination environments via linear combination fitting of the EXAFS spectra of these coordination states computed from AIMD to the experimental EXAFS spectrum. In a case study of dilute Ni(II) dissolved in the ZnCl2+KCl melts, our method reveals heterogeneous distributions of coordination states of Ni(II) that are sensitive to variations in temperature and melt composition. These results are fully explained by the difference in the chloride exchange dynamics at varied temperatures and melt compositions. This insight will enable a better understanding and control of ion solubility and transport in MS.

Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process.

Chromium (VI) and arsenic (V) oxoanions are major toxic heavy metal pollutants in water threatening both human health and environmental safety. Herein, the development is reported of a bifunctional ionic covalent organic network (iCON) with integrated guanidinium and phenol units to simultaneously sequester chromate and arsenate in water via a synergistic ion-exchange-redox process. The guanidinium groups facilitate the ion-exchange-based adsorption of chromate and arsenate at neutral pH with fast kinetics and high uptake capacity, whereas the integrated phenol motifs mediate the Cr(VI)/Cr(III) redox process that immobilizes chromate and promotes the adsorption of arsenate via the formation of Cr(III)-As(V) cluster/complex. The synergistic ion-exchange-redox approach not only pushes high adsorption efficiency for both chromate and arsenate but also upholds a balanced Cr/As uptake ratio regardless of the change in concentration and the presence of interfering oxoanions.

Improving the theoretical description of Ln(III)/An(III) separation with phosphinic acid ligands: a benchmarking study of structure and selectivity.

The efficient separation of trivalent lanthanides from minor actinides with soft-donor ligands, while showing experimental promise, has theorists continuing to search for suitable approaches for describing and interpreting selectivity. To remedy this, we employ several computational methods in describing the structure of model M(H2PX2)3 complexes, with M = Eu and Am, and X = O, S, Se, and Te, and predicting the selectivity of model phosphinic acid ligands in Eu(III)/Am(III) separation. After first establishing a set of MP2 and CCSD(T)-DKH3 results as benchmarks, we evaluate several density functionals and descriptions of valence shells for their accuracy with respect to metal-ligand bonding and selectivity. We find that commonly employed functionals with a 0-27% range of exact exchange used with small-core effective core potentials or with an explicit treatment of the relativistic effects (DKH2) incorrectly predict a decrease in the metal-ligand bond distance in going from Eu(III) to Am(III) and completely fail to track a selectivity trend, even giving a wrong sign for some or all ligands. Surprisingly, when these functionals are used in conjunction with an f-in-core description of metal ions, the correct trend in selectivity is recovered, though Am-X distances are overestimated in relation to Eu-X. Functionals with high components of exact exchange (50%) and double-hybrid functionals are reasonably aligned with benchmark results, pointing to the problems of DFT with small exact exchange fractions to handle f-electrons. Natural bond orbital analyses reveal that these poorly performing functionals disproportionately overpopulate outer f orbitals in the model complexes. We anticipate that recommendations resulting from this work will lead to more accurate theoretical descriptions of lanthanide/actinide selectivity with soft-donor chalcogen-based ligands in the future.

Spectroscopic characterization of neptunium(VI), plutonium(VI), americium(VI) and neptunium(V) encapsulated in uranyl nitrate hexahydrate.

The coordination of crystalline products resulting from the co-crystallization of Np(vi), Pu(vi), Am(vi), and Np(v) with uranyl nitrate hexahydrate (UNH) has been revealed through solid-state spectroscopic characterization via diffuse reflectance UV-Vis-NIR spectroscopy, SEM-EDS, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Density functional and multireference wavefunction calculations were performed to analyze the An(vi/v)O2(NO3)2·2H2O electronic structures and to help assign the observed transitions in the absorption spectra. EXAFS show a similar coordination between the U(VI) in UNH and Np(vi) and Pu(vi); while Am resulted in a similar coordination to Am(iii), as reduction of Am(vi) occurred prior to EXAFS data being obtained. The co-crystallization of the oxidized transuranic species-penta- and hexavalent-with UNH, represents a significant advance from not only a practical standpoint in providing an elegant solution for used nuclear fuel recycle, but also as an avenue to expand the fundamental understanding of the 5f electronic behavior in the solid-state.

Theoretical, Solid-State, and Solution Quantification of the Hydrogen Bond-Enhanced Halogen Bond.

Proximal noncovalent forces are commonplace in natural systems and understanding the consequences of their juxtaposition is critical. This paper experimentally quantifies for the first time a Hydrogen Bond-Enhanced Halogen Bond (HBeXB) without the complexities of protein structure or preorganization. An HBeXB is a halogen bond that has been strengthened when the halogen donor simultaneously accepts a hydrogen bond. Our theoretical studies suggest that electron-rich halogen bond donors are strengthened most by an adjacent hydrogen bond. Furthermore, stronger hydrogen bond donors enhance the halogen bond the most. X-ray crystal structures of halide complexes (X- =Br- , I- ) reveal that HBeXBs produce shorter halogen bonds than non-hydrogen bond analogues. 19 F NMR titrations with chloride highlight that the HBeXB analogue exhibits stronger binding. Together, these results form the foundation for future studies concerning hydrogen bonds and halogen bonds in close proximity.

Exploring Soft Donor Character of the N-2-Pyrazinylmethyl Group by Coordinating Trivalent Actinides and Lanthanides Using Aminopolycarboxylates.

The trivalent f-element coordination chemistry of a novel aminopolycarboxylate complexant was investigated. The novel reagent is an octadentate complexant that resembles diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA), but a single N-acetate pendant arm was substituted with a N-2-pyrazinylmethyl functional group. Thermodynamic studies of ligand protonation and trivalent lanthanide, americium and curium, complexation by N-2-pyrazinylmethyldiethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-PzM) emphasize the strong electron withdrawing influence of the N-2-pyrazinylmethyl group. Specifically, DTTA-PzM is more acidic compared to a N-2-pyridinylmethyl-substituted structural equivalent, DTTA-PyM, with a substantial lowering of pK7, corresponding to the protonation of a second aliphatic amine site. The participation of the pyrizyl nitrogen in the metal ion coordination sphere is evident from the fluorescence lifetime decay measurements of metal hydration and the interpretation of the stability constants for ML- and MHL(aq) complexes. The overall conditional stability constants for the trivalent f-element complexation by DTTA-PzM complexes decrease, relative to DTTA-PyM, as expected based on the lower basicity of pyrazine in water relative to pyridine. Replacement of the N-2-pyridinylmethyl group with N-2-pyrazinylmethyl, while enhancing the total acidity of DTTA-PzM, also reduces its softness, as manifested by a small lowering of ß101Am/Nd and liquid-liquid separation of trivalent lanthanides from trivalent americium. Despite this, the 4f/5f separation is doubled when DTTA-PzM replaces DTPA as an aqueous complexant in solvent extraction.

Structure Activity Relationship Approach toward the Improved Separation of Rare-Earth Elements Using Diglycolamides.

The separation of adjacent lanthanides continues to be a challenge worldwide because of the similar physical and chemical properties of these elements and a necessity to advance the use of clean-energy applications. Herein, a systematic structure-performance relationship approach toward understanding the effect of N-alkyl group characteristics in diglycolamides (DGAs) on the separation of lanthanides(III) from a hydrochloric acid medium is presented. In addition to the three most extensively studied DGA complexants [N,N,N',N'-tetra(n-octyl)diglycolamide, TODGA; N,N,N',N'-tetra(2-ethylhexyl)diglycolamide, TEHDGA; N,N'-dimethyl-N,N'-di(n-octyl)diglycolamide, DMDODGA], 12 new extracting agents with varying substitution patterns were designed to study the interplay of steric and electronic effects that control rare-earth element extraction. Subtle changes in the structure around diglycolamide carbonyl oxygen atoms result in dramatic shifts in the lanthanide extraction strength and selectivity. The effects of the chain length and branching position of N-alkyl substituents in DGAs are elaborated on with the use of experimental, computational, and solution-structure characterization techniques.

Hydration structure and water exchange kinetics at xenotime-water interfaces: implications for rare earth minerals separation.

Hydration of surface ions gives rise to structural heterogeneity and variable exchange kinetics of water at complex mineral-water interfaces. Here, we employ ab initio molecular dynamics (AIMD) simulations and water adsorption calorimetry to examine the aqueous interfaces of xenotime, a phosphate mineral that contains predominantly Y3+ and heavy rare earth elements. Consistent with natural crystal morphology, xenotime is predicted to have a tetragonal prismatic shape, dominated by the {100} surface. Hydration of this surface induces multilayer interfacial water structures with distinct OH orientations, which agrees with recent crystal truncation rod measurements. The exchange kinetics between two adjacent water layers exhibits a wide range of underlying timescales (5-180 picoseconds), dictated by ion-water electrostatics. Adsorption of a bidentate hydroxamate ligand reveals that {100} xenotime surface can only accommodate monodentate coordination with water exchange kinetics strongly depending on specific ligand orientation, prompting us to reconsider traditional strategies for selective separation of rare-earth minerals.