Electron-rich Regions Research Articles

Robust Negative Electrode Materials Derived from Carbon Dots and Porous Hydrogels for High-Performance Hybrid Supercapacitors.

Hybrid supercapacitors generally show high power and long life spans but inferior energy densities, which are mainly caused by carbon negative electrodes with low specific capacitances. To improve the energy densities, the traditional methods include optimizing pore structures and modifying pseudocapacitive groups on the carbon materials. Here, another promising way is suggested, which has no adverse effects to the carbon materials, that is, constructing electron-rich regions on the electrode surfaces for absorbing cations as much as possible. For this aim, a series of hierarchical porous carbon materials are produced by calcinating carbon dots-hydrogel composites, which have controllable surface states including electron-rich regions. The optimal sample is employed as the negative electrode to fabricate hybrid supercapacitors, which show remarkable specific energy densities (up to 62.8-90.1 Wh kg-1 ) in different systems.

Outer electronic shell visualization by NMR chemical shift laplacian of a helium probe.

A new method of visualization of lone pairs is proposed using fluorine atoms in fluoroacetylene, trifluoroethylene, and fluoroform (sp, sp2 , and sp3 hybridized carbons) as an example. The space around fluorines was probed by a helium atom and at each point 3 He NMR chemical shift δHe and its Laplacian ∇2 δHe were calculated. The ∇2 δHe isosurfaces have symmetrical toroidal shapes along CF axes; the maximum values of ∇2 δHe are reached at about 1.5 Å from fluorine atom center in the direction roughly coinciding with the lone pair localization. The absolute values of the ∇2 δHe could be used to quantify the electron-rich regions of lone pairs. The results are compared with more common visualization methods-3D maps of the molecular electrostatic potential and the electron localization function. We show that a helium atom could be used for evaluation of fine features of molecular electronic shell, since 3 He NMR spectral characteristics remain sensitive at relatively large distances. © 2018 Wiley Periodicals, Inc.

A combined experimental and theoretical study on 4‑hydroxy carbazole by FT-IR, FT-Raman, NMR, UV–visible and quantum chemical investigations

Abstract Spectroscopic properties of 4‑hydroxy carbazole have been characterized by analytical spectroscopic techniques with help of high level quantum chemical calculations. For the structural confirmation and to gain better insight on 4‑hydroxy carbazole, the structural properties, vibrational assignments, chemical shifts, electronic properties and other molecular parameters were determined and comprehensively discussed. The molecular electrostatic potential surfaces (MESP) have been generated to isolate the reactive sites for electron rich and electron poor regions in this compound, and additionally confirmed with Mullikan atomic charge distribution analysis. The obtained experimental and theoretical results show an excellent correlation to confirm the molecular structure of 4‑hydroxy carbazole.

Cocrystals of pyrazinamide with p-toluenesulfonic and ferulic acids: DFT investigations and molecular docking studies

Cocrystals of pyrazinamide with p-toluenesulfonic acid (PZTSA) and ferulic acid (PZFER) are investigated by density functional calculations to find geometrical parameters, wavenumbers and different molecular properties. The vibrational modes associated with the phenyl and pyrazine rings have small changes during the cocrystal formation, while changes are associated with the amide group of pyrazinamide, hydroxyl and carboxylic groups of ferulic acid and SO3 group of PZTSA. Using density functional theory method the molecular geometries of the co-crystals were optimized in the ground state and a comparison is made with the reported experimental data. The theoretically obtained wavenumbers are assigned by means of potential energy distribution. The downshift of different modes in the infrared spectrum is due to hydrogen bonding and this is supported by the strong hyper conjugative interactions given by natural bond orbital analysis. HOMO is delocalized over the entire molecule except phenyl ring and methyl group and LUMO is delocalized over the entire molecule except CONH2 group for PZTSA while for PZFER, HOMO and LUMO are delocalized over the ferulic acid. From the MEP plot, electron rich regions are mainly localized over CO and SO3 groups of PZTSA while for PZFER, all the oxygen atoms electron rich regions. The first hyperpolarizability of PZTSA and PZFER are 5.99 and 89.66 times that of urea. Different molecular properties like, global chemical descriptors, frontier molecular orbital analysis, nonlinear optical properties and natural bond orbital analysis are also discussed in the present work. The different functional groups in the title compounds are identified by configurations markers using the theoretical VCD spectra analysis. Light harvesting efficiency analysis shows that PZFER is suitable for photo sensitizers in DSSC's. Docking results suggest that the compounds might exhibit inhibitory activity against mycobacterium tuberculosis type II and the compounds can be developed as a new anti-TB drug.

Role Reversal of the Carboxylate Group from Coordination to Hydrogen Bonding Only, in Structurally Diverse Metal-2-amino,5-Nitro-benzoates: A First Report

The remarkable structural diversities arising from a small ligand, 2-amino,5-nitro-benzoic acid (L1H), equipped with different functional groups, conferring tunable coordination sites as well as H-bonding abilities, have been explored. Six new crystal structures of Cs(I), Na(I), K(I), and Li(I) ions with L1 have been realized. Notably, for the medium-sized alkali metal ions, a role reversal between the coordination and H-bonding nature of −COO– and −NO2 groups, respectively, has been observed for the first time. Since L1 possesses three potential sites for coordination as well as H-bonding interactions, we realized a delicate control of noncovalent interactions on the resulting supramolecular assemblies. Structural diversities observed range from one-dimensional helices and linear threads to two-dimensional brick-wall types or three-dimensional networks. The electrostatic surface potential (ESP) of three representative complexes provided an insight into the electronic-deficient and electron-rich regions. The coordination of Na(I) and K(I) ions through the nitro groups extends the electron-deficient region of the complexes. The electron densities at the bond critical paths of the representative complexes were calculated to understand the supramolecular outcome of the coordination polymers. Unravelling of such critical electronic information is paramount toward the systematic construction of a new generation of complex coordination polymers.

Investigation and modeling of photocurrent collection process in multiple quantum well solar cells

Solar cells employing quantum wells can enhance the light absorption but suffer from the difficulty in photocarrier extraction. Here, we analyzed the spectral response and the photocarrier collection mechanism of p-i-n multiple quantum well (MQW) solar cells using the effective-mobility model. Both the simulation and experimental results imply that the spatial profiles of electron and hole densities in MQWs play an important role in the carrier collection process. By considering the recombination increment under illumination, our findings suggest that the concept of the majority/minority carriers is important even in the intrinsic region: photogenerated electrons and holes only experience significant recombination when passing through the hole-rich and electron-rich regions, respectively. This can accurately explain the photocurrent behavior in cells with background doping, background illumination, and different MQW positions. Based on the experimental findings, we derived analytical formulae for carrier collection efficiency, which directly show the impact of each cell parameter and can be used for the systematic cell design.

Revealing Germylene Compounds to Attain Superbasicity with Sigma Donor Substituents: A Density Functional Theory Study.

Compounds of GeII are shown for the first time to function as superbases. Two B(N=PiPr3 )2 groups attached to a germanium(II) center show a gas-phase proton affinity of 296.2 kcal mol-1 , which is close to the range of a hyperbase as revealed by B3LYP-D3/6-31G(2d,p) level of theory. These DFT calculations showed better agreement of geometrical parameters for the reported stable germylene compound 1 than previously reported calculations. A systematic study with different substitutions of GeII revealed that such a system can achieve basicity close to a hyperbase. The stabilities of these superbases were examined with dimerization energy and singlet-triplet state energy difference (ΔES-T ). Furthermore, the calculated gas-phase proton affinity values also show good correlation with the most negative valued point (Vmin ) in electron-rich regions from the molecular electrostatic potential. The high PA values of compounds were also supported by ionization potential, electron affinity, absolute electronegativity, and absolute hardness calculations. The energetics for the reaction with BH3 and AlMe3 further suggest that the lone pair of GeII can act as a Lewis base and display higher donor-acceptor bond strengths.

Electrocatalytic activity of lithium polysulfides adsorbed into porous TiO2 coated MWCNTs hybrid structure for lithium-sulfur batteries

Lithium-sulfur batteries have attracted great attention because of their high energy density, environmental friendliness, natural abundance and intrinsically low cost of sulfur. However, their commercial applications are greatly hindered by rapid capacity decay due to poor conductivity of electrode, fast dissolution of the intermediate polysulfides into the electrolyte, and the volume expansion of sulfur. Herein, we report a novel composite MWCNTs@TiO2-S nanostructure by grafting TiO2 onto the surface of MWCNTs, followed by incorporating sulfur into the composite. The inner MWCNTs improved the mechanical strength and conductivity of the electrode and the outer TiO2 provided the adsorption sites to immobilize polysulfides due to bonding interaction between TiO2 and polysulfides. The MWCNTs@TiO2-S composite with a mass ratio of 50% (MWCNTs in MWCNTs@TiO2) exhibited the highest electrochemistry performance among all compositing ratios of MWCNTs/TiO2. The performance improvement might be attributed to the downward shift of the apparent Fermi level to a more positive potential and electron rich space region at the interface of MWCNTs-TiO2 that facilitates the reduction of lithium polysulfide at a higher potential. Such a novel hybrid structure can be applicable for electrode design in other energy storage applications.

H2O incorporation in the phosphorene/a-SiO2 interface: a first-principles study

Based on first-principles calculations, we investigate (i) the energetic stability and electronic properties of single-layer phosphorene (SLP) adsorbed on an amorphous SiO2 surface (SLP/a-SiO2), and (ii) the further incorporation of water molecules at the phosphorene/a-SiO2 interface. In (i), we find that the phosphorene sheet binds to a-SiO2 through van der Waals interactions, even in the presence of oxygen vacancies on the surface. The SLP/a-SiO2 system presents a type-I band alignment, with the valence (conduction) band maximum (minimum) of the phosphorene lying within the energy gap of the a-SiO2 substrate. The structure and the surface-potential corrugations promote the formation of electron-rich and electron-poor regions on the phosphorene sheet and at the SLP/a-SiO2 interface. Such charge density puddles are strengthened by the presence of oxygen vacancies in a-SiO2. In (ii), because of the amorphous structure of the surface, we consider a number of plausible geometries for H2O embedded in the SLP/a-SiO2 interface. There is an energetic preference for the formation of hydroxyl (OH) groups on the a-SiO2 surface. Meanwhile, in the presence of oxygenated water or interstitial oxygen in the phosphorene sheet, we observe the formation of metastable OH bonded to the phosphorene, and the formation of energetically stable P–O–Si chemical bonds at the SLP/a-SiO2 interface. Further x-ray absorption spectra simulations are performed, which aim to provide additional structural/electronic information on the oxygen atoms forming hydroxyl groups or P–O–Si chemical bonds at the interface region.

ChemInform Abstract: Geometric Isomerism in Coordination Cages Based on Tris‐chelate Vertices: a Tool to Control Both Assembly and Host/Guest Chemistry

AbstractReview: 77 refs.

Inside Cover: A Non‐Exploding Alkali Metal Drop on Water: From Blue Solvated Electrons to Bursting Molten Hydroxide (Angew. Chem. Int. Ed. 42/2016)

Hydrated electrons (blue) can been seen by the naked eye despite their very short lifetime in water. Gently placing a drop of water on a sodium/potassium alloy drop leads to a vigorous reaction, but avoids an explosion, as shown by S. Bauerecker, P. Jungwirth, and co-workers in their Communication on page 13019 ff. The snapshot depicts the glass reaction vessel with a shiny alkali metal drop surrounded by a blue, electron-rich region and white hydroxide as the final product of the reaction.

Cooperative Hydrogen Bonding, Molecular Electrostatic Potentials, and Spectral Characteristics of Partial Thia-Substituted Calix[4]arene Macrocycles.

Structure and spectral characteristics of the 2,14-dithiacalix[4]arene and its homooxa derivatives are obtained employing the dispersion-corrected ωB97X-D-based density functional theory. The conformational behavior of these receptors is governed by the nature and number of substituents at the bridging positions. The partial thia-substituted calix[4]arene scaffolds reveal the electron-rich regions reside near heteroatoms which emerge with deeper minima in molecular electrostatic potential topography. Underlying cooperativity of intramolecular hydrogen bonding manifests in characteristic OH vibrations of their infrared spectra. Moreover, the (1)H NMR reveals that hydrogen-bonded protons are deshielded, unlike those from tertiary butyl substituents. These inferences are in agreement with the experimental data.

Self-powered graphene thermistor

Self-powered flexible graphene thermistor device is successfully fabricated by modifying the graphene layers with ferroelectric polarization. Chemical vapor deposited graphene monolayer was transferred to the surface of a ferroelectric polymer poly[(vinylidenefluoride-co-trifluoroethylene)] P(VDF-TrFE) thin film. By utilizing alternating polarization charge, the graphene surface contains polarization induced hole-rich, intrinsic and electron-rich regions (p-i-n regions). The potential difference created in the graphene layer leads to the drift of the extra carriers created in the graphene monolayer with increasing temperature and gives rise to an exemplary self-powered or autonomous thermistor.

Comparison of pillar[5]arene and calix[4]arene anion receptor ability in aqueous media

Recognition ability of both cationic pillar[5]arene and calix[4]arene has been studied in aqueous media. Anion complexation can be evaluated from their ability to complex their counterions as well as an added external organic anion. DOSY NMR experiments and fluorescence quenching show that pillararenes have a larger ability for including their own counterions than calixarenes irrespective of the anion (tetrafluoroborate or chloride or bromide) and the structure of the cationic moiety (trimethylammonium or methylimidazolium). Counterion complexation shows a picture where four to five positive charges of the pillar[5]arene are neutralised, meanwhile only one positive charge of the calixarene is neutralised for a 1 mM solution of the macrocycle. Irrespective of the smaller net positive charge in the pillar[5]arene, its binding ability for organic anions (toluenesulfonate or hydroxybenzoate) is larger than for calix[4]arene allowing a better accommodation of the guest in its cavity. The larger separation between the cationic groups of the receptor and its electron-rich aromatic region improves the anion recognition ability for pillar[5]arene.

Geometric isomerism in coordination cages based on tris-chelate vertices: a tool to control both assembly and host/guest chemistry.

This 'Perspective' article summarises recent work from the authors' research group on the exploitation of the simple fac/mer geometric isomerism of octahedral metal tris-chelates as a tool to control the chemistry of coordination cages based on bis(pyrazolyl-pyridine) ligands, in two different respects. Firstly this geometric isomerism plays a major role in controlling the guest binding properties of cages because a fac tris-chelate arrangement of pyrazolyl-pyridine chelates around a metal ion vertex results in formation of a convergent set of inwardly-directed C-H protons in a region of high positive electrostatic potential close to a metal cation. This collection of δ+ protons therefore provides a charge-assisted hydrogen-bond donor site, which interacts with the electron-rich regions of guest molecules that are of the correct size and shape to occupy the cage cavity, and the strength of this hydrogen-bonding interaction plays a major role in guest recognition in non-aqueous solvents. Secondly the ability to prepare mononuclear complexes with either a fac or mer arrangement of ligands provides an entry into the controlled, stepwise assembly of heterometallic cages based on a combination of kinetically inert and kinetically labile metal ions at different sites. This has allowed introduction of useful physical properties such as redox activity or luminescence, commonly associated with inert metal ions which are not amenable to participation in thermodynamic self-assembly processes, to be incorporated in a predictable way into the superstructures of coordination cages at specific sites.

Pentacycloundecane lactam vs lactone norstatine type protease HIV inhibitors: binding energy calculations and DFT study.

BackgroundNovel pentacycloundecane (PCU)-lactone-CO-EAIS peptide inhibitors were designed, synthesized, and evaluated against wild-type C-South African (C-SA) HIV-1 protease. Three compounds are reported herein, two of which displayed IC50 values of less than 1.00 μM. A comparative MM-PB(GB)SA binding free energy of solvation values of PCU-lactam and lactone models and their enantiomers as well as the PCU-lactam-NH-EAIS and lactone-CO-EAIS peptide inhibitors and their corresponding diastereomers complexed with South African HIV protease (C-SA) was performed. This will enable us to rationalize the considerable difference between inhibitory concentration (IC50) of PCU-lactam-NH-EAIS and PCU-lactone-CO-EAIS peptides.ResultsThe PCU-lactam model exhibited more negative calculated binding free energies of solvation than the PCU-lactone model. The same trend was observed for the PCU-peptide inhibitors, which correspond to the experimental activities for the PCU-lactam-NH-EAIS peptide (IC50 = 0.076 μM) and the PCU-lactone-CO-EAIS peptide inhibitors (IC50 = 0.850 μM). Furthermore, a density functional theory (DFT) study on the natural atomic charges of the nitrogen and oxygen atoms of the three PCU-lactam, PCU-lactim and PCU-lactone models were performed using natural bond orbital (NBO) analysis. Electrostatic potential maps were also used to visualize the electron density around electron-rich regions. The asymmetry parameter (η) and quadrupole coupling constant (χ) values of the nitrogen and oxygen nuclei of the model compounds were calculated at the same level of theory. Electronic molecular properties including polarizability and electric dipole moments were also calculated and compared. The Gibbs theoretical free solvation energies of solvation (∆Gsolv) were also considered.ConclusionsA general trend is observed that the lactam species appears to have a larger negative charge distribution around the heteroatoms, larger quadrupole constant, dipole moment and better solvation energy, in comparison to the PCU-lactone model. It can be argued that these characteristics will ensure better eletronic interaction between the lactam and the receptor, corresponding to the observed HIV protease activities in terms of experimental IC50 data.Electronic supplementary materialThe online version of this article (doi:10.1186/s12929-015-0115-5) contains supplementary material, which is available to authorized users.

Intermolecular carbon-carbon, nitrogen-nitrogen and oxygen-oxygen non-covalent bonding in dipolar molecules.

Clear evidence for the existence of intermolecular carbon-carbon (C···C), nitrogen-nitrogen (N···N) and oxygen-oxygen (O···O) interactions between atoms in similar chemical environments in homogeneous dimers of organic dipolar molecules has been obtained from molecular orbital (MO), natural bond orbital (NBO) and atoms-in-molecule (AIM) electron density analyses at the M06L/6-311++G(d,p) level of density functional theory (DFT). These X···X type interactions are mainly the result of local polarization effects, causing segregation of electron-rich and electron-deficient regions in the X atoms, leading to complementary electrostatic interactions. NBO analysis provides evidence of charge transfer between the two X atoms. Even in symmetrical molecules such as acetylene, induced dipoles in the dimer create C···C bonding interactions. The strength of this type of interaction increases with increase in the dipole moment of the molecule. Energy decomposition analysis (EDA) shows that the electrostatic component of the interaction energy (Eint) is very high, up to 95.86%. The C···C interactions between similar carbon atoms are located for several crystal structures obtained from the literature. In addition, MO, AIM and electrostatic potential analyses support interactions between similar oxygen (O···O) and nitrogen (N···N) atoms in a variety of molecular dimers. Good prediction of Eint is achieved in terms of the total gain in electron density at non-covalently interacting intermolecular bonds (∑ρ) and the monomer dipole moment (μ). A rigorously tested QSAR equation has been derived to predict Eint for all dimer systems: Eint (kcal mol(-1)) = -138.395∑ρ(au) - 0.551μ (Debye). This equation suggests that the polarization-induced bonding interaction between atoms in a similar chemical environment could well be a general chemical phenomenon. The results have been further validated by different density functional methods and also by G3MP2 method.

Small-Molecule Activation Driven by Confinement Effects

Electron-rich environments may arise from interactions between nanostructures at subnanometer separations and have been shown able to facilitate molecular dissociation of simple diatomic molecules. Electrons in these regions have energies close to the Fermi energy of the structures contributing to the electron-rich environment. These energies can be tuned to achieve specific interactions with the LUMO of molecules of interest. In this work, we focus on electron-rich regions generated between platinum nanoclusters and carbon nanotubes in the confined space defined by a nanopillared graphene (NPG) structure decorated with Pt22 nanoparticles. First, the reactivity of these regions is evaluated using density functional theory by comparing the bond strengths of O2, CO, and N2 adsorbed on Pt22 with that of the molecules adsorbed on similar surfaces in absence of surrounding electron-rich regions. Results show larger charge transfer to the adsorbed molecules in the presence of the electron-rich environments. The...

Response of the plasma to the size of an anode electrode biased near the plasma potential

As the size of a positively biased electrode increases, the nature of the interface formed between the electrode and the host plasma undergoes a transition from an electron-rich structure (electron sheath) to an intermediate structure containing both ion and electron rich regions (double layer) and ultimately forms an electron-depleted structure (ion sheath). In this study, measurements are performed to further test how the size of an electron-collecting electrode impacts the plasma discharge the electrode is immersed in. This is accomplished using a segmented disk electrode in which individual segments are individually biased to change the effective surface area of the anode. Measurements of bulk plasma parameters such as the collected current density, plasma potential, electron density, electron temperature and optical emission are made as both the size and the bias placed on the electrode are varied. Abrupt transitions in the plasma parameters resulting from changing the electrode surface area are identified in both argon and helium discharges and are compared to the interface transitions predicted by global current balance [S. D. Baalrud, N. Hershkowitz, and B. Longmier, Phys. Plasmas 14, 042109 (2007)]. While the size-dependent transitions in argon agree, the size-dependent transitions observed in helium systematically occur at lower electrode sizes than those nominally derived from prediction. The discrepancy in helium is anticipated to be caused by the finite size of the interface that increases the effective area offered to the plasma for electron loss to the electrode.

Supramolecular Assemblies Obtained by Mixing Different Cyclodextrins and AOT or BHDC Reverse Micelles

In this contribution we show the effect of the surfactant polar head and the external solvent on the incorporation of different cyclodextrins (CDs) {α-CD, β-CD, γ-CD, decenylsuccinyl-β-CD (Mod-β-CD), and hydroxypropyl-β-CD (hp-β-CD)} in different reverse micelles (RMs) {benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate(AOT)/water, and benzene/benzyl-n-hexadecyldimethylammonium chloride (BHDC)/water} and compare them with previous results obtained in n-heptane/AOT/water RMs. To investigate the different systems, we have used UV-vis spectrophotometry, induced circular dichroism spectroscopy (ICD), and the achiral molecular probe methyl orange (MO). The results show dramatic differences changing the external solvent and the surfactant, which are explained by considering the differences in the RMs interface composition, the water-surfactant interaction, and the CDs' location in the different media investigated. None of the CDs were incorporated into the benzene/AOT/water RMs at any [H2O]/[surfactant] ratio studied (W0) whereas it was previously shown that Mod-β-CD and hp-β-CD could be included in n-heptane/AOT/water RMs. However, all of the CDs are incorporated in benzene/BHDC/water RMs at W0> 10 and hp-β-CD is dissolved even at W0 = 0. Different from what was found in n-heptane/AOT RMs, in BHDC RMs MO showed ICD signals with two different CDs: Mod-β-CD and hp-β-CD. The results are explained by considering the known difference in the interfacial water structure for AOT and BHDC RMs and the electron-rich region on the secondary hydroxyl (wider side of the CDs), which helps to solubilize all CDs in BHDC. This study shows that chiral cyclodextrin could be available for a guest in an organic medium such as the RMs. Therefore we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.