Calculated Electron Density Distribution Research Articles

On the photophysics of nanographenes - investigation of functionalized hexa-peri-hexabenzocoronenes as model systems.

In this study, we report on hexa-peri-hexabenzocoronenes (HBCs) as representative models for nanographenes. To this end, we synthesized a family of functionalized HBCs and investigated the impact of the substituents on the π-extended systems of the HBCs. DFT and TD-DFT calculations suggested a charge transfer character, which intensified as the electron density withdrawing effects of the substituents (-M-effect) increased. Unambiguous corroboration of the charge transfer character in the case of NO2-substituents was realized via steady-state absorption and fluorescence experiments, which focused on the dependencies on the solvent polarity and temperature featuring. Going beyond HBCs with NO2-substituents, time-correlated single photon counting, and femtosecond and nanosecond transient absorption spectroscopy unveiled long-lived singlet and triplet excited states. As a complement, we performed electrochemical and spectroelectrochemical measurements. These measurements were carried out to shed light onto the nature of the functionalized HBCs as electron acceptors and/or donors, on the one hand, and their corresponding spectroscopic signatures, on the other hand. All of the aforementioned information enabled intermolecular charge separation assays with, for example, suitable electron acceptors by steady-state and time-resolved spectroscopy.

Refined Structure of Langmuir Lysozyme Films on Single-Crystal Silicon Wafers According to X-Ray Reflectivity Data

Thin lysozyme films on single-crystal silicon wafers have been studied by X-ray reflectivity. The films have been formed using the modified Langmuir–Schaefer method with potassium chloride as a precipitant. A layered model of the structure of the investigated films based on the calculated electron density distribution in a lysozyme molecule is proposed. It is shown that the model obtained in this way allows one to refine the structure of investigated films when processing reflectivity data.

Experimental charge density of grossular under pressure - a feasibility study.

X-ray diffraction studies of crystals under pressure and quantitative experimental charge density analysis are among the most demanding types of crystallographic research. A successful feasibility study of the electron density in the mineral grossular under 1 GPa pressure conducted at the CRISTAL beamline at the SOLEIL synchrotron is presented in this work. A single crystal was placed in a diamond anvil cell, but owing to its special design (wide opening angle), short synchrotron wavelength and the high symmetry of the crystal, data with high completeness and high resolution were collected. This allowed refinement of a full multipole model of experimental electron distribution. Results are consistent with the benchmark measurement conducted without a diamond-anvil cell and also with the literature describing investigations of similar structures. Results of theoretical calculations of electron density distribution on the basis of dynamic structure factors mimic experimental findings very well. Such studies allow for laboratory simulations of processes which take place in the Earth's mantle.

Significantly Enhanced Electrical Performances of Eco-Friendly Dielectric Liquids for Harsh Conditions with Fullerene.

The eco-friendly vegetable liquid is increasingly used because of the growing demand for environmentally friendly dielectric liquid. A vegetable liquid/fullerene nanofluid was fabricated via ultrasonic processing with good dispersion of the fullerene nanoparticles. It was observed that a small amount of fullerene (~100 mg/L) can significantly improve the electrical properties of vegetable insulating liquid (dissipation factor decreased by 20.1%, volume resistivity increased by 23.3%, and Alternating Current (AC) dielectric breakdown strength increased by 8.6%). Meanwhile, the trace amount of fullerene is also able to improve the electrical performances (i.e., dissipation factor and electrical resistivity) of the vegetable nanofluid under harsh conditions of long-term thermal aging compared with the blank contrast. The reduced acid values (25%) and dissolved decomposition gases (58.2% for hydrogen) in the aged vegetable nanofluid indicate the inhibition of molecule decomposition of vegetable liquid with fullerene. The improved electrical performances and thermal resistance of the vegetable nanofluid contribute to the electron affinity of fullerene proved by calculation of electron density distribution on the surface. The thermogravimetric analysis of the nanofluid under different atmospheres interprets that the oxygen absorbed inevitably in the fullerene contributes to the performance deterioration of the nanofluids during the initial aging. This work provides a potential method towards eco-friendly dielectric liquid with great electrical performances for harsh environments.

Hydrogen Bond-Driven Self-Assembly between Single-Layer MoS2 and Alkyldiamine Molecules

We report the synthesis, structure determination, and quantum-chemical analysis of a new family of layered nanocrystals (NCs) obtained by a liquid-phase assembly reaction of exfoliated, negatively charged MoS2 sheets with alkyldiammonium ions. A combined PXRD, TEM, FTIR and DFT study allowed us to determine the atomic structure of these turbostratically disordered NCs and to reveal the topology of cation-MoS2 binding interactions. The diamine molecules sandwiched between the sulfur layers of the adjacent 1T-MoS2 sheets were found to interlink these sheets through the hydrogen bonding interaction network. Quantification of these interactions on the basis of the analysis of calculated electron density distribution showed that the strong NH···S bonds contribute 40–80% of the total cation-MoS2 hydrogen bonding interaction energy (33–38 kcal/mol), being accompanied by the contribution of the weaker, but more numerous CH···S bonds. The short-range ordering in the positions of neighboring MoS2 layers was identif...

Elastic, Optoelectronic and Thermoelectric Properties of the Lead-Free Halide Semiconductors Cs2AgBiX 6 (X = Cl, Br): Ab Initio Investigation

We report a detailed investigation of the elastic moduli, electronic band structure, density of states, chemical bonding, electron and hole effective masses, optical response functions and thermoelectric properties of the lead-free halide double perovskites Cs2AgBiCl6 and Cs2AgBiBr6 using the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA-PBEsol) and the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential. Because of the presence of heavy elements in the studied compounds, we include the spin–orbit coupling (SOC) effect. Our calculated structural parameters agree very well with the available experimental and theoretical findings. Single-crystal and polycrystalline elastic constants are predicted using the total-energy versus strain approach. Three-dimensional representations of the crystallographic direction dependence on the shear modulus, Young’s modulus and Poisson’s ratio demonstrate a noticeable elastic anisotropy. The TB-mBJ potential with SOC yields an indirect band gap of 2.44 (1.93) eV for Cs2AgBiCl6 (Cs2AgBiBr6), in good agreement with the existing experimental data. The chemical bonding features are probed via density of states and valence electron density distribution calculations. Optical response functions were predicted from the calculated band structure. Both of the investigated compounds have a significant absorption coefficient (∼ $$ 25 \times 10^{4} \;{\hbox{cm}}^{ - 1} $$ ) in the visible range of sunlight. The thermoelectric properties of the title compounds were investigated using the FP-LAPW approach in combination with the semi-classical Boltzmann transport theory. The Cs2AgBiCl6 and Cs2AgBiBr6 compounds have a large thermopower S, which makes them potential candidates for thermoelectric applications.

Criticality-Enhanced Magnetocaloric Effect in Quantum Spin Chain Material Copper Nitrate

In this work, a systematic study of Cu(NO3)2·2.5 H2O (copper nitrate hemipentahydrate, CN), an alternating Heisenberg antiferromagnetic chain model material, is performed with multi-technique approach including thermal tensor network (TTN) simulations, first-principles calculations, as well as magnetization measurements. Employing a cutting-edge TTN method developed in the present work, we verify the couplings J = 5.13 K, α = 0.23(1) and Landé factors g∥= 2.31, g⊥ = 2.14 in CN, with which the magnetothermal properties have been fitted strikingly well. Based on first-principles calculations, we reveal explicitly the spin chain scenario in CN by displaying the calculated electron density distributions, from which the distinct superexchange paths are visualized. On top of that, we investigated the magnetocaloric effect (MCE) in CN by calculating its isentropes and magnetic Grüneisen parameter. Prominent quantum criticality-enhanced MCE was uncovered near both critical fields of intermediate strengths as 2.87 and 4.08 T, respectively. We propose that CN is potentially a very promising quantum critical coolant.

Ga–In intermixing, intrinsic doping, and Wigner localization in the emission spectra of self-organized InP/GaInP quantum dots

We present study of structural and optical properties of InP/GaInP quantum (QDs) providing a weak quantum confinement and creating a platform to study Wigner localization (WL) effects using high spatial resolution optical spectroscopy. Self-organized QD structures were grown using metal-organic chemical phase epitaxy by using different substrate misorientations and cap layer deposition temperatures. Using transmission electron microscopy and energy dispersive x-ray spectroscopy, we demonstrated a bimodal height distribution with peaks at ~5 and ~20 nm and a control of both the lateral size distribution, peaked from ~100 to ~160 nm, and the amount of Ga–In intermixing in the QDs (up to 20%). Using photoluminescence (PL) spectroscopy in combination with circular polarization degree and time resolved micro-PL measurements, we demonstrated control of the emission energy, the intrinsic doping, and the emission decay of these In(Ga)P QDs. Using high-spatial-resolution near-field PL spectra and imaging of single dots, we demonstrated WL effects in dots having a population of up to nine electrons and a parabolic confinement down to ħω0 ~ 1 meV. We performed a self-consistent calculation of exciton transitions using an effective mass, mean field theory with an isotropic elasticity model to describe the effect of Ga–In intermixing on the emission properties of these dots; and we used calculations of shell splitting, using mean field Hartree–Fock approach and calculations of electron density distribution using configuration interaction approach, to described effects of enhancement of WL in non-circular dots with hard-wall potentials.

Maximum Entropy Method Applied in the Experimental Visualization of Electron Density Distributions in BiFeO3

The BiFeO3 is a largely studied compound with magnetoeletric coupling at room temperature. This is due its potential technological applications that can be directly used in advanced technologies. BiFeO3 is an oxide with perovskite structure and R3c space group (rhombohedral symmetry). In this work, synchrotron radiation X-ray powder diffraction data was used to obtain a detailed description of BiFeO3 crystal structure. Thereafter, experimental electron density distribution calculations were obtained using by the maximum entropy method (MEM) statistical approach. The ionic/covalent nature of the bonding and the interaction between the ions are clearly revealed by constructing electron density maps. The Fe3+-O2− and Bi3+-O2− bonding were studied in 1 and 2 dimensional electron density distributions. Due the high resolution of the obtained maps, it can be regarded as the most precise electron density distributions seen inside the material.

Stabilization of 1T-MoS₂ Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals.

We report a facile, room-temperature assembly of MoS2-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methylimidazolium (BuMeIm), 2-phenylimidazolium, and 2-methylbenzimidazolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS2 sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5-20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS2 sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS2 sheets of the NCs are of the nonconventional 1T-MoS2 (metallically conducting) structure. The sheets' puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS2 modification and raises the temperature for its transition to the conventional 2H-MoS2 (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS2 (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS2.

Calculation of Electron Density Distribution in Cd<sub>0.5</sub>Zn<sub>0.5</sub>S by Density Functional Theory: Exploring its Bonding Character and Stability

The topology of electron density is a sketch that gives a clear picture about electronic distribution and details information of natural molecular bonding. In quantum mechanics or particularly in quantum chemistry, the electron density is a measure of the probability of an electron occupying at any infinitesimal element of space surrounding at any given point.Studies about the topology of electron density in Cd0.5Zn0.5S at plane (101) were conducted by the application of density functional theory (DFT) within linear augmented plane wave (LAPW) by using the WIEN2k software. In this calculation, generalized gradient approximation (GGA) methods were used to calculate the exchange-correlation potential of the electrons. Based on this calculation, the topology of electron density were presented in a contour plot (2D) and also inarelief map plot (3D) in order to see the way electron density, ρ of Cd0.5Zn0.5S defines the gradient field and hence the bonding type. From these plots, we identified the bond paths that coincide with the contours of electron to suggest that Cd0.5Zn0.5S exhibited characteristics of covalent and a slightly ionic bond character.

A study on AB2O6 compounds, Part V: a DFT study on charge balance as driving force for structural organisation

Abstract In continuation of earlier investigations on the structures of AB2O6 compounds we have remodelled those with A=Ca, Sr, Ba and B=As, Sb, V, Nb, Ta by relaxation in DFT calculations with the VASP code. The results match the real structures very closely. We have then calculated charges of the constituent atoms by means of a Bader analysis of the calculated electron density distribution. The results of these calculations are discussed with the aim to establish a correlation between the charges of the atoms and the vicinity structure around the atoms. Similar calculations were done first for all these compounds in a rosiaite type structure and second for CaAs2O6 in all the other structures found for these compounds. We discuss how the balance of charges changes on forcing the systems into another topology, and we finally address the issue that structural organisation is strongly determined by the need to balance charges of the constituting elements in their specific stoichiometric relation.

Insights into the crystal chemistry of Earth materials rendered by electron density distributions: Pauling's rules revisited

Experimental and calculated electron density distributions determined for oxide and silicate crystals and siloxane molecules provide a new basis for addressing the classic foundation of the crystal chemistry of silicates, including atomic/ionic radii, the radius ratio rule and the nexus between the Pauling’s bond strength, resonance bond number, and bond length. The distributions indicate that the charge density of a bonded oxygen atom is highly distorted with its bonded radius decreasing systematically from 1.50 Å when bonded to highly electropositive atoms like sodium to ~ 0.65 Å when bonded to highly electronegative atoms like nitrogen. Rather than a single radius, the atom has as many bonded radii as it has bonded interactions. Bonded radii determined for the metal atoms match the Shannon effective ionic radii for the more electropositive atoms, but they depart and decrease systematically as the electronegativity of the M atoms increases. Pauling’s first rule is considered to be irrelevant given the asphericity and the range of the bonded radii displayed by the O atom. A power law regression expression is formulated between the average M-O bond lengths, <R(M-O)>, and the average value of the electron density, <ρ( r c )> = r[1.41/<R(M-O)>] 4.76 , at the bond critical point, r c , between pairs of bonded M-O atoms. The expression applies to a host of crystals and molecules comprising M-atoms for all rows, r, of the periodic table. The <ρ( r c )> values correlate with bond strength and resonance bond strength for the M-O bonded interactions on a one-to-one basis, demonstrating that the Pauling bond strength is a direct measure of the electron density involved in a bonded interaction and the accumulation of the electron density between the bonded pair. The widespread applications of the Brown-Shannon bond valence model in the Earth sciences and material science owes much of its success to the direct connection that exists between bond strength and the quantum mechanical observable, the electron density distribution. Compelling evidence is presented that supports the argument that the Si-O bonded interactions within siloxane molecules and silicate crystals are fundamentally the same, and that the local Si-O bonded interactions comprising molecules are, at the very core, equivalent to the Si-O bonded interactions observed in silicate crystals. Bond paths between the O atoms comprising shared polyhedral edges are consistent with Pauling’s third rule, the shorter the O-O shared edges, the greater the accumulation of the electron density between the O atoms, the greater the stabilization of the shared edges.

Topology of the Electron Density ofd0Transition Metal Compounds at Subatomic Resolution

Accurate X-ray diffraction experiments allow for a reconstruction of the electron density distribution of solids and molecules in a crystal. The basis for the reconstruction of the electron density is in many cases a multipolar expansion of the X-ray scattering factors in terms of spherical harmonics, a so-called multipolar model. This commonly used ansatz splits the total electron density of each pseudoatom in the crystal into (i) a spherical core, (ii) a spherical valence, and (iii) a nonspherical valence contribution. Previous studies, for example, on diamond and α-silicon have already shown that this approximation is no longer valid when ultrahigh-resolution diffraction data is taken into account. We report here the results of an analysis of the calculated electron density distribution in the d(0) transition metal compounds [TMCH3](2+) (TM = Sc, Y, and La) at subatomic resolution. By a detailed molecular orbital analysis, it is demonstrated that due to the radial nodal structure of the 3d, 4d, and 5d orbitals involved in the TM-C bond formation a significant polarization of the electron density in the inner electronic shells of the TM atoms is observed. We further show that these polarizations have to be taken into account by an extended multipolar model in order to recover accurate electron density distributions from high-resolution structure factors calculated for the title compounds.

Density functional theory study on configurations and electronic properties of periodic nanoridges

Periodic nanoridges (PNRs), a new kind of hybrid nanostructures which consists of graphene monolayer and zigzag single-wall carbon nanotube (SWCNT), were engineered in this work. With density functional theory methods, two stable hybrid carbon nanostructures coming from two different types of absorption between SWCNT and graphene were obtained. Based on calculations of electron density distribution and binding energy, the first structure (PNR-1) is revealed to be more thermodynamically favorable, compared with the second one (PNR-2). Electronic band structures analyses suggest that both PNR-1 and PNR-2 are narrow-band-gap semiconducting with a direct band gap, which indicates promising applications for photocatalyst.

The azide anion as a building block in crystal engineering from a charge density point of view

The geometrical preferences of hydrogen bonds with azide anion are studied using topological analysis of experimental and calculated electron density distribution. The comparison of electron localization function (ELF) and QTAIM analysis shows the reasons behind the apparent unidirectionality of azide anion's lone pairs. The similarity between hydrogen and coordinated bonds involving azide species is demonstrated via the analysis of the CSD.

First principles study of the electron density distribution in a pair of bare metallic electrodes

Self-consistent calculations of electron density distribution from first principles for a series of semi-infinite metals show that the electron density almost drops to zero at 8.5 a.u. away from a metal surface. The electron densities in a series of bimetallic-electrode systems with a distance between the two electrodes of 21.7 a.u. are further investigated. Spin-polarized calculations of electron density for nonmagnetic and magnetic bimetallic-electrode systems are compared. Our work is helpful for first principles investigation of spin-dependent metal–molecule–metal tunneling junctions.

Interaction of ethyltrichlorostannane withN,N‐dimethylamides ofO‐ trimethylsilyl‐α‐hydroxyacids

AbstractReactions of ethyltrichlorostannane withN,N‐dimethylamides of 2‐(trimethylsilyl)glycolic (2a), 2(S)‐lactic [(S)‐2b] and mandelic (2c) acids led to the first representatives of tin(IV)O,O‐monochelates containing the five‐membered SnOCC(O) monoanionic chelate cycles as a result of the substitution of one halogen atom. According to the X‐ray data, in the case of the derivatives of glycolic and mandelic acids, the isolated complexes have dimeric structures {EtCl2Sn[OCH(R)C(O)NMe2]}2[R = H (3a), Ph (3c)] with the bridge oxygen atoms and hexacoordinated tin atoms. By contrast, in the case of the derivative of lactic acid (S)‐2b, the product of the reaction is the cationic–anionic complex 2EtCl2Sn[OCH(Me)C(O)NMe2]·HCl (4), which is the adduct of two molecules of the respective monomeric complex with HCl. In addition,N,N‐dimethylamide ofO‐triethylstannylglycolic acid (5) with tetracoordinated tin is described. To analyze the stability of complex 4, the experimental charge density study and the quantum chemical calculations [PBE0/6‐311G(d,p)] were carried out. Topological analysis of the experimental and calculated electron density distribution functions allowed us to conclude that the strong OH···O bond with pronounced covalent contribution stabilizes the dimeric structure of 4 to a greater extent than the bridged SnCl bonds. Copyright © 2010 John Wiley & Sons, Ltd.

Bond Paths and van der Waals Interactions in Orpiment, As2S3

The calculated electron density distribution for orpiment, As(2)S(3), reveals that As-S, S-S, and As-As bond paths are associated with the experimental interlayer directed bonded interactions detected in a combined infrared and Raman study. The successful modeling of the infrared- and Raman-determined interlayer bonded interactions together with bond paths and the structuralization of a variety of inorganic molecules in terms of "key-lock" bond path mainstays support the argument that van der Waals forces in inorganic molecular crystals are directional.

Unsymmetrical squaraine dimer with an extended π-electron framework: An approach in harvesting near infra-red photons for energy conversion

A fully conjugated unsymmetrical dimeric squaraine dye was synthesized. The synthetic key step for this dye, a selective Knoevenagel type condensation of end groups to the core moiety, allows the incorporation of a carboxylic acid function essentially for the strong coupling to the titanium dioxide (TiO 2) surface. Moreover, the dimer exhibit outstanding optical properties such as absorption maximum in the NIR region with a huge molar absorption coefficient. The solvatochromic effects were also studied. Furthermore the electrochemical properties were determined with cyclic voltammetry. The reduction potential is just high enough to allow an efficient photo induced charge injection into the conduction band of the TiO 2 semiconductor and the oxidation potential lies about 300 mV below the potential of the iodide / triiodide redox couple providing a good regeneration driving force. From electron density distribution calculations of the optimized BSQ01 structure we could see the directional charge displacement from the solvent exposed side in the HOMO to the carboxyl anchor group in the LUMO. The dimer was immobilised on TiO 2 nanoparticles and a first dye sensitized solar cell was made as a proof-of-concept.