Formation of Rh island on Pd-supported α-Fe2O3 (0001)

A Rhodium ultra-thin film was deposited on Pd-supported α-Fe2O3 (0001) film by molecular beam epitaxy (MBE). The surface atomic and electronic structures were studied using x-ray photoelectron diffraction (XPD), x-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). For a oxide thin film (~16.5Å), an ordered (√3×√3)R30° structure was observed. The multiple scattering calculations approach combined with a genetic algorithm for surface structure optimization was used to investigate precisely the atomic structure of the oxide support as well as the Rh nanoislands or nanoparticles (NPs). Rh exhibited a 3D growth on α-Fe2O3 thin film forming NPs with a lattice constant 5.26% expanded relative to Rh bulk value. The coexistence of 60orotated domains were observed and also evidence of Fe termination at the surface of the α-Fe2O3.Read more

Study of asymmetrical mixed-valent Mo2−Mo2 complexes in the class III regime.

Dalton Transactions
Three novel asymmetrical dimolybdenum dimers [Mo2(DAniF)3]2(μ-OOCCOS) (DAniF =N, N′-di(p-anisyl)formamidinate) ([OO−OS]), [Mo2(DAniF)3]2(μ-S2CCO2) ([SS−OO]), and [Mo2(DAniF)3]2(μ-SSCCOS) ([SS−OS]) have been synthesized and characterized either by single-crystal X-ray crystallography or 1H NMR spectroscopy.The structural asymmetry for these compounds gives rise to a redox asymmetry which enlarges the potential separation (ΔE1/2) between the two [Mo2] units. The mixed-valance (MV) species [OO−OS]+, [SS−OO]+ and...  Read more

Geometrical and Electronic Properties of the Manganese(IV)/Iron(III) Cofactor of Chlamydia Trachomatis Ribonucleotide Reductase Unveiled by Simulations of XAS Spectra

Dalton Transactions
Using simulations of Mn/Fe K‐edge X‐ray absorption spectroscopy (XAS), combined with DFT-optimized structural models and direct comparisons with available experimental data, we determine geometrical and electronic propeties of the Mn-Fe active site of Chlamydia trachomatis (Ct) of ribonucleotide reductase (RNR). In the post-edge XAS energy range, we use extended X-ray absorption fine structure (EXAFS) data, to acquire absorber-scatterer geometrical information around each absorber metal center. For this task, we apply a protocol that evaluates Debye-Waller factors in scattering paths instead of scattering shells to fit the experimental EXAFS. The model of the Manganese(IV)/Iron(III) cofactor that best fit Mn and Fe K-edge EXAFS experimental data is a structure with Mn at metal site 1 (proximal to Phe-127), a μ-oxo/μ-hydroxo/μ-1,3-carboxylato core, and a terminal hydroxo ligand, i.e....  Read more

On the Lewis acidic character of bis(salicylaldiminato)zinc(II) Schiff-base complexes: a computational and experimental investigation on a series of compounds varying the bridging diimine

Dalton Transactions
This contribution explores the effect of the 1,2-diimine bridge upon the Lewis acidic character of a series of bis(salicylaldiminato)zinc(II), ZnL, Schiff-base complexes. The structure of the monomeric and dimeric ZnL complexes, and of the 1:1 adducts with pyridine, ZnL·py, is fully optimized by means of DFT calculations. The Gibbs free energy for the dimerization of ZnL complexes and for the formation of ZnL·py adducts is evaluated by accurate composite calculations. It accounts for their spontaneous dimerization and for the greater stability of the ZnL·py adducts with respect to the dimers. Calculated binding constants for the formation ZnL·py adducts are in excellent agreement with experimentally derived values, thus allowing establishing a relative Lewis acidity scale within this series. While the complex derived from the non-conjugated ethylenediamine reveals the lowest Lewis acidity, the complex derived from the diaminomaleonitrile represents the stronger Lewis acidic...  Read more

“Protrusions” or “holes” in graphene: which is the better choice for sodium ion storage?

Energy & Environmental Science
The main challenge associated with sodium-ion battery (SIBs) anodes is a search for novel candidate materials with high capacity and excellent rate capability. The most commonly used and effective route for a graphene-based anode design is the introduction of in-plane “hole” defects via nitrogen-doping; this creates a spacious reservoir for storing more energy. Inspired by mountains in nature, herein, we propose another way – introduction of blistering in graphene instead of making “holes”; this facilitates adsorbing/inserting more Na+ ions. In order to properly answer the key question: “protrusions” or “holes” in graphene, which is better for sodium ion storage?, two types of anode materials with a similar doping level were designed: a phosphorus-doped graphene (GP, with protrusions) and a nitrogen-doped graphene (GN, with holes). As compared with GN, GP anode perfectly satisfies all the desired criteria, it reveals an ultrahigh capacity (374 mAh/g after 120 cycles...  Read more

Structure and unconventional dihydrogen bonding of a pressure-stabilized hydrogen-rich (NH3BH3)(H2)x (x = 1.5) compound

Journal of Materials Chemistry A
Combining X-ray diffraction, Raman spectroscopy, and ab initio simulations we characterize an extremely hydrogen-rich phase with the chemical formula (NH3BH3)(H2)x (x = 1.5). This phase was formed by compressing ammonia borane (AB, NH3BH3) in an environment with excess of molecular hydrogen (H2). This compound can store a total of 26.8 wt% hydrogen, both as molecular hydrogen and chemically bonded hydrogen in AB, making it one of the most hydrogen-rich solids currently known. The new compound possesses a layered AB structure where additional H2 molecules reside in channels created through the weaving of AB layers. The unconventional dihydrogen bonding network of the new compound is significantly modified from its parent AB phase and contains H…H contacts between adjacent AB molecules and between AB and H2 molecules. H-H can be either a proton donor or a proton acceptor that forms new types of dihydrogen bonding with the host AB molecules, which are depicted as H-H…H-B or...  Read more

Capacity Retention of Lithium Sulfur Batteries enhanced with Nano-sized TiO2-Embedded Polyethylene Oxide

Journal of Materials Chemistry A
The shuttle effect of polysulfides in the Li/S battery causes the loss of active materials and capacity decay. The phenomenon has retarded the practical application of Li/S batteries. Herein, we demonstrate that the undesired shuttle mechanism owing to dissolved lithium polysulfides can be effectively suppressed by the incorporation of TiO2 nanoparticles (NPs) within a solid nanocomposite polymer electrolyte of polyethylene oxide (PEO). The approach shows enhanced capacity retention, whereby the cell with hybrid solid electrolyte of TiO2 NPs embedded in PEO has delivered more than 1450 mAh g-1 (first cycle discharge capacity) with ~87% capacity retention after 100 cycles; compared to only 38% capacity retention in the absence of TiO2. Density Function Theory (DFT) computational results based on bonding interactions and Raman characterization of the different components reveal that the undesired diffusion shuttle mechanism causing Li anode passivation can be effectively reduced by the...  Read more

Formation of surface nanodroplets facing a structured microchannel wall

Lab on a Chip
Surface nanodroplets are important units for lab-on-chip devices, compartmentalised catalytic reactions, high-resolution near-field imaging, and many others. Solvent exchange is a simple solution-based bottom-up approach for producing surface nanodroplets by displacing a good solvent of the droplet liquid by a poor one in a narrow channel in the laminar regime. The droplet size is controlled by the solution composition and the flow conditions during the solvent exchange. In this paper, we investigated the effects of local microfluidic structures on the formation of surface nanodroplets. The microstructures consist of a microgap with well-defined geometry, embedded on the opposite microchannel wall, facing the substrate where nucleation takes place. For given channel height, the dimensionless control parameters were the Peclect number of the flow, the ratio between gap height and channel height, and the aspect ratio between gap length and channel height. We found and explained three...  Read more

Enantioselective Diels-Alder-Lactamization Organocascades Employing a Furan-Based Diene

Organic & Biomolecular Chemistry
Alpha,beta-Unsaturated acylammonium salts are useful dienophiles enabling highly enantioselective and stereodivergent Diels-Alder-initiated organocascades with furan-based dienes. Complex polycyclic systems can thus be obtained from readily prepared dienes, commodity acid chlorides, and a chiral isothiourea organocatalyst under mild conditions. We describe the use of a furan-based diene bearing pendant sulfonamides leading to the generation of oxa-bridged, trans-fused tricyclic gamma-lactams. This process constitutes the first highly enantio- and diastereoselective, organocatalytic Diels-Alder cycloadditions with these typically problematic dienes due to their reversibility. Computational studies suggest that high diastereoselectivity with these furan dienes may be due to a reversible Diels-Alder cycloaddition for the endo adducts. In addition, the utility of this methodology is demonstrated through a concise approach to a core structure with similarity to the natural product...  Read more

The Mechanism of Hydrogen Adsorption on Transition Metal Dichalcogenides as Hydrogen Evolution Reaction Catalyst

Physical Chemistry Chemical Physics
Two-dimension transition-metal dichalcogenides (TMDs) have been widely concerned as potential hydrogen evolution reaction (HER) catalysts due to their low cost and good electrochemical stability in acid condition. The mechanism of hydrogen adsorption on TMDs plays an important role to optimize HER activity. Here, a series of TMDs (MX2, M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Tc, W, Re, X=S, Se, Te) in 2H- and 1T-phases are investigated using density functional theory to determine the relationship between hydrogen adsorption free energy (ΔGH) and electronic structure through a simple descriptor. The results shown a positive linear relation between ΔGH and the work required of H electron to fill unoccupied electronic states of TMDs. Based on such linear relations, the various defect (B-, C-, N-, O-, F-, P-, Se- doping and S-vacancy) were used to activate the inert basal planes of 2H-phase MoS2, which can introduce impurity states in lower energy level to effectively accommodate H...  Read more

Chemical states of 3d transition metal impurities in liquid lead-bismuth eutectic by first principles calculations

Physical Chemistry Chemical Physics
Steels are easily corroded in liquid lead-bismuth eutectic (LBE) because their components, such as Fe, Cr and Ni, have a high solubility in liquid LBE. To understand the reason for such high solubility of these 3d transition metals, we have performed first-principles molecular dynamics calculations and analyzed pair-correlation functions, electronic densities of states, and Bader charges and volumes of 3d transition metals dissolved in liquid LBE as impurities. The calculations show that the 4s and 3d orbitals of 3d impurity atoms largely interact with the 6p band of LBE, which generates bonding orbitals. We suggest that the high stability of 3d metals in liquid LBE is caused by the interactions of the 4s and 3d orbitals with the 6p band. Spin polarization is induced with V, Cr, Mn, Fe and Co impurity atoms in a similar manner to the Slater-Pauling curve of solid transition metals, which exhibits...  Read more

The role of ion-water interactions in determining the Soret coefficient of LiCl aqueous solutions

Physical Chemistry Chemical Physics
The application of a thermal gradient to an aqueous electrolyte solution induces the Soret effect, and the salt migrates towards hot (thermophilic) or cold regions (thermophobic). Experimental studies of LiCl reported changes in the sign of the Soret coefficient as well as minima in this coefficient at specific salt concentrations and temperatures. At the minimum the thermodiffusive response of the solution is enhanced significantly. We have performed non-equilibrium molecular dynamics simulations of LiCl solutions to quantify the dependence of the sign change and minimum of the Soret coefficient with salt concentration and temperature. Ion isotopic effects are shown to play a secondary role in determining the Soret coefficient, while the diameter of the cation has a significant impact on the coefficient and on the observation of the minimum. Our simulations show that the ordering of water around Li+ plays a key role in determining the Soret coefficient of...  Read more

An adaptive mutation simulated annealing based investigation on Coulombic explosion and identification of dissociation patterns in (CO2)n2+ clusters

Physical Chemistry Chemical Physics
In this communication, we would like to discuss the advantages of adaptive mutation simulated annealing (AMSA) over standard simulated annealing (SA) in studying the Coulombic explosion of (CO2)n2+ clusters for n=20-68, where 'n' is the size of the cluster. We have demonstrated how AMSA itself can over come the predicaments which can arise in conventional SA and carry on the search for better results by adapting the parameters (only when needed) dynamically during the simulation so that the search process can come out of high energy basins and also does not go astray for better exploration and convergence respectively. This technique also has the in-built properties for getting more than one minimum in a single run. For (CO2)n2+ clusters system we have found out the critical limit to be n=43, above which the attractive forces...  Read more

A quantum-rovibrational-state-selected study of the H2O+(X2B1; v1+v2+v3+; N+Ka+Kc+ ) + CO reaction in the collision energy range of 0.05-10.00 eV: translational, rotational, and vibrational energy effects

Physical Chemistry Chemical Physics
We report detailed absolute integral cross sections (σ’s) for the quantum-rovibrational-state-selected ion-molecule reaction H2O+(X2B1: v1+v2+v3+; N+Ka+Kc+) + CO in the center-of-mass collision energy (Ecm) range of 0.05-10.00 eV, where (v1+v2+v3+) = (000), (100), and (020), and N+Ka+Kc+ = 000, 111, and 211. Three product channels, HCO++ OH, HOCO++ H, and CO+ + H2O, are identified. The measured σ(HCO+) curve [σ(HCO+) versus Ecm plot] supports that the formation of the HCO+ + OH channel follows an exothermic pathway with no potential energy barriers. Although the HOCO+ + H channel is the most exothermic, the σ(HOCO+) is found to be significantly lower than the σ(HCO+). The σ(HOCO+) curve is bimodal, indicating two distinct mechanisms for the formation of HOCO+. The σ(HOCO+) is strongly inhibited at Ecm < 0.4 eV, but is enhanced at Ecm > 0.4 eV by (100) vibrational excitation. The Ecm onsets of σ(CO+) determined for the (000) and (100) vibrational states are in excellent...  Read more

Structural and energetic study of cation–π–cation interactions in proteins

Physical Chemistry Chemical Physics
Cation–π interactions of aromatic rings and positively charged groups are among the most important interactions in structural biology. Although the role and energetic characteristics of these interactions is well established, the occurrence of cation–π–cation interactions is an unexpected motif, which raises intriguing questions about its functional role in proteins. We present a statistical analysis of the occurrence, composition and geometrical preferences of cation–π–cation interactions identified in a set of non-redundant protein structures taken from the Protein Data Bank. Our results demonstrate that this structural motif is observed at a small, albeit non-negligible frequency in proteins, and suggests a preference to establish cation–π–cation motifs with Trp, followed by Tyr and Phe. Furthermore, we have found that cation–π–cation interactions tend to be highly conserved, which supports their structural or functional role. Finally, we have performed an...  Read more

Phonon Bottleneck and Long-Lived Excited States in π-Conjugated Pyrene Hoop

Physical Chemistry Chemical Physics
In the last decade, recent synthetic advances have launched carbon-based p-conjugated hoops to the forefront of theoretical and experimental investigation not only for their potential use as bottom-up templates for carbon nanotube (CNT) growth, but also for the interesting excitonic effects arising from the cyclic geometry, unique π-system orientation, and unusual electronic interactions and couplings. In particular, cyclic materials based on pyrene, a common component in organic electronics, are popular candidates for the future design of π-conjugated nanorings for optoelectronic applications. Understanding the photophysical response in cyclic oligopyrenes can be achieved using Non-Adiabatic Excited State molecular Dynamics (NA-ESMD). Through NA-ESMD modeling, we reveal details of the nonradiative relaxation processes in the circular pyrene tetramer [4]cyclo-2,7-pyrenylene ([4]CPY) where we find that the strong non-adiabatic coupling combined with the dense manifold of excited...  Read more

Effect of microsolvation on a SN2 reaction. Indirect atomistic dynamics and weakened suppression of reactivity

Physical Chemistry Chemical Physics
The systematic studies of microsolvation in the gas phase have enriched our knowledge of solvent effects. Here, the dynamics of a prototype SN2 reaction of hydrated fluoride ion with methyl iodide is uncovered employing direct dynamics simulations that show strikingly distinct features from those determined for unsolvated system. An indirect scattering is found to prevail, which occurs dominantly by forming hydrated F-(H2O)-HCH2I and F-(H2O)-CH3I pre-reaction complexes at low energies, but proceeds through the water-free counterparts at higher energies. This finding is in strong contrast to a general evolution from indirect to direct dynamics with enhancing energy for the unsolvated substitution reactions, and this discrepancy is understood by the substantial steric hindrance introduced by water molecule. As established in experiments, solvation suppresses the reactivity, whereas we find this depression is remarkably frustrated upon raising energy given that collision-induced...  Read more

The role of electron interfacial transfer in mesoporous nano-TiO2 photocatalysis: a combined study of in situ photoconductivity and numerical kinetic simulation

Physical Chemistry Chemical Physics
In this research, a combination of in situ photoconductivity (σ) and kinetic simulations was used to study the role of electron interfacial transfer (IT) in the gaseous photocatalysis of formic acid by mesoporous nanocrystalline TiO2. The effects of light intensity, initial formic acid concentrations, oxygen amounts, and temperature on the in situ σ and the photocatalytic courses were studied in detail. The temperature dependence of in situ σ clearly shows that the electron transfer is determined by the IT of electrons to O2 rather than by the transport. It was seen that the electron IT limits the photocatalysis by correlating with the recombination and the hole IT via the dynamic change in electron densities. The numerical simulation of in situ σ shows that the IT of electrons belongs to a thermally activated process that presents a thermal barrier of 0.5 eV. It is considered...  Read more

Polymers from sugars and CO2: ring-opening polymerisation and copolymerisation of cyclic carbonates derived from 2-deoxy-D-ribose

Polymer Chemistry
Bio-based aliphatic polycarbonates (APCs) are attractive synthetic materials for biomedical applications because of their biodegradabilty and biocompatability properties. A high yielding 3-step process that utilises CO2 as a C1 synthon is presented for converting raw sugar, 2-deoxy-D-ribose into a novel 6-membered cyclic carbonate for ring-opening polymerisation (ROP) into carbohydrate-based APCs. The α- and β-anomers of the monomer could be isolated and revealed very different polymerisability, as rationalised by DFT calculations. Whereas the β-anomer could not be polymerised under the conditions tested, organocatalytic homopolymerisation of the α-anomer, in solution at room temperature (rt) or under melt conditions, yielded highly insoluble polycarbonates, composed of both cyclic and linear topologies, and exhibiting a glass transition temperature (Tg) of ∼58 °C. Random copolymers with controllable...  Read more

Solution of an Elusive Pigment Crystal Structure from a Thin Film: a Combined X-ray Diffraction and Computational Study

Epindolidione, a hydrogen-bonded derivative of the organic semiconductor tetracene, is an organic pigment which has previously been used to produce stable OFETs with relatively high hole mobilities. Despite its use as an inkjet pigment and organic semiconductor, the crystal structure of epindolidione has proved elusive and is currently unknown. In this work, we report a crystal structure solution of epindolidione determined from vapor deposited thin films using a combined experimental and theoretical approach. The structure is found to be similar to one of the previously reported epindolidione derivatives and is most likely a surface-mediated polymorph, with a slightly different crystal packing compared to the bulk powder. The effect of substrate temperature on film morphology and structure is also investigated, where it is found that the crystallite orientation can be tuned by deposition at different substrate temperatures. The results also illustrate the possibilities for crystal...  Read more