Stability of n-alkanes and n-perfluoroalkanes against horizontal displacement on a graphite surface.

The stability of adsorbed molecules on surfaces is fundamental and important for various applications, such as coating, lubrication, friction, and self-assembled structure formation. In this study, we investigated the structures and interaction energies (Eint) of propane, n-pentane, n-heptane, perfluoropropane, n-perfluoropentane, and n-perfluoroheptane adsorbed on the surface of C96H24 (a model surface of graphite). The changes in Eint (ΔEint = Eint - Eint(0)) associated with the horizontal displacement from the stable position were calculated using dispersion-corrected density functional theory (DFT; B3LYP-D3), where Eint(0) is the Eint at the stable position. The maximum value of ΔEint (ΔEint(max)) associated with the horizontal displacement increased as the chain length increased. The ΔEint(max) for the three n-alkanes were 1.10, 1.82, and 2.35 kcal mol-1, respectively. The values for n-perfluoroalkanes were 0.57, 0.83, and 1.04 kcal mol-1, respectively. The ΔEint(max) values for the n-alkanes were significantly larger than those for the corresponding n-perfluoroalkanes. The Eint(max) value per carbon atom of the n-alkanes (ca. 0.30 kcal mol-1) is approximately 2.5 times as large as that of n-perfluoroalkanes (ca. 0.12 kcal mol-1). The ΔEint associated with the horizontal displacement of propane and perfluoropropane on circumcoronene (C54H18) obtained by the B3LYP-D3 calculations are close to those obtained by the second order Møller-Plesset (MP2) and dispersion-corrected double hybrid DFT calculations, suggesting the sufficient accuracy of the ΔEint obtained by the B3LYP-D3. Thus, our quantitative analysis revealed the higher stability of n-alkanes against horizontal displacement on a graphite surface than that of n-perfluoroalkanes.

Stabilization of the Protein Structure by the Many-Body Cooperative Effect in the NH/π Hydrogen-bonding Tryptophan Triad.

The indole ring of tryptophan can form NH/π hydrogen bonds, acting both as a hydrogen donor at the NH group in the pyrrole subring and as a hydrogen acceptor at the benzene subring. In the structural core of the trimeric stable protein Pholiota squarrosa lectin (PhoSL), three indoles are symmetrically arranged and form NH/π hydrogen bonds among each other. Here, we conducted quantum chemical calculations on this indole triad by using various methods and basis sets. The analyses revealed cooperativity in triad formation, with the many-body effect contributing approximately -2 kcal mol-1, which significantly stabilizes this protein. Symmetry-adapted perturbation theory ascribed this effect to the induced polarization. The electrostatic potential and atomic charges indeed revealed a charge redistribution through the NH/π hydrogen bond, which was favorable for triad formation.

Linear ether-based highly concentrated electrolytes for Li-sulfur batteries.

Li-S batteries have attracted attention as next-generation rechargeable batteries owing to their high theoretical capacity and cost-effectiveness. Sparingly solvating electrolytes hold promise because they suppress the dissolution and shuttling of polysulfide intermediates to increase the coulombic efficiency and extend the cycle life. This study investigated the solubility of polysulfide (Li2S8) in a range of liquid electrolytes, including organic electrolytes, highly concentrated electrolytes, and ionic liquids. The Li2S8 solubility was well correlated with the donor number (DNNMR), estimated via23Na-NMR, and was lower than 100 mM_(elemental sulfur) in electrolytes with DNNMR < 14, regardless of the type of electrolyte. Highly concentrated electrolytes comprising lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and linear chain dialkyl ethers such as methyl propyl ether (MPE), n-butyl methyl ether (BME), and ethyl propyl ether (EPE) were studied as sparingly solvating electrolytes for Li-S batteries. Monomethyl ethers, such as BME, showed more pronounced Li-ion coordination and higher ionic conductivity, whereas the steric hindrance of the longer alkyl chains in EPE lowered the solvation number, enhanced ion association, and lowered the ionic conductivity despite the solvents having similar dielectric constants. The charge-discharge rate capabilities of Li-S cells with dialkyl ether-based electrolytes were more impressive than those of cells with a localized high-concentration electrolyte using sulfolane (SL) and hydrofluoroether (HFE), [Li(SL)2][TFSA]-2HFE. The higher rate performance was attributed to the superior Li-ion transport properties of the dialkyl ether-based electrolytes. A pouch-type cell using lightweight [Li(BME)3][TFSA] demonstrated an energy density exceeding 300 W h kg-1 under lean electrolyte conditions.

Control of Polar/Antipolar Layered Organic Semiconductors by the Odd-Even Effect of Alkyl Chain.

Some rodlike organic molecules exhibit exceptionally high layered crystallinity when composed of a link between π-conjugated backbone (head) and alkyl chain (tail). These molecules are aligned side-by-side unidirectionally to form self-organized polar monomolecular layers, providing promising 2D materials and devices. However, their interlayer stacking arrangements have never been tunable, preventing the unidirectional arrangements of molecules in whole crystals. Here, it is demonstrated that polar/antipolar interlayer stacking can be systematically controlled by the alkyl carbon number n, when the molecules are designed to involve effectively weakened head-to-head affinity. They exhibit remarkable odd-even effect in the interlayer stacking: alternating head-to-head and tail-to-tail (antipolar) arrangement in odd-n crystals, and uniform head-to-tail (polar) arrangement in even-n crystals. The films show excellent field-effect transistor characteristics presenting unique polar/antipolar dependence and considerably improved subthreshold swing in the polar films. Additionally, the polar films present enhanced second-order nonlinear optical response along normal to the film plane. These findings are key for creating polarity-controlled optoelectronic materials and devices.

Molecular Level Origin of Ion Dynamics in Highly Concentrated Electrolytes.

Single-ion conducting liquid electrolytes are key to achieving rapid charge/discharge in Li secondary batteries. The Li+ transference (or transport) numbers are the defining properties of such electrolytes and have been discussed in the framework of concentrated solution theories. However, the connection between macroscopic transference and microscopic ion dynamics remains unclear. Molecular dynamics simulations were performed to obtain direct information regarding the microscopic behaviors in highly concentrated electrolytes, and the relationships between these behaviors and the transference number were determined under anion-blocking conditions. Various solvents with different donor numbers (DNs) were used along with a Li salt of the weakly Lewis basic bis(fluorosulfonyl)amide anion for electrolyte preparation. Favorable ordered Li+ structuring and a continuous Li+ conduction pathway were observed for the fluoroethylene carbonate-based electrolyte due to its low DN. The properties were less pronounced at higher DNs, e.g., for the dimethyl sulfoxide-based electrolyte. The τLi-solventlife/τdipolerelax ratio was introduced as a factor for ion dynamics, and the two mechanisms of ion transport were considered an exchange mechanism (τLi-solventlife/τdipolerelax < 1) and a vehicle mechanism (translational motion of solvated Li+) (τLi-solventlife/τdipolerelax ≥ 1). Vehicle-type transport was dominant with high DNs, while exchangeable transport was preferable at lower DNs. These findings should aid the further selection of solvents and Li salts to prepare single-ion conducting electrolytes.

Molecular Dynamics Simulations of High-Concentration Li[TFSA] Sulfone Solution: Effect of Easy Conformation Change of Sulfolane on Fast Diffusion of Li Ion.

The parameters of the polarizable force field used for molecular dynamics simulations of Li diffusion in high-concentration lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) sulfone (sulfolane, dimethylsulfone, ethylmethylsulfone, and ethyl-i-propylsulfone) solutions were refined. The densities of the solutions obtained by molecular dynamics simulations reproduced well the experimental values. The calculated concentration, temperature, and solvent dependencies of self-diffusion coefficients of ions and solvents in the mixtures well reproduce the experimentally observed dependencies. Ab initio calculations show that the intermolecular interactions between Li ions and four sulfones are not largely different. Conformational analyses show that sulfolane can change the conformation more easily owing to lower barrier height for pseudorotation compared to the rotational barrier heights of diethylsulfone and ethylmethylsulfone. Molecular dynamics simulations indicate that the easy conformation change of solvent affects the rotational relaxation of the solvent and the diffusion of Li ion in the mixture. The easy conformation change of sulfolane is one of the causes of faster diffusion of Li ion in the mixture of Li[TFSA] and sulfolane compared to the mixtures of smaller dimethylsulfone and ethylmethylsulfone.

Tuning the odd-even effect on two-dimensional assemblies of curcumin derivatives by alkyl chain substitution: a scanning tunnelling microscopy study.

Well-ordered molecular arrangement on surfaces is fundamental for fabrication of functional molecular devices which are of particular interest in nanotechnology. In addition to nano-manufacturing, the production of useful materials from natural resources has recently attracted increasing attention. Herein, we focused on the two-dimensional (2D) self-assemblies of curcumin derivatives. The effects of the number, length, and substitution of the alkyl chains on the 2D structures of curcumin derivatives were studied by scanning tunnelling microscopy at the highly oriented pyrolytic graphite/1,2,4-trichlorobenzene interface. Curcumin derivatives containing both methoxy and alkoxy chain groups and those possessing four alkoxy chains exhibit linear structures with and without interdigitation of alkoxy chains, respectively. These 2D structure formations are independent of the alkyl chain length. However, the bisdemethoxycurcumin derivatives periodically form stair-like and linear structures depending on the alkyl chain length, which indicates the existence of the odd-even effect. These results suggest that the 2D structural modulation of curcumin derivatives caused by the odd-even effect can be tuned by the number of alkyl chain substituents. The appearance and disappearance of the odd-even effect in curcumin derivatives are discussed in terms of the balance between intermolecular and molecule-substrate interactions.

Analysis of intermolecular interactions of n-perfluoroalkanes with circumcoronene using dispersion-corrected DFT calculations: comparison with those of n-alkanes.

Understanding the interactions between the adsorbate and substrate is critical in basic and advanced scientific fields, including the formation of well-organised nanoarchitectures via self-assembly on surfaces. In this study, the interactions of n-alkanes and n-perfluoroalkanes with circumcoronene were studied using dispersion-corrected density functional theory calculations as models of their adsorption on graphite. The interactions of n-perfluoroalkanes with circumcoronene were significantly weaker than those of the corresponding n-alkanes, e.g. the calculated adsorption energies of n-perfluorohexane and n-hexane were -9.05 and -13.06 kcal mol-1, respectively. The dispersion interactions were the major source of attraction between circumcoronene and the adsorbed molecules. Larger steric repulsion of n-perfluoroalkanes compared to those of n-alkanes increased their equilibrium distances from circumcoronene and decreased the dispersion interactions, resulting in weaker interactions. The interactions between two adsorbed n-perfluorohexane molecules and those of n-hexane molecules were -2.96 and -2.98 kcal mol-1, respectively, which are not negligible in the stabilisation of adsorbed molecules. The geometries of adsorbed n-perfluoroalkane dimers revealed that the equilibrium distance between two n-perfluoroalkane molecules did not match the width of the six-membered rings in circumcoronene, in contrast to that between n-alkanes. The lattice mismatch also destabilised the adsorbed n-perfluoroalkane dimers. The difference in the adsorption energy between flat-on and edge-on orientations of n-perfluorohexane was smaller than that of corresponding n-hexane.

Liquid Structures and Ion Dynamics of Ionic Liquids Viewed from Intermolecular Interactions.

The elucidation of the factors determining liquid structures and transport properties of ionic liquids is important for the design and development of ionic liquid electrolytes. This personal account introduces the importance of computational methods for studying ionic liquids. Molecular dynamics simulations provide detailed information on liquid structures of ionic liquid such as the structures of solvated cation complexes in equimolar mixtures of glymes and Li[TFSA] and the effects of the charges of electrode on liquid structure near the electrode. Ab initio calculations reveal that the magnitude of the attraction between ions and conformational flexibility ions play important roles in determining transport properties of ionic liquids. First principle molecular dynamics simulations elucidate why solvated cation complex is stable in the equimolar mixtures, although the Li+ -[TFSA]- interaction is greater than Li+ -glyme interaction.

On the concentration polarisation in molten Li salts and borate-based Li ionic liquids.

Electrolytes that transport only Li ions play a crucial role in improving rapid charge and discharge properties in Li secondary batteries. Single Li-ion conduction can be achieved via liquid materials such as Li ionic liquids containing Li+ as the only cations because solvent-free fused Li salts do not polarise in electrochemical cells, owing to the absence of neutral solvents that allow polarisation in the salt concentration and the inevitably homogeneous density in the cells under anion-blocking conditions. However, we found that borate-based Li ionic liquids induce concentration polarisation in a Li/Li symmetric cell, which results in their transference (transport) numbers under anion-blocking conditions (tabcLi) being well below unity. The electrochemical polarisation of the borate-based Li ionic liquids was attributed to an equilibrium shift caused by exchangeable B-O coordination bonds in the anions to generate Li salts and borate-ester solvents at the electrode/electrolyte interface. By comparing borate-based Li ionic liquids containing different ligands, the B-O bond strength and extent of ligand exchange were found to be directly linked to the tabcLi values. This study confirms that the presence of dynamic exchangeable bonds causes electrochemical polarisation and provides a reference for the rational molecular design of Li ionic liquids aimed at achieving single-ion conducting liquid electrolytes.

Carbonaceous-Material-Induced Gelation of Concentrated Electrolyte Solutions for Application in Lithium-Sulfur Battery Cathodes.

Lithium-sulfur (Li-S) batteries can theoretically deliver high energy densities exceeding 2500 Wh kg-1. However, high sulfur loading and lean electrolyte conditions are two major requirements to enhance the actual energy density of the Li-S batteries. Herein, the use of carbon-dispersed highly concentrated electrolyte (HCE) gels with sparingly solvating characteristics as sulfur hosts in Li-S batteries is proposed as a unique approach to construct continuous electron-transport and ion-conduction paths in sulfur cathodes as well as achieve high energy density under lean-electrolyte conditions. The sol-gel behavior of carbon-dispersed sulfolane-based HCEs was investigated using phase diagrams. The sol-to-gel transition was mainly dependent on the amount of the carbonaceous material and the Li salt content. The gelation was caused by the carbonaceous-material-induced formation of an integrated network. Density functional theory (DFT) calculations revealed that the strong cation-π interactions between Li+ and the induced dipole of graphitic carbon were responsible for facilitating the dispersion of the carbonaceous material into the HCEs, thereby permitting gel formation at high Li-salt concentrations. The as-prepared carbon-dispersed sulfolane-based composite gels were employed as efficient sulfur hosts in Li-S batteries. The use of gel-type sulfur hosts eliminates the requirement for excess electrolytes and thus facilitates the practical realization of Li-S batteries under lean-electrolyte conditions. A Li-S pouch cell that achieved a high cell-energy density (up to 253 Wh kg-1) at a high sulfur loading (4.1 mg cm-2) and low electrolyte/sulfur ratio (4.2 µL mg-1) was developed. Furthermore, a Li-S polymer battery was fabricated by combining the composite gel cathode and a polymer gel electrolyte.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: effect of the alkyl chain substitution position.

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronic and molecular devices. In this study, we investigated the effect of substitution positions of alkyl chains on the formation of halogen-bonded molecular networks. For this purpose, building blocks with different head groups (i.e., pyridine (Py) or tetrafluoro-iodobenzene (FI)) were substituted with hexadecyloxy chains at either the 3,4- or the 3,5-positions. The two-dimensional assembly of each compound as a single-component system was studied using scanning tunnelling microscopy (STM) at the highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface. All compounds displayed linear structures in which the alkyl chains were aligned along one of the HOPG axes. In the exceptional case of FI bearing hexadecyloxy chains at the 3,5-positions (denoted as FI-3,5), hexagonal arrays were tentatively formed owing to the triangular molecular arrangement induced by halogen bonding. A bicomponent blend of Py-3,4/FI-3,5 (1 : 1 molar ratio) enabled the formation of a honeycomb structure, whereas that of Py-3,5/FI-3,4 (1 : 1 molar ratio) produced a rectangular assembly that was periodically arranged in a zig-zag fashion. Finally, based on the observed blend ratio dependence, the formation of these different two-dimensional structures by variation in the substitution positions of the alkyl chains was discussed in terms of molecule-molecule and molecule-substrate interactions.

Tools for studying ion solvation and ion pair formation in ionic liquids: isotopic substitution Raman spectroscopy.

Isotopic H/D or 6/7Li substitution Raman spectroscopy was applied to new kinds of ionic liquids; N-methylimidazole (C1Im) and acetic acid (CH3COOH) as the pseudo-protic ionic liquid (pPIL), and both of the neat and the 2,2,3,3-tetrafluoropropyl ether (HFE) diluted Li-glyme solvate ionic liquids (SIL) [Li(Gn)][TFSA] (Gn, glyme n = 3 or 4); TFSA, bis(trifluoromethanesulfonyl)amide) to clarify the proton transfer or the Li+ solvation/ion pair formation. The isotopic substitution Raman (ISR) spectra were obtained as the difference between the samples containing the same composition except the substituted isotope. The calculated and theoretical ISR spectra were also evaluated for comparison. With the C1Im-CH3COOH(D) pPIL, the Raman bands attributable to the C1Im/C1HIm+ gave signals of differential shape, and they were well reproduced with the curve fitting by taking the small amount of C1HIm+ and CH3COO- generation into consideration. The ISR spectra for the SIL were well explained by the formation of the Li-TFSA contact ion pair (CIP) and the solvent shared ion pair (SSIP) in the [Li(G3)][TFSA] SIL. In addition, the ISR spectra for the HFE-diluted [Li(G4)][TFSA] SIL clearly proved that the HFE hardly coordinates to the Li+ in the HFE-diluted SIL. Here, the ISR spectroscopy is proposed as a new tool for studying the ion solvation and the ion pair formation in ionic liquids.

LiNi0.5Mn1.5O4-Hybridized Gel Polymer Cathode and Gel Polymer Electrolyte Containing a Sulfolane-Based Highly Concentrated Electrolyte for the Fabrication of a 5 V Class of Flexible Lithium Batteries.

The design and fabrication of lithium secondary batteries with a high energy density and shape flexibility are essential for flexible and wearable electronics. In this study, we fabricated a high-voltage (5 V class) flexible lithium polymer battery using a lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode. A LiNi0.5Mn1.5O4-hybridized gel polymer cathode (GPC) and a gel polymer electrolyte (GPE) membrane, both containing a sulfolane (SL)-based highly concentrated electrolyte (HCE), enabled the fabrication of a polymer battery by simple lamination with a metallic lithium anode, where the injection of the electrolyte solution was not required. GPC with high flexibility has a hierarchically continuous three-dimensional porous architecture, which is advantageous for forming continuous ion-conduction paths. The GPE membrane has significant ionic conductivity enough for reliable capacity delivery. Therefore, the fabricated lithium polymer pouch cells demonstrated excellent capacity retention under continuous deformation conditions. This study provides a promising strategy for the fabrication of scalable and flexible 5 V class batteries using GPC and GPE containing SL-based HCE.

Well-organised two-dimensional self-assembly controlled by in situ formation of a Cu(II)-coordinated rufigallol derivative: a scanning tunnelling microscopy study.

The two-dimensional self-assembly of rufigallol derivatives and their metal coordination were studied by scanning tunnelling microscopy. Ex situ Cu(II)-coordinated rufigallol derivatives exhibited columnar structures with some defects, whereas regular and linear structures were formed upon in situ metal coordination at solid/liquid interfaces.

Dicationic oligotelluroxane or mononuclear telluronium cation? Elucidation of the true catalytic species and activation mechanism of the benzylic carbon-halogen bond.

The application of diaryltelluronium cations as chalcogen bonding organocatalysts was investigated for the Ritter-like reaction using time-course NMR analysis. The resistance to water of dicationic oligotelluroxanes differed depending on the oligomer chain length and counter anions. The activation mechanism of the substrate was discussed based on DFT calculations.

Chiral anthranilic pyrrolidine as custom-made amine catalyst for enantioselective Michael reaction of nitroalkenes with carbonyl compounds.

A chiral anthranilic pyrrolidine catalyst as a custom-made amine-catalyst was developed for the enantio- and diastereo selective Michael reaction of nitroalkenes with carbonyl compounds. In particular, a peptide-like catalyst in which an α-amino acid is attached to the anthranilic acid skeleton induced the high stereoselectivity of the reaction with aldehydes. Studies of the reaction mechanism indicated that the catalyst exhibits a divergent stereocontrol in the reaction, namely, steric control by a 2-substituted group on the catalyst and hydrogen-bonding control by a carboxylic acid group on the catalyst.

Thermodynamic aspect of sulfur, polysulfide anion and lithium polysulfide: plausible reaction path during discharge of lithium-sulfur battery.

The elucidation of elemental redox reactions of sulfur is important for improving the performance of lithium-sulfur batteries. The energies of stable structures of Sn, Sn˙-, Sn2-, [LiSn]- and Li2Sn (n = 1-8) were calculated at the CCSD(T)/cc-pVTZ//MP3/cc-pVDZ level. The heats of reduction reactions of S8 and Li2Sn with Li in the solid phase were estimated from the calculated energies and sublimation energies. The estimated heats of the redox reactions show that there are several redox reactions with nearly identical heats of reaction, suggesting that several reactions can proceed simultaneously at the same discharge voltage, although the discharging process was often explained by stepwise reduction reactions. The reduction reaction for the formation of Li2Sn (n = 2-6 and 8) from S8 normalized as a one electron reaction is more exothermic than that for the formation of Li2S directly from S8, while the reduction reactions for the formation of Li2S from Li2Sn are slightly less exothermic than that for the formation of Li2S directly from S8. If the reduction reactions with large exotherm occur first, these results suggest that the reduction reactions forming Li2Sn (n = 2-6 and 8) from S8 occur first, then Li2S is formed, and therefore, a two-step discharge-curve is observed.

Construction of poly-N-heterocyclic scaffolds via the controlled reactivity of Cu-allenylidene intermediates.

Controlling the sequence of the three consecutive reactive carbon centres of Cu-allenylidene remains a challenge. One of the impressive achievements in this area is the Cu-catalyzed annulation of 4-ethynyl benzoxazinanones, which are transformed into zwitterionic Cu-stabilized allenylidenes that are trapped by interceptors to provide the annulation products. In principle, the reaction proceeds via a preferential γ-attack, while annulation reactions via an α- or ß-attack are infrequent. Herein, we describe a method for controlling the annulation mode, by the manipulation of a CF3 or CH3 substituent, to make it proceed via either a γ-attack or an α- or ß-attack. The annulation of CF3-substituted substrates with sulfamate-imines furnished densely functionalized N-heterocycles with excellent enantioselectivity via a cascade of an internal ß-attack and an external α-attack. CH3-variants were transformed into different heterocycles that possess a spiral skeleton, via a cascade of an internal ß-attack and a hydride α-migration followed by a Diels-Alder reaction.

Nitroxyl Catalysts for Six-Membered Ring Bromolactonization and Intermolecular Bromoesterification of Alkenes with Carboxylic Acids.

We developed a nitroxyl-catalyzed bromoesterification of alkenes with bromo reagents, which includes a six-membered ring bromolactonization of alkenyl carboxylic acids catalyzed by AZADO as the nitroxyl radical catalyst, and an intermolecular bromoesterification of alkenes with carboxylic acids using NMO as the N-oxide catalyst. We also accomplished a remote diastereoselective bromohydroxylation via an AZADO-catalyzed six-membered ring bromolactonization and a subsequent ring cleavage reaction with alkylamines to furnish ε-bromo-δ-hydroxy amides with high diastereoselectivity.