Computational Insights into Iron Heterometal Installation in Polyoxovanadate-Alkoxide Clusters.

Polyoxovanadate-alkoxide clusters are a new class of electroactive species with applications in a wide variety of fields from redox catalysis to energy storage. Heterometallic installation in these species can be used to modulate the redox properties of polyoxovanadate-alkoxide clusters and thus their applications. However, the formation mechanism of heterometallic polyoxovanadate alkoxides during the solvothermal process is unknown, limiting our understanding regarding what thermodynamic driving forces and/or kinetic barriers are present in the heterometal insertion. Here, we present a computational study on the nucleation pathways of the iron-functionalized mixed-valent hexameric [VV2VIV3O5(µ6-O)(µ2-OCH3)12(FeIIICl)] polyoxovanadate-alkoxide cluster.

Structure, Properties, and Reactivity of Polyoxocationic Zirconium and Hafnium Clusters: A Computational Investigation.

Tetravalent zirconium and hafnium molecular metal oxides clusters are key building blocks of many metal-organic frameworks; however, the chemical space to form all possible MOF nodes is vast, containing many potential new clusters. Our computational study provides a complete picture of the structure, properties, and reactivity of two tetrameric zirconium and hafnium [M4(µ2-η2:η2-O2)x(µ2-OH)8-2x(H2O)16]8+ polycationic clusters. The electronic structure of the studied species has characteristic polyoxometalate oxygen-based and metal-based bands in the valence region. The energetics for the evolution of pure metal clusters into mixed-metal clusters revealed that only the incorporation of zirconium into hafnium clusters is thermodynamically favorable. We confirmed that the incorporation of up to four peroxide ligands is thermodynamically favorable; however, the experimental absence of rich peroxide species with three or more peroxides is attributed to their thermal degradation. The mechanism for peroxide incorporation involves the partial dissociation of the cluster rather than complete dissociation.

Aqueous Speciation of Tetravalent Actinides in the Presence of Chloride and Nitrate Ligands.

Speciation of hexachloride tetravalent uranium, neptunium, and plutonium species in aqueous media has been investigated using density functional theory in the presence of inner sphere ligands such as chloride, nitrate, and solvent molecules. All possible structures with the formula [AnIV(Cl)x(H2O)y(NO3)z]4-x-z (An = U, Np, and Pu; x = 0-6, y = 0-8, and z = 0-6) were considered to explore the speciation chemical space of each actinide. The nature of the mixed-ligand complexes present in solution is controlled by the concentration of free ligands in solution. A low chloride concentration is suitable to drive the speciation away from the highly thermodynamically stable hexachloride species. Furthermore, the formation of dimeric species can proceed through both olation and oxolation mechanisms. Oxolation is preferred for monomers that contain fewer water ligands, while olation becomes favorable for complexes with more water ligands.

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.

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster.

The isolation of the oxygen-deficient, polyoxovanadate-alkoxide (POV-alkoxide) cluster, [nBu4N][V6O6(OMe)12(MeCN)], and its subsequent reactivity with oxygen (O2), has demonstrated the utility of these assemblies as molecular models for heterogeneous metal oxide catalysts. However, the mechanism through which this cluster activates and reduces O2 to generate the oxygenated species is poorly understood. Currently it is speculated that this POV-alkoxide mediates the four-electron O-O bond cleavage through an O2 bridged dimeric intermediate, a mechanism which is not viable for O2 reduction at solid-state metal oxide surfaces. Here, we report the successful activation and reduction of O2 by the calix-functionalized POV-alkoxide cluster, [nBu4N][(calix)V6O6(OMe)8](MeCN)] (calix = 4-tert-butylcalix[4]arene). The steric hindrance imparted to the open vanadium site by the calix motif eliminates the possibility of cooperative, bimolecular O2 activation, allowing for a comparison of the reactivity of this system with that of the nonfunctionalized POV-alkoxide described previously. Rigorous characterization of the calix-substituted assembly, enabled by its newfound solubility in organic solvent, reveals that the incorporation of the tetradentate aryloxide ligand into the POV-alkoxide scaffold perturbs the electronic communication between the site-differentiated vanadium(III) ion and the cluster core. Collectively, our results provide insight into the physiochemical factors that are important during the O2 reduction reaction at oxygen-deficient sites in reduced POV-alkoxide clusters.

Computational Insights into the Nucleation of Mixed-Valent Polyoxovanadate Alkoxide Clusters.

The synthesis of novel tunable electroactive species remains a key challenge for a wide range of chemical applications such as redox catalysis, energy storage, and optoelectronics. In recent years, polyoxovanadate (POV) alkoxide clusters have emerged as a new class of compounds with highly promising electrochemical applications. However, our knowledge of the formation pathways of POV alkoxides is rather limited. Understanding the speciation of POV alkoxides is fundamental for controlling and manipulating the evolution of transient species during their nucleation and therefore tuning the properties of the final product. Here, we present a computational study of the nucleation pathways of a mixed-valent [(VV6-nVIVnO6)(O)(O-CH3)12](4-n)+ POV alkoxide cluster in the absence of reducing agents other than methanol.

Plausible Emergence and Self Assembly of a Primitive Phospholipid from Reduced Phosphorus on the Primordial Earth.

How life arose on the primitive Earth is one of the biggest questions in science. Biomolecular emergence scenarios have proliferated in the literature but accounting for the ubiquity of oxidized (+ 5) phosphate (PO43-) in extant biochemistries has been challenging due to the dearth of phosphate and molecular oxygen on the primordial Earth. A compelling body of work suggests that exogenous schreibersite ((Fe,Ni)3P) was delivered to Earth via meteorite impacts during the Heavy Bombardment (ca. 4.1-3.8 Gya) and there converted to reduced P oxyanions (e.g., phosphite (HPO32-) and hypophosphite (H2PO2-)) and phosphonates. Inspired by this idea, we review the relevant literature to deduce a plausible reduced phospholipid analog of modern phosphatidylcholines that could have emerged in a primordial hydrothermal setting. A shallow alkaline lacustrine basin underlain by active hydrothermal fissures and meteoritic schreibersite-, clay-, and metal-enriched sediments is envisioned. The water column is laden with known and putative primordial hydrothermal reagents. Small system dimensions and thermal- and UV-driven evaporation further concentrate chemical precursors. We hypothesize that a reduced phospholipid arises from Fischer-Tropsch-type (FTT) production of a C8 alkanoic acid, which condenses with an organophosphinate (derived from schreibersite corrosion to hypophosphite with subsequent methylation/oxidation), to yield a reduced protophospholipid. This then condenses with an α-amino nitrile (derived from Strecker-type reactions) to form the polar head. Preliminary modeling results indicate that reduced phospholipids do not aggregate rapidly; however, single layer micelles are stable up to aggregates with approximately 100 molecules.

Application of Symmetry Functions to Large Chemical Spaces Using a Convolutional Neural Network.

The use of machine learning in chemistry is on the rise for the prediction of chemical properties. The input feature representation or descriptor in these applications is an important factor that affects the accuracy as well as the extent of the explored chemical space. Here, we present the periodic table tensor descriptor that combines features from Behler-Parrinello's symmetry functions and a periodic table representation. Using our descriptor and a convolutional neural network model, we achieved 2.2 kcal/mol and 94 meV/atom mean absolute error for the prediction of the atomization energy of organic molecules in the QM9 data set and the formation energy of materials from Materials Project data set, respectively. We also show that structures optimized with a force field derived from this modelcan be used as input to predict the atomization energies of molecules at density functional theory level. Our approach extends the application of Behler-Parrinello's symmetry functions without a limitation on the number of elements, which is highly promising for universal property calculators in large chemical spaces.

Prediction of optoelectronic properties of Cu2O using neural network potential.

Neural network potentials (NNPs) trained against density functional theory (DFT) are capable of reproducing the potential energy surface at a fraction of the computational cost. However, most NNP implementations focus on energy and forces. In this work, we modified the NNP model introduced by Behler and Parrinello to predict Fermi energy, band edges, and partial density of states of Cu2O. Our NNP can reproduce the DFT potential energy surface and properties at a fraction of the computational cost. We used our NNP to perform molecular dynamics (MD) simulations and validated the predicted properties against DFT calculations. Our model achieved a root mean squared error of 16 meV for the energy prediction. Furthermore, we show that the standard deviation of the energies predicted by the ensemble of training snapshots can be used to estimate the uncertainty in the predictions. This allows us to switch from the NNP to DFT on-the-fly during the MD simulation to evaluate the forces when the uncertainty is high.

Self-Assembly of Uranyl-Peroxide Nanocapsules in Basic Peroxidic Environments.

A wide range of uranyl-peroxide nanocapsules have been synthesized using very simple reactants in basic media; however, little is known about the process to form these species. We have performed a density functional theory study of the speciation of the uranyl ions under different experimental conditions and explored the formation of dimeric species via a ligand exchange mechanism. We shed some light onto the importance of the excess of peroxide and alkali counterions as a thermodynamic driving force towards the formation of larger uranyl-peroxide species.

Colloidal synthesis of single-layer MSe2 (M = Mo, W) nanosheets via anisotropic solution-phase growth approach.

The generation of single-layer 2-dimensional (2D) nanosheets has been challenging, especially in solution-phase, since it requires highly anisotropic growth processes that exclusively promote planar directionality during nanocrystal formation. In this study, we discovered that such selective growth pathways can be achieved by modulating the binding affinities of coordinating capping ligands to the edge facets of 2D layered transition-metal chalcogenides (TMCs). Upon changing the functional groups of the capping ligands from carboxylic acid to alcohol and amine with accordingly modulated binding affinities to the edges, the number of layers of nanosheets is controlled. Single-layer MSe2 (M = Mo, W) TMC nanosheets are obtained with the use of oleic acid, while multilayer nanosheets are formed with relatively strong binding ligands such as oleyl alcohol and oleylamine. With the choice of appropriate capping ligands in the 2D anisotropic growth regime, our solution-based synthetic method can serve a new guideline for obtaining single-layer TMC nanosheets.

An atlas of two-dimensional materials.

The discovery of graphene and other two-dimensional (2D) materials together with recent advances in exfoliation techniques have set the foundations for the manufacturing of single layered sheets from any layered 3D material. The family of 2D materials encompasses a wide selection of compositions including almost all the elements of the periodic table. This derives into a rich variety of electronic properties including metals, semimetals, insulators and semiconductors with direct and indirect band gaps ranging from ultraviolet to infrared throughout the visible range. Thus, they have the potential to play a fundamental role in the future of nanoelectronics, optoelectronics and the assembly of novel ultrathin and flexible devices. We categorize the 2D materials according to their structure, composition and electronic properties. In this review we distinguish atomically thin materials (graphene, silicene, germanene, and their saturated forms; hexagonal boron nitride; silicon carbide), rare earth, semimetals, transition metal chalcogenides and halides, and finally synthetic organic 2D materials, exemplified by 2D covalent organic frameworks. Our exhaustive data collection presented in this Atlas demonstrates the large diversity of electronic properties, including band gaps and electron mobilities. The key points of modern computational approaches applied to 2D materials are presented with special emphasis to cover their range of application, peculiarities and pitfalls.

Oxygenation by ruthenium monosubstituted polyoxotungstates in aqueous solution: experimental and computational dissection of a Ru(III)-Ru(V) catalytic cycle.

Molecular polyoxometalates with one embedded ruthenium center, with general formula [Ru(II/III)(DMSO)XW11O39](n-) (X = P, Si; n = 4-6), are readily synthesized in gram scale under microwave irradiation by a flash hydrothermal protocol. These nanodimensional and polyanionic complexes enable aerobic oxygenation in water. Catalytic oxygen transfer to dimethylsulfoxide (DMSO) yielding the corresponding sulfone (DMSO2 ) has been investigated with a combined kinetic, spectroscopic and computational approach addressing: (i ) the Ru(III) catalyst resting state; (ii ) the bimolecular event dictating its transformation in the rate-determining step; (iii ) its aerobic evolution to a high-valent ruthenium oxene species; (iv ) the terminal fate to diamagnetic dimers. This pathway is reminiscent of natural heme systems and of bioinspired artificial porphyrins. The in silico characterization of a key bis-Ru(IV)-µ-peroxo-POM dimeric intermediate has been accessed by density functional theory. This observation indicates a new landmark for tracing POM-based manifolds for multiredox oxygen reduction/activation, where metal-centered oxygenated species play a pivotal role.

Hexagonal transition-metal chalcogenide nanoflakes with pronounced lateral quantum confinement.

Transition-metal chalcogenide (TMC) nanoflakes of composition MX2 (where M=Ti, Zr and Hf; X=S and Se) crystallize preferentially in equilateral hexagons and exhibit a pronounced lateral quantum confinement. The hexagonal shape of octahedral (1T) TMC nanoflakes is the result of charge localization at the edges/vertices and the resulting Coulomb repulsion. Independent of their size, all nanoflakes have the Mn X2n-2 stoichiometry and thus an unoxidized metal center which results in dopant states. These states become relevant for small nanoflakes and lead to metallic character, but for larger nanoflakes (>6 nm) the 2D monolayer properties dominate. Finally, coordination of Lewis bases at the nanoflake edges has no significant effect on the electronic structure of these species confirming the viability of colloidal synthetic approaches.

Two dimensional materials beyond MoS2: noble-transition-metal dichalcogenides.

The structure and electronic structure of layered noble-transition-metal dichalcogenides MX2 (M=Pt and Pd, and chalcogenides X=S, Se, and Te) have been investigated by periodic density functional theory (DFT) calculations. The MS2 monolayers are indirect band-gap semiconductors whereas the MSe2 and MTe2 analogues show significantly smaller band gap and can even become semimetallic or metallic materials. Under mechanical strain these MX2 materials become quasi-direct band-gap semiconductors. The mechanical-deformation and electron-transport properties of these materials indicate their potential application in flexible nanoelectronics.

Water clusters to nanodrops: a tight-binding density functional study.

We predict structures and energies of water clusters containing up to 100 waters with tight-binding density functional theory (DFTB). A per-hydrogen-bond energy correction is found to correct for systematic errors in the DFTB cluster energies. We compare the DFTB structures and energies to density functional theory (DFT) calculations and to the most accurate wave function theoretical (WFT) values available (ranging from coupled-cluster theory to second-order perturbation theory). After including the simple hydrogen bond correction, we achieve a root-mean-square difference of less than one kcal mol(-1) with the best estimates. As DFTB optimizations are orders of magnitude faster than DFT or canonical MP2, it is apparent that DFTB is a very practical method for calculating large water cluster structures and, with the hydrogen bond correction, also energies.

A journey inside the U28 nanocapsule.

Anionic uranyl-peroxide U(28) nanocapsules trap cations and other anions inside, whose structures cannot be resolved by X-ray diffraction, owing to crystallographic disorder. DFT calculations enabled the complete characterization of the geometry of these complex systems and also explained the origin of the disorder. The stability of the capsules was strongly influenced by the entrapped cations. Excellent agreement between experiment and theory was also obtained for the electronic character and redox properties.

Uranyl-peroxide nanocapsules: electronic structure and cation complexation in [(UO2)20(µ-O2)30]20-.

The pentagonal K(10)[(UO(2))(5)(µ-O(2))(5)(C(2)O(4))(5)] species have been identified as the building blocks of uranyl-peroxide nanocapsules. The computed complexation energies of different alkali cations (Li(+), Na(+), K(+), Rb(+), and Cs(+)) with [(UO(2))(5)(µ-O(2))(5)(O(2))(5)](10-) and [(UO(2))(20)(µ-O(2))(30)](20-) species suggest a strong cation templating effect. In the studied species, the largest complexation energy occurs for the experimentally used alkali cations (Na(+) and K(+)).

Experimental and computational study of a new wheel-shaped {[W5O21]3[(U(VI)O))2(µ-O2)]3}30- polyoxometalate.

A new wheel-shaped polyoxometalate {[W(5)O(21)](3)[(U(VI)O(2))(2)(µ-O(2))](3)}(30-) has been synthesized and structurally characterized. The calculated electrostatic potential reveals the protonation of several µ-oxo bridges reducing the polyoxometalate total charge. A protonated structure computed at the density functional level of theory (DFT) is in good agreement with the experimental fit. This species presents a classical polyoxometalate electronic structure with well-defined metal and oxo bands belonging to its U/W and oxo/peroxo constituents, respectively. Furthermore, fragment calculations indicate that the electronic structures of the uranyl-peroxide and polyoxotugstate fragments are little affected by the nanowheel assembly.

Effect of axially projected oligothiophene pendants and nitro-functionalized diimine ligands on the lowest excited state in cationic Ir(III) bis-cyclometalates.

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C^N)(2)(N^N)]PF(6) 9-12 were prepared. The synthesis, characterization, electrochemical, and photophysical properties are reported. The cyclometalating ligands (C^N) are 2-phenylpyridinato (ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in the 5 and 5" positions with the ppy and mesityl groups. The diimine ligands (N^N) are 2,2'-bipyridine (bpy) or 4-NO(2)-bipyridine (4-NO(2)-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO(2) functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO(2) substituent and pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO(2) substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10 and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λ(ex) = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φ(em) = 0.11. In stark contrast the NO(2) and 3T functionalized complexes 10-12 display near total quenching of luminescence. Computations of the ground and excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both the NO(2) and 3T substituents play an important role in excited state deactivation of complexes 10-12. A substantial electronic contribution of the NO(2) substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient nonradiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, nonluminescent, triplet states centered on the 3T-pendants.