Temperature-dependent Li vacancy diffusion in Li4Ti5O12 by means of first principles molecular dynamic simulations.

Lithium-ion batteries (LIBs) are a key electrochemical energy storage technology for mobile applications. In this context lithium titanate (LTO) is an attractive anode material for fast-charging LIBs and solid-state batteries (SSBs). The Li ion transport within LTO has a major impact on the performance of the anode in LIBs or SSBs. The Li vacancy diffusion in lithium-poor Li4Ti5O12 can take place either via 8ainit ↔ 16c ↔ 8afinal or a 8ainit ↔ 16c ↔ 48f ↔ 16dfinal diffusion path. To gain a more detailed understanding of the Li vacancy transport in LTO, we performed first principles molecular dynamics (FPMD) simulations in the temperature range from 800 K to 1000 K. To track the Li vacancies through the FPMD simulations, we introduce a method to distinguish the positions of multiple (Li) vacancies at each time. This method is used to characterize the diffusion path and the number of different diffusion steps. As a result, the majority of Li vacancy diffusion steps occur along the 8ainit ↔ 16c ↔ 8afinal. Moreover, the results indicate that the 16d Wyckoff position is a trapping site for Li vacancies. The dominant 8ainit ↔ 16c ↔ 8afinal path appears to compete with its back diffusion, which can be identified by the lifetime t16c of the 16c site. Our studies show that for t16c < 100 fs the back diffusion dominates, whereas for 100 fs ≤ t16c < 200 fs the 8ainit ↔ 16c ↔ 8afinal path dominates. In addition, the temperature-independent pre-factor D0 of the diffusion coefficient, as well as the attempt frequency Γ0 and the activation energy EA in lithium-poor LTO have been determined to be D0 = 1.5 × 10-3 cm2 s-1, as well as Γ0 = 6.6 THz and EA = 0.33 eV.

Elucidating the Role of Halides and Iron during Radiolysis-Driven Oxidative Etching of Gold Nanocrystals Using Liquid Cell Transmission Electron Microscopy and Pulse Radiolysis.

Graphene liquid cell transmission electron microscopy (TEM) has enabled the observation of a variety of nanoscale transformations. Yet understanding the chemistry of the liquid cell solution and its impact on the observed transformations remains an important step toward translating insights from liquid cell TEM to benchtop chemistry. Gold nanocrystal etching can be used as a model system to probe the reactivity of the solution. FeCl3 has been widely used to promote gold oxidation in bulk and liquid cell TEM studies, but the roles of the halide and iron species have not been fully elucidated. In this work, we observed the etching trajectories of gold nanocrystals in different iron halide solutions. We observed an increase in gold nanocrystal etch rate going from Cl-- to Br-- to I--containing solutions. This is consistent with a mechanism in which the dominant role of halides is as complexation agents for oxidized gold species. Additionally, the mechanism through which FeCl3 induces etching in liquid cell TEM remains unclear. Ground-state bleaching of the Fe(III) absorption band observed through pulse radiolysis indicates that iron may react with Cl2·- radicals to form an oxidized transient species under irradiation. Complete active space self-consistent field (CASSCF) calculations indicate that the FeCl3 complex is oxidized to an Fe species with an OH radical ligand. Together our data indicate that an oxidized Fe species may be the active oxidant, while halides modulate the etch rate by tuning the reduction potential of gold nanocrystals.

Bridging the Green Gap: Metal-Organic Framework Heteromultilayers Assembled from Porphyrinic Linkers Identified by Using Computational Screening.

In organic photovoltaics, porphyrins (PPs) are among the most promising compounds owing to their large absorption cross-section, wide spectral range, and stability. Nevertheless, a precise adjustment of absorption band positions to reach a full coverage of the so-called green gap has not been achieved yet. We demonstrate that a tuning of the PP Q- and Soret bands can be carried out by using a computational approach for which substitution patterns are optimized in silico. The most promising candidate structures were then synthesized. The experimental UV/Vis data for the solvated compounds were in excellent agreement with the theoretical predictions. By attaching further functionalities, which allow the use of PP chromophores as linkers for the assembly of metal-organic frameworks (MOFs), we were able to exploit packing effects resulting in pronounced redshifts, which allowed further optimization of the photophysical properties of PP assemblies. Finally, we use a layer-by-layer method to assemble the PP linkers into surface-mounted MOFs (SURMOFs), thus obtaining high optical quality, homogeneous and crystalline multilayer films. Experimental results are in full accord with the calculations, demonstrating the huge potential of computational screening methods in tailoring MOF and SURMOF photophysical properties.

Synthesis of an Iron(IV) Aqua-Oxido Complex Using Ozone as an Oxidant.

The iron(IV) oxido complex [(tmc)Fe=O(OTf)]OTf with the macrocyclic ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclo-tetradecane (tmc) has been synthesized using ozone as an oxidant. By adding water to this compound the complex [(H2 O)(tmc)Fe=O)](OTf)2 could be prepared. This complex is important in regard to a better understanding of the reactivity of FeIV oxido complexes. Mössbauer measurements using the solid compound showed an isomer shift of δ=0.19 mm s-1 and a quadrupole splitting ΔEQ =1.38 mm s-1 , confirming the high-valent FeIV state. DFT calculations were performed and led to an assignment of triplet spin multiplicity. Crystallographic characterization of [(H2 O)(tmc)Fe=O)](OTf)2 as well as of starting materials [(tmc)Fe(CH3 CN)](OTf)2 and [(tmc)Fe(OTf)]OTf together with previous results strongly suggest that [(H2 O)(tmc)Fe=O)](OTf)2 was formed similar to the oxido-hydroxido tautomerism analogous to heme systems.

Deep eutectic solvents: similia similibus solvuntur?

Deep eutectic solvents, mixtures of an organic compound and a salt with a deep eutectic melting point, are promising cheap and eco-friendly alternatives to ionic liquids. Ab initio molecular dynamics simulations of reline, a mixture consisting of urea and choline chloride, reveal that not solely hydrogen bonds allow similar interactions between both constituents. The chloride anion and the oxygen atom of urea also show a similar spatial distribution close to the cationic core of choline due to a similar charge located on both atoms. As a result of multiple similar interactions, clusters migrating together cannot be observed in reline which supports the hypothesis similia similibus solvuntur. In contrast to previous suggestions, the interaction of the hydroxyl group of choline with a hydrogen bond acceptor is overall rigid. Fast hydrogen bond acceptor dynamics is facilitated by the hydrogen atoms in the trans position to the carbonyl group of urea which contributes to the low melting point of reline.

Charge Spreading in Deep Eutectic Solvents.

Ab initio molecular dynamic simulations reveal significantly reduced ion charges in several choline-based deep eutectic solvents, which are cheap and eco-friendly alternatives to ionic liquids. Increasing hydrogen bond strength between the anion and the organic compound enhances charge spreading from the anion to the organic compound while the positive charge is stronger located at the cation. Nonetheless, the negative charge transferred from chloride to urea in choline chloride urea mixtures is negligible. Thus, it seems questionable if charge delocalization occurring through hydrogen bonding between the halide anion and the organic compound is responsible for the deep eutectic melting point.

Using Surface Scans for the Evaluation of Halogen Bonds toward the Side Chains of Aspartate, Asparagine, Glutamate, and Glutamine.

Using halogen-specific Connolly type molecular surfaces, we herein invented a new type of surface-based interaction analysis employed for the study of halogen bonding toward model systems of biologically relevant carboxylates (ASP/GLU) and carboxamides (ASN/GLN). Database mining and statistical assessment of the PDB revealed that such interactions are widely underrepresented at the moment. We observed important distance-dependent adaptions of the binding modes of halobenzenes from a preferential oxygen-directed to a bifurcated interaction geometry of the carboxylate. In addition, halogen···π contacts perpendicular to the nitrogen atom of the carboxamide become increasingly important for the lighter halogens. Our analysis on a MP2/TZVPP level of theory is backed by CCSD(T)/CBS reference calculations. To put the vast interaction energies into perspective, we also performed COSMO-RS calculations of the solvation free energy. Facilitating the visualization of our results mapped onto any binding site of choice, we aim to inspire more design studies showcasing these underrepresented interactions.

Targeting the Gatekeeper MET146 of C-Jun N-Terminal Kinase 3 Induces a Bivalent Halogen/Chalcogen Bond.

We target the gatekeeper MET146 of c-Jun N-terminal kinase 3 (JNK3) to exemplify the applicability of X···S halogen bonds in molecular design using computational, synthetic, structural and biophysical techniques. In a designed series of aminopyrimidine-based inhibitors, we unexpectedly encounter a plateau of affinity. Compared to their QM-calculated interaction energies, particularly bromine and iodine fail to reach the full potential according to the size of their σ-hole. Instead, mutation of the gatekeeper residue into leucine, alanine, or threonine reveals that the heavier halides can significantly influence selectivity in the human kinome. Thus, we demonstrate that, although the choice of halogen may not always increase affinity, it can still be relevant for inducing selectivity. Determining the crystal structure of the iodine derivative in complex with JNK3 (4X21) reveals an unusual bivalent halogen/chalcogen bond donated by the ligand and the back-pocket residue MET115. Incipient repulsion from the too short halogen bond increases the flexibility of Cε of MET146, whereas the rest of the residue fails to adapt being fixed by the chalcogen bond. This effect can be useful to induce selectivity, as the necessary combination of methionine residues only occurs in 9.3% of human kinases, while methionine is the predominant gatekeeper (39%).

Order in the chaos: the secret of the large negative entropy of dissolving 1-alkyl-3-methylimidazolium chloride in trihexyltetradecylphosphonium chloride.

The dissolution of 1-alkyl-3-methylimidazolium chloride ILs with short alkyl chains in trihexyltetradecylphosphonium chloride does not only exhibit a large negative entropy. Also, in the resulting mixtures, the phosphonium cation diffuses faster than the much smaller imidazolium cation. Both unexpected features originate from the formation of a large symmetric ion cluster cage in which the imidazolium cation is caught by three chloride anions and four phosphonium cations.

Molecular features contributing to the lower viscosity of phosphonium ionic liquids compared to their ammonium analogues.

Molecular features contributing to the lower viscosity of phosphonium based ionic liquids (ILs) compared to ammonium based ILs are investigated by static quantum chemistry calculations and classical molecular dynamics simulations. The larger bond distance and the higher flexibility of bond angles and dihedral angles in the phosphonium compounds tend to reduce their viscosity compared to ammonium analogues, while the strongly localized charge at the central atom has the opposite effect. Fast translational ion dynamics is also found to be related to a short counter-ion association lifetime in the investigated compounds. Furthermore, a weak structuring between the center of charges also seems to increase mobility. Interestingly, the order of ion pair interaction energies in the gas phase is reversed compared to the order of counter-ion association lifetimes in the liquid, which highlights the important role of solvation in ILs. Overall, the higher flexibility of the bond and dihedral angles of the phosphonium compounds appears to be the most important factor in producing the lower viscosity of these ILs compared to their ammonium analogues.

Cobalt Boryl Complexes: Enabling and Exploiting Migratory Insertion in Base-Metal-Mediated Borylation.

Cobalt boryl complexes, which have only been sporadically reported, can be accessed systematically with remarkable (but controllable) variation in the nature of the M-B bond. Complexes incorporating a very strong trans σ-donor display unparalleled inertness, reflected in retention of the M-B bond even in the presence of extremely strong acid. By contrast, the use of the strong π-acceptor CO in the trans position, results in significant Co-B elongation and to labilization of the boryl ligand via unprecedented CO migratory insertion. Such chemistry provides a pathway for the generation of coordinative unsaturation, thereby enabling ligand substitution and/or substrate assimilation. Alkene functionalization by boryl transfer, a well-known reaction for noble metals such as Rh or Pt, can thus be effected by an 18-electron base-metal complex.

Electrical conductivity and glass formation in nitrile-functionalized pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids: chain length and odd-even effects of the alkyl spacer between the pyrrolidinium ring and the nitrile group.

The electrical conductivity of a series of pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids, functionalized with a nitrile (cyano) group at the end of an alkyl chain attached to the cation, was studied in the temperature range between 173 K and 393 K. The glass formation of the ionic liquids is influenced by the length of the alkyl spacer separating the nitrile function from the pyrrolidinium ring. The electrical conductivity and the viscosity do not show a monotonic dependence on the alkyl spacer length, but rather an odd-even effect. An explanation for this behavior is given, including the potential energy landscape picture for the glass transition.

Dilithium hexaorganostannate(IV) compounds.

Hypercoordination of main-group elements such as the heavier Group 14 elements (silicon, germanium, tin, and lead) usually requires strong electron-withdrawing ligands and/or donating groups. Herein, we present the synthesis and characterization of two hexaaryltin(IV) dianions in form of their dilithium salts [Li2(thf)2{Sn(2-py(Me))6}] (py(Me)=C5H3N-5-Me) (2) and [Li2{Sn(2-py(OtBu))6}] (py(OtBu)=C5H3N-6-OtBu) (3). Both complexes are stable in the solid state and solution under inert conditions. Theoretical investigations of compound 2 reveal a significant valence 5s-orbital contribution of the tin atom forming six strongly polarized tin-carbon bonds.

Immobilization of Bi2WO6 on Polymer Membranes for Photocatalytic Removal of Micropollutants from Water - A Stable and Visible Light Active Alternative.

In this work, bismuth tungstate Bi2WO6 is immobilized on polymer membranes to photocatalytically remove micropollutants from water as an alternative to titanium dioxide TiO2. A synthesis method for Bi2WO6 preparation and its immobilization on a polymer membrane is developed. Bi2WO6 is characterized using X-ray diffraction and UV-vis reflectance spectroscopy, while the membrane undergoes analysis through scanning electron microscopy, X-ray photoelectron spectroscopy, and degradation experiments. The density of states calculations for TiO2 and Bi2WO6, along with PVDF reactions with potential reactive species, are investigated by density functional theory. The generation of hydroxyl radicals OH• is investigated via the reaction of coumarin to umbelliferone via fluorescence probe detection and electron paramagnetic resonance. Increasing reactant concentration enhances Bi2WO6 crystallinity. Under UV light at pH 7 and 11, the Bi2WO6 membrane completely degrades propranolol in 3 and 1 h, respectively, remaining stable and reusable for over 10 cycles (30 h). Active under visible light with a bandgap of 2.91 eV, the Bi2WO6 membrane demonstrates superior stability compared to a TiO2 membrane during a 7-day exposure to UV light as Bi2WO6 does not generate OH• radicals. The Bi2WO6 membrane is an alternative for water pollutant degradation due to its visible light activity and long-term stability.

Reversible Optical Switching of Polyoxovanadates and Their Communication via Photoexcited States.

The 2-bit Lindqvist-type polyoxometalate (POM) [V6O13((OCH2)3CCH2N3)2]2- with a diamagnetic {V6O19} core and azide termini shows six fully oxidized VV centers in solution as well as the solid state, according to 51V NMR spectroscopy. Under UV irradiation, it exhibits reversible switching between its ground S0 state and the energetically higher lying states in acetonitrile and water solutions. TD-DFT calculations demonstrate that this process is mainly initialized by excitation from the S0 to S9 state. Pulse radiolysis transient absorption spectroscopy experiments with a solvated electron point out photochemically induced charge disproportionation of VV into VIV and electron communication between the POM molecules via their excited states. The existence of this unique POM-to-POM electron communication is also indicated by X-ray photoelectron spectroscopy (XPS) studies on gold-metalized silicon wafers (Au//SiO2//Si) under ambient conditions. The amount of reduced vanadium centers in the "confined" environment increases substantially after beam irradiation with soft X-rays compared to non-irradiated samples. The excited state of one POM anion seems to give rise to subsequent electron transfer from another POM anion. However, this reaction is prohibited as soon as the relaxed T1 state of the POM is reached.

Cobalt(III)-containing penta-dentate "helmet"-type phthalogens: synthesis, solid-state structures and their thermal and electrochemical characterization.

Treatment of unsubstituted and substituted phthalonitrile (1a-d) with appropriate equivalents of sodium methoxide and ammonia afforded the corresponding 1,3-diiminoisoindolines (2a-d), which were converted to cobalt(III)-containing penta-dentate "helmet"-type phthalogens (3a-d) by the reaction with CoCl2·6H2O as templating agent in the inert solvent 1,2,4-trichlorobenzene. The identities of 2a-d and 3a-d were established by elemental analysis, infrared spectroscopy (IR), nuclear magnetic resonance (NMR), and electrospray ionization mass spectrometry (ESI-MS). A computational study was performed to determine the most stable tautomeric form of 2a-c in the gas phase. The solid-state structures of 2b and 2c were determined by single crystal X-ray diffraction (SC-XRD) studies to confirm their existence in the stereoisomeric anti-form, which is aligned with quantum chemical computations. SC-XRD studies of 3a and 3b revealed a slightly distorted octahedral geometry around the CoIII ions which are coordinated by five N-donor atoms and one extra co-ligand, resulting in a coordination environment of CoN5Cl (3a) and CoN5O (3b), respectively. The thermal stabilities of 2a-d and 3a-d were investigated by thermogravimetric analysis (TGA) in the temperature range of 40-500 °C and 40-800 °C, respectively, revealing that 3a-d were converted to the parent cobalt(II)-containing phthalocyanines (4a-d), which was verified independently by furnace heating experiments. Moreover, the electrochemical behavior of 3a was studied exemplarily for the phthalogens by cyclic voltammetry and square wave voltammetry. This study showed that 4a (CoPc) is formed irreversibly by reducing 3a electrochemically.

Targeting histidine side chains in molecular design through nitrogen-halogen bonds.

Halogen bonds are directional noncovalent interactions that can be used to target electron donors in a protein binding site. In this study, we employ quantum chemical calculations to explore halogen···nitrogen contacts involving histidine side chains. We characterize the energetics on the MP2 level of theory using SCS-MP2 and CCSD(T)/CBS as reference calculations and elucidate their energy profile in suboptimal geometries. We derive simple rules allowing medicinal chemists and chemical biologists to easily determine preferred areas of interaction in a binding site and exploit them for scaffold decoration and design. Our work shows that nitrogen-halogen bonds are valuable interactions that are this far underexploited in patent applications, lead structure, and clinical candidate selection. We highlight their potential to increase binding affinities and suggest that they can significantly contribute to inducing and tuning subtype selectivities.

Assessment of Kohn-Sham density functional theory and Møller-Plesset perturbation theory for ionic liquids.

We present high-level benchmark calculations of interaction energies of 236 ion pair structures of ionic liquids constituting a new IL-2013 set. 33 different approaches using various basis sets are validated against these benchmark data. Overall, traditional functionals like B3LYP, without an explicit dispersion correction, should be avoided when investigating ionic liquids. We can recommend the third version of Grimme's empirical dispersion correction (DFT-D3) and the LC-BOP functional, as well as most functionals of the Minnesota family of the M0X type. Our results highlight the importance of diffuse basis set functions for the accurate prediction of the IL energetics using any DFT functional. The best combination of reasonable accuracy and reasonable cost was found to be the M06-L functional in combination with the 6-31++G** basis set, producing a remarkable mean absolute deviation of only 4.2 kJ mol(-1) and a maximum deviation of -12.5 kJ mol(-1). Second-order Møller-Plesset perturbation theory (MP2) in combination with counterpoise-corrected triple-ζ basis sets can also be recommended for reliable calculations of energetics of ionic liquids.

Efficient Molecular Dynamics Simulations of Deep Eutectic Solvents with First-Principles Accuracy Using Machine Learning Interatomic Potentials.

In recent years, deep eutectic solvents emerged as highly tunable and ecofriendly alternatives to common organic solvents and liquid electrolytes. In the present work, the ability of machine learning (ML) interatomic potentials for molecular dynamics (MD) simulations of these liquids is explored, showcasing a trained neural network potential for a 1:2 ratio mixture of choline chloride and urea (reline). Using the ML potentials trained on density functional theory data, MD simulations for large systems of thousands of atoms and nanosecond-long time scales are feasible at a fraction of the computational cost of the target first-principles simulations. The obtained structural and dynamical properties of reline from MD simulations using our machine learning models are in good agreement with the first-principles MD simulations and experimental results. Running a single MD simulation is highlighted as a general shortcoming of typical first-principles studies if the dynamic properties are investigated. Furthermore, velocity cross-correlation functions are employed to study the collective dynamics of the molecular components in reline.