Diastereoselective Dearomatizing Cyclizations of 5-Arylpentan-2-ones by Samarium Diiodide - A Computational Analysis.

This study analyzes the samarium diiodide-promoted cyclizations of 5-arylpentan-2-ones to dearomatized bicyclic products utilizing density functional theory. The reaction involves a single electron transfer to the carbonyl group, which occurs synchronously with the rate determining cyclization event, and a second subsequent proton-coupled electron transfer. These redox reactions are accurately computed employing small core pseudo potentials explicitly involving all f-electrons of samarium. Comparison of the energies of the possible final products rules out thermodynamic control of the observed regio- and diastereoselectivities. Kinetic control via appropriate transition states is correctly predicted, but to obtain reasonable energy levels the influence of the co-solvent hexamethylphosphortriamide has to be estimated by using a correction term. The steric effect of the bulky samarium ligands is decisive for the observed stereoselectivity. Carbonyl groups in para-position of the aryl group change the regioselectivity of the cyclization and lead to spiro compounds. The computations suggest again kinetic control of this deviating outcome. However, the standard mechanism has to be modified and the involvement of a complex activated by two SmI2 moieties is proposed in which two electrons are transferred simultaneously to form the new C-C bond. Computation of model intermediates show the feasibility of this alternative+ mechanism.

Power of the Disulfide Bond: An Ideal Random Copolymerization of Biodegradable Redox-Responsive Hyperbranched Polyglycerols.

The development of copolymerization techniques that can randomly incorporate biodegradable moieties into the hyperbranched polyglycerol backbone is an option to prevent its bioaccumulation in vivo. In this study, redox-responsive and biocompatible hyperbranched polyglycerol copolymers of glycidol and 1,4,5-oxadithiepan-2-one were synthesized with an adjustable molecular weight and a defined disulfide bond content through anionic and coordination-insertion ring-opening polymerization. A truly random incorporation of the monomers was achieved under both copolymerization mechanisms. The copolymers were further characterized in terms of their aggregation behavior in solution, degradability, in vitro cell viability, and blood compatibility for potential future biomedical applications. Transmission electron microscopy revealed that the copolymer assembled into nanoparticles with a size range of 20 nm. The copolymers underwent degradation when incubated with two different reducing agents, resulting in smaller fragments of the polymer with thiol end groups. The copolymers demonstrated good biocompatibility, making them suitable for further investigation in biomedical applications.

Graphitic Armor: A Natural Molecular Sieve for Robust Hydrogen Electroxidation.

Carbon coating layers have been found to improve the catalytic performance of transition metals, which is usually explained as an outcome of electronic synergistic effect. Herein we reveal that the defective graphitic carbon, with a unique interlayer gap of 0.342 nm, can be a highly selective natural molecular sieve. It allows efficient diffusion of hydrogen molecules or radicals both along the in-plane and out-of-plane direction, but sterically hinders the diffusion of molecules with larger kinetic diameter (e.g., CO and O2) along the in-plane direction. As a result, poisonous species lager than 0.342 nm are sieved out, even when their adsorption on the metal is thermodynamically strong; at the same time, the interaction between H2 and the metal is not affected. This natural molecular sieve provides a very chance for constructing robust metal catalysts for hydrogen-relevant processes, which are more tolerant to chemical or electrochemical oxidation or CO-relevant poisoning.

Chemical bonding of HF, HCl, and H 2 O onto YF 3 surfaces: Quantification by first principles.

The surfaces of waimirite ß- YF 3 have been studied for their fluorine and chlorine versus water affinity. Bonding patterns of HF, HCl, and H 2 O chemically adsorbed onto surfaces of (010), (100), (011), and (101) have been quantified by density functional theory applying energy decomposition analysis. We found that the adsorption of H 2 O is dominated by about 65% of electrostatics, which causes a low surface sensitivity and weak interactions. On the contrary, the adsorptions of HF and HCl are driven by strong hydrogen bonds resulting in a highly surface-dependent ratio of 30-60% electrostatic versus orbital contribution. Among the stoichiometric surfaces, the shortest and strongest hydrogen bonds and consequently most covalent bonding patterns are found within YF 3 · HCl. However, when including the preparation energy, each surface favors the adsorption of HF over HCl, which reproduces the higher affinity of yttrium towards fluoride over chloride, previously known for solutions, also for the solid state.

The Interplay of Weakly Coordinating Anions and the Mechanical Bond: A Systematic Study of the Explicit Influence of Counterions on the Properties of (Pseudo)rotaxanes.

Weakly coordinating anions (WCAs) have attracted much attention in recent years due to their ability to stabilise highly reactive cations. It may well be argued, however, that a profound understanding of what truly defines a WCA is still lacking, and systematic studies to unravel counterion effects are scarce. In this work, we investigate a supramolecular pseudorotaxane formation reaction, subject to a selection of anions, ranging from strongly to weakly coordinating, which not only aids in fostering our knowledge about anion coordination properties, but also provides valuable theoretical insight into the nature of the mechanical bond. We employ state-of-the-art DFT-based methods and tools, combined with isothermal calorimetry and 1H NMR experiments, to compute anion-dependent Gibbs free association energies ΔGa, as well as to evaluate intermolecular interactions. We find correlations between ΔGa and the anions' solvation energies, which are exploited to calculate physico-chemical reaction parameters in the context of coordinating anions. Furthermore, we show that the binding situation within the (pseudo)rotaxanes can be mostly understood by straight-forward electrostatic considerations. However, quantum-chemical effects such as dispersion and charge-transfer interactions become more and more relevant when WCAs are employed.

Phospholenes from Phosphabenzenes by Selective Ring Contraction.

A 3-amino-functionalized phosphabenzene (phosphinine) has been synthesized and structurally characterized. The pyramidalized nitrogen atom of the dimethylamino substituent indicates only a weak interaction between the lone pair of the nitrogen atom and the aromatic phosphorus heterocycle, resulting in somewhat basic character. It turned out that the amino group can indeed be protonated by HCl. In contrast to pyridines, however, the phosphabenzene-ammonium salt undergoes a selective ring contraction to form a hydroxylphospholene oxide in the presence of additional water. Based on deuterium labeling experiments and quantum chemical calculations, a rational mechanism for this hitherto unknown conversion is proposed.

Catalytic Potential of Post-Transition Metal Doped Graphene-Based Single-Atom Catalysts for the CO2 Electroreduction Reaction.

Catalysts are required to ensure electrochemical reduction of CO2 to fuels proceeds at industrially acceptable rates and yields. As such, highly active and selective catalysts must be developed. Herein, a density functional theory study of p-block element and noble metal doped graphene-based single-atom catalysts in two defect sites for the electrochemical reduction of CO2 to CO and HCOOH is systematically undertaken. It is found that on all of the systems considered, the thermodynamic product is HCOOH. Pb/C3 , Pb/N4 and Sn/C3 are identified as having the lowest overpotential for HCOOH production while Al/C3 , Al/N4 , Au/C3 and Ga/C3 are identified as having the potential to form higher order products due to the strength of binding of adsorbed HCOOH.

Theoretical modeling of molecules in weakly interacting environments: trifluoride anions in argon.

The properties of molecules can be affected by the presence of a host environment. Even in inert rare gas matrices such effects are observable, as for instance in matrix isolation spectroscopy. In this work we study the trifluoride anion in cryogenic argon environments. To investigate the structure and vibrational properties of the guest-host systems, a potential energy surface of compound F-3-argon structures is determined from ab initio calculations with the CCSD(T)-F12b approach. Argon environments are probed with minima hopping optimizations of extended trifluoride-argon clusters. The vibrations of F-3 within the optimized environments are examined with anharmonic vibrational analyses. Among the three identified structural surroundings for the trifluoride, two are characterized by relatively favorable guest-host and host-host interactions as well as vibrational zero-point energies. A striking dependence of the trifluoride properties on the particular argon environment reveals the delicate influence of the host atoms on the guest molecule. Very good agreement with measured data suggests that in experiment F-3 occupies a double-vacancy site.

Samarium Diiodide Acting on Acetone-Modeling Single Electron Transfer Energetics in Solution.

Samarium diiodide is a versatile single electron transfer (SET) agent with various applications in organic chemistry. Lewis structures regularly insinuate the existence of a ketyl radical when samarium diiodide binds a carbonyl group. The study presented here investigates this electron transfer by the means of computational chemistry. All electron CASPT2 calculations with the inclusion of scalar relativistic effects predict an endotherm electron transfer from samarium diiodide to acetone. Energies calculated with the PBE0-D3(BJ) functional and a small core pseudopotential are in good agreement with CASPT2. The calculations confirm the experimentally measured increase of the samarium diiodide reduction potential through the addition of hexamethylphosphoramide also known as HMPA.

Resolving the Dilemma of Fe-N-C Catalysts by the Selective Synthesis of Tetrapyrrolic Active Sites via an Imprinting Strategy.

Combining the abundance and inexpensiveness of their constituent elements with their atomic dispersion, atomically dispersed Fe-N-C catalysts represent the most promising alternative to precious-metal-based materials in proton exchange membrane (PEM) fuel cells. Due to the high temperatures involved in their synthesis and the sensitivity of Fe ions toward carbothermal reduction, current synthetic methods are intrinsically limited in type and amount of the desired, catalytically active Fe-N4 sites, and high active site densities have been out of reach (dilemma of Fe-N-C catalysts). We herein identify a paradigm change in the synthesis of Fe-N-C catalysts arising from the developments of other M-N-C single-atom catalysts. Supported by DFT calculations we propose fundamental principles for the synthesis of M-N-C materials. We further exploit the proposed principles in a novel synthetic strategy to surpass the dilemma of Fe-N-C catalysts. The selective formation of tetrapyrrolic Zn-N4 sites in a tailor-made Zn-N-C material is utilized as an active-site imprint for the preparation of a corresponding Fe-N-C catalyst. By successive low- and high-temperature ion exchange reactions, we obtain a phase-pure Fe-N-C catalyst, with a high loading of atomically dispersed Fe (>3 wt %). Moreover, the catalyst is entirely composed of tetrapyrrolic Fe-N4 sites. The density of tetrapyrrolic Fe-N4 sites is more than six times as high as for previously reported tetrapyrrolic single-site Fe-N-C fuel cell catalysts.

Local current analysis on defective zigzag graphene nanoribbons devices for biosensor material applications.

In this contribution, we aim at investigating the mechanism of biosensing in graphene-based materials from first principles. Inspired by recent experiments, we construct an atomistic model composed of a pyrene molecule serving as a linker fragment, which is used in experiment to attach certain aptamers, and a defective zigzag graphene nanoribbons (ZGNRs). Density functional theory including dispersive interaction is employed to study the energetics of the linker absorption on the defective ZGNRs. Combining non-equilibrium Green's function and the Landauer formalism, the total current-bias voltage dependence through the device is evaluated. Modifying the distance between the linker molecule and the nanojunction plane reveals a quantitative change in the total current-bias voltage dependence, which correlates to the experimental measurements. In order to illuminate the geometric origin of these variation observed in the considered systems, the local currents through the device are investigated using the method originally introduced by Evers and co-workers. In our new implementation, the numerical efficiency is improved by applying sparse matrix storage and spectral filtering techniques, without compromising the resolution of the local currents. Local current density maps qualitatively demonstrate the local variation of the interference between the linker molecule and the nanojunction plane.

Ino-Chloridolithates from Ionothermal Synthesis.

Anionic lithium-containing species were predicted to impact ionic liquid-based electrochemical applications but have hitherto never been isolated from ionic liquid systems. Here, we report the first representatives of this class of compounds, ino-chloridolithates, comprising [LiCl2]- and [Li2Cl3]- polyanions from ionothermal reactions. Such compounds are obtained at moderate temperatures with imidazolium-based ionic liquids and LiCl. The addition of an auxiliary ammonium salt enhances the lattice energy to yield an ammonium lithate in good yields, which enables extensive investigations including solid-state nuclear magnetic resonance, infrared, and Raman spectroscopy. The structural motifs of ino-lithates are related to ino-silicates, as 1D-extended anionic substructures are formed. Despite this analogy, according to density functional theory calculations with periodic boundary conditions, no evidence of covalent bonding in the anionic moieties is found-indicating packing effects to be the main cause for the formation. Based on an in-depth analysis of the different synthetic parameters, this class of compounds is discussed as an intermediate in ionic liquid applications and could serve as a model system for electrochemical lithium-based systems.

A pair potential modeling study of F3- in neon matrices.

In this study, the structural and vibrational properties of a trifluoride anion trapped in solid neon are investigated. For that, a potential energy surface based on a truncated many-body expansion scheme is constructed from explicitly correlated coupled cluster calculations. Cluster modeling and minima hopping optimizations are used to evaluate different neon environments, revealing a dominant underlying structural motif in the guest-host system. Moreover, vibrational analyses of the trapped trifluoride anion are performed. These show the subtle ways in which the neon matrix affects the vibrational properties of the trifluoride. In particular, the vibrational states are slightly compressed and fundamental transitions are blue-shifted within the matrix. Also, the calculated vibrational transition energies are in quantitative agreement with available experimental observations, validating the employed procedure for future applications.

Clustering of metal dopants in defect sites of graphene-based materials.

Single-atom catalysts are promising candidates for many industrial reactions. However, making true single-atom catalysts is an experimental dilemma, due to the difficulty of keeping dopant single atoms stable at temperature and under pressure. This difficulty can lead to clustering of the metal dopant atoms in defect sites. However, the electronic and geometric structure of sub-nanoscale clusters in single-atom defects has not yet been explored. Furthermore, recent studies have proven sub-nanoscale clusters of dopants in single-atom defect sites can be equally good or better catalysts than their single-atom counterparts. Here, a comprehensive DFT study is undertaken to determine the geometric and electronic structure effects that influence clustering of noble and p-block dopants in C3- and N4-defect sites in graphene-based systems. We find that the defect site is the primary driver in determining clustering dynamics in these systems.

Aggregation-induced emission leading to two distinct emissive species in the solid-state structure of high-dipole organic chromophores.

The concept of aggregation-induced emission represents a means to rationalise photoluminescence of usually nonfluorescent excimers in solid-state materials. In this publication, we study the photophysical properties of selected diaminodicyanoquinone (DADQ) derivatives in the solid state using a combined approach of experiment and theory. DADQs are a class of high-dipole organic chromophores promising for applications in non-linear optics and light-harvesting devices. Among the compounds investigated, we find both aggregation-induced emission and aggregation-caused quenching effects rationalised by calculated energy transfer rates. Analysis of fluorescence spectra and lifetime measurements provide the interesting result that (at least) two emissive species seem to contribute to the photophysical properties of DADQs. The main emission peak is notably broadened in the long-wavelength limit and exhibits a blue-shifted shoulder. We employ high-level quantum-chemical methods to validate a molecular approach to a solid-state problem and show that the complex emission features of DADQs can be attributed to a combination of H-type aggregates, monomers, and crystal structure defects.

The Peculiar Interaction of Trifluoride Anions with Cryogenic Rare Gas Matrices.

In this contribution, we present theoretical modeling of the interaction between rare gas matrices and a trifluoride guest anion, as well as its quantitative effect on measured vibrational spectra. Using a combination of coupled-cluster electronic structure calculations and a many-body potential expansion coupled with permutation invariant polynomial fitting and anharmonic vibrational spectrum simulations, we shed light on the origin of the trifluoride matrix effects observed experimentally. The theoretical spectra are found to reproduce accurately the measured data while providing deeper insights into the effects of the guest-host interaction. The investigations reveal that neon can only stabilize trifluoride in hexagonal cavities formed by double vacancies, while argon can host the anion in a variety of cavities ranging from zero to two defects in the matrix. The origin of this structural variability can be traced back to the disparate strengths of the host-host interactions in neon and argon. The present work demonstrates the importance of theoretical modeling to complement matrix isolation experiments, which alone do not provide direct information about the structure of the matrices or about the physical origin of their interaction and of their spectroscopic signature.

Electronic and optical properties of fluorinated graphene within many-body Green's function framework.

In this article, a systematic examination of the electronic and optical properties of partially fluorinated graphene is presented. In order to capture a large variety of fluorination degrees and configurations, different sizes of the supercell combining with various degrees of fluorination are considered. On top of periodic density functional theory, the G0W0 method and the G0W0Γ method within many-body Green's function framework are employed. Including the description of electron-hole interactions, the optical spectra based on the Bethe-Salpeter equation are calculated. Two-sided fluorination with compact fluorination arrangements is energetically most favorable. The fluorination degree has a determined impact on the bandgap value in the system, while the fluorination pattern strongly influences the characteristics of the bands in the electronic structures. Depending on the polarization of the applied electromagnetic field, the optical absorption spectra of the same structure could vary significantly. These interesting results suggest the potential applications of partially fluorinated graphene as optoelectronic materials.

Metal-Assisted and Solvent-Mediated Synthesis of Two-Dimensional Triazine Structures on Gram Scale.

Covalent triazine frameworks are an emerging material class that have shown promising performance for a range of applications. In this work, we report on a metal-assisted and solvent-mediated reaction between calcium carbide and cyanuric chloride, as cheap and commercially available precursors, to synthesize two-dimensional triazine structures (2DTSs). The reaction between the solvent, dimethylformamide, and cyanuric chloride was promoted by calcium carbide and resulted in dimethylamino-s-triazine intermediates, which in turn undergo nucleophilic substitutions. This reaction was directed into two dimensions by calcium ions derived from calcium carbide and induced the formation of 2DTSs. The role of calcium ions to direct the two-dimensionality of the final structure was simulated using DFT and further proven by synthesizing molecular intermediates. The water content of the reaction medium was found to be a crucial factor that affected the structure of the products dramatically. While 2DTSs were obtained under anhydrous conditions, a mixture of graphitic material/2DTSs or only graphitic material (GM) was obtained in aqueous solutions. Due to the straightforward and gram-scale synthesis of 2DTSs, as well as their photothermal and photodynamic properties, they are promising materials for a wide range of future applications, including bacteria and virus incapacitation.

A Validation of Cluster Modeling in the Description of Matrix Isolation Spectroscopy.

Matrix isolation is a fundamental tool for the synthesis and characterization of highly reactive novel species and investigation of unusual bonding situations. Ab initio descriptions of guest-host interactions in matrix isolation are highly demanding, as the weak interactions between guest and host can influence the former's oftentimes challenging electronic structure. In this study, the matrix effects on a single CO2 molecule in an argon matrix were investigated with dispersion-corrected density functional theory calculations. Three different guest-host structures were described by bulk models employing periodic boundary conditions as well as cluster models. The calculations were analyzed with respect to structural features of the CO2 molecule and its immediate surroundings. Also, the molecule's harmonic frequencies were determined. The calculated frequencies were in qualitative agreement with experimental observations. The cluster models produced comparable results given that the clusters were large enough to reproduce the structural features of the bulk model. © 2019 Wiley Periodicals, Inc.

Combining Theory and Experiment to Characterize the Voltammetric Behavior of Nickel Anodes in the Simons Process.

The Simons process, otherwise known as the electrochemical fluorination (ECF) method, is widely used in industry to electrolytically synthesize chemicals for various purposes. Even to this day, the exact mechanism of the ECF reaction remains unknown, but is believed to involve the formation of an anodic nickel fluoride film with highly oxidized nickel centers. In this study, experiments and density functional theory calculations are combined to characterize the initial anodic peak occurring at potentials typically required in an ECF cell. NiF2 is believed to form a passivating layer at low potentials. The calculations show that a potential of +3.1 V is required to oxidize surface Ni2+ centers to Ni3+ . This is in good agreement with the measured anodic peak at +3.57 V.