Redistribution Of Electron Density Research Articles

Ab initio insight into graphene nanofibers to destabilize hydrazine borane for hydrogen release

We report the potential destabilizing effects of graphene nanofibers on the hydrogen release property of hydrazine borane via state-of-the-art ab initio calculations for the first time. Interactions of a hydrazine borane cluster with two types of graphene patch edges which exist abundantly in our synthesized graphene nanofibers have been investigated. It is found that both zigzag and armchair edges can greatly weaken the H-host bonds (especially the middle NH bond) of hydrazine borane. The dramatic decrease in hydrogen removal energy is caused by the strong interaction between hydrazine borane and the graphene patch edges concerning the electronic charge density redistribution.

Estimation of Ground-State and Singlet Excited-State Dipole Moments of Substituted Schiff Bases Containing Oxazolidin-2-one Moiety through Solvatochromic Methods

Absorption and fluorescence studies on novel Schiff bases (E)-4-(4-(4-nitro benzylideneamino)benzyl)oxazolidin-2-one (NBOA) and (E)-4-(4-(4-chlorobenzylidene amino)benzyl)oxazolidin-2-one (CBOA) were recorded in a series of twelve solvents upon increasing polarity at room temperature. Large Stokes shift indicates bathochromic fluorescence band for both the molecules. The photoluminescence properties of Schiff bases containing electron withdrawing and donating substituents were analyzed. Intramolecular charge transfer behavior can be studied based on the influence of different substituents in Schiff bases. Changes in position and intensity of absorption and fluorescence spectra are responsible for the stabilization of singlet excited-states of Schiff base molecules with different substituents, in polar solvents. This is attributed to the Intramolecular charge transfer (ICT) mechanism. In case of electron donating (-Cl) substituent, ICT contributes largely to positive solvatochromism when compared to electron withdrawing (-NO2) substituent. Ground-state and singlet excited-state dipole moments of NBOA and CBOA were calculated experimentally using solvent polarity function approaches given by Lippert-Mataga, Bakhshiev, Kawskii-Chamma-Viallet and Reichardt. Due to considerable π- electron density redistribution, singlet excited-state dipole moment was found to be greater than ground-state dipole moment. Ground-state dipole moment value which was determined by quantum chemical method was used to estimate excited-state dipole moment using solvatochromic correlations. Kamlet-Abboud-Taft and Catalan multiple linear regression approaches were used to study non-specific solute-solvent interaction and hydrogen bonding interactions in detail. Optimized geometry and HOMO-LUMO energies of NBOA and CBOA have been determined by DFT and TD-DFT/PCM (B3LYP/6-311G (d, p)). Mulliken charges and molecular electrostatic potential have also been evaluated from DFT calculations.

Relaxation method for constructing the interaction potentials of metal–nonmetal atomic pairs

An algorithm that employs the method of successive relaxation for determining the parameters of the pairwise interaction potential of iron and nonmetallic atoms that implicitly considers the redistribution of electron density between atoms is developed. The parameters of the interatomic interaction potential are calculated for ten pairs of elements: Fe–P, Fe–S, Fe–B, Fe–V, Fe–Mo, Fe–Cr, Fe–Mn, Fe–Si, Fe–Ni, and Fe–Al. The structural and thermodynamic properties of solid solutions based on iron and pure materials, and some pairwise interaction potentials constructed earlier in the Lennard–Jones form for identical metal atoms and homogeneous pairs of different metal–metal atoms, are used in these calculations.

Through space and through bridge channels of charge transfer at p-n nano-junctions: A DFT study

Details of charge density distribution at p-n nano interface are analyzed with density functional theory techniques using model system of dimers of doped silicon quantum dots interacting through bond and through space. Spatial distributions of transition densities between the ground and excited states suggest the character of essential electronic excitations, which have a Fӧrster, bound, unbound, or charge transfer character. A redistribution of electronic density from n-impurities to p-impurities results in a ground state polarization and creates an offset of energies of the bands localized on p-doped quantum dot and the bands localized on n-doped quantum dot. Although impurities contribute very few orbitals to the total density, a ground state charge redistribution and polarization are both responsible for the presence of a large number of charge transfer excitations involving solely silicon orbitals.

ChemInform Abstract: Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

AbstractReview: 362 refs.

Theoretical Study of the ESIPT Process for a New Natural Product Quercetin.

The investigation of excited-state intramolecular proton transfer (ESIPT) has been carried out via the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) method for natural product quercetin in dichloromethane (DCM) solvent. For distinguishing different types of intramolecular interaction, the reduced density gradient (RDG) function also has been used. In this study, we have clearly clarified the viewpoint that two kinds of tautomeric forms (K1, K2)originated from ESIPT processconsist inthe first electronic excited state (S1). The phenomenon of hydrogen bonding interaction strengtheninghas been proved by comparing the changes of infrared (IR) vibrational spectra and bond parameters of the hydrogen bonding groups in the ground state with that in the first excited state. The frontier molecular orbitals (MOs)provided visual electron density redistribution have further verified the hydrogen bond strengthening mechanism. It should be noted that the ESIPT process of the K2 form is easier to occur than that of the K1 form via observing the potential energy profiles. Furthermore, the RDG isosurfaces has indicated that hydrogen bonding interaction of the K2 form is stronger than that of the K1 formin the S1 state, which is also the reason why the ESIPT process of the K2 form is easier to occur.

First-principles study of organically modified muscovite mica with ammonium (NH $$_4^+$$ 4 + ) or methylammonium (CH $$_3$$ 3 NH $$_3^+$$ 3 + ) ion

Using density functional theory calculations, we have investigated the interlayer cation exchange phenomena in muscovite mica, which is motivated by a necessity to develop flexible high-insulating covering materials. The crystalline structures, chemical bonding properties, energetics, and electronic properties of muscovites before and after exchange of interlayer K$^+$ cation with ammonium (NH$_4^+$) or methylammonium (CH$_3$NH$_3^+$) ion were calculated. It was found that the unit cell volume changes are negligibly small upon exchange with NH$_4^+$ ion, while the unit cells are expanded with about 4 \% relative rate when replacing the interlayer K$^+$ cation with CH$_3$NH$_3^+$ ion. The energy band gap of pre-exchanged muscovite was calculated to be about 5 eV, which hardly changes upon interlayer cation exchange with either NH$_4^+$ or CH$_3$NH$_3^+$ ion, indicating the preservation of high insulating property of muscovite. The exchange energies were found to be about -100 kJ/mol for NH$_4^+$ and about -50 kJ/mol for CH$_3$NH$_3^+$ exchange, indicating that the exchange reactions are exothermic. A detailed analysis of atomic resolved density of states and electron density redistribution was provided.

Photophysical study of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile and estimation of ground-state and singlet excited-state dipole moments by solvatochromic approaches

An investigation of absorption and fluorescence behaviour of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (NDPPC) at room temperature was carried out using a series of twelve organic solvents with different polarities. Ground and excited state dipole moments were determined experimentally by using Lippert–Mataga polarity function, Bakhshiev solvent polarity parameter, Kawski-Chamma-Viallet solvent polarity parameter, Reichardt's microscopic solvent polarity parameter, Kamlet-Abboud-Taft multiple linear regression and Catalan dipolar polarizibility approach. Due to considerable π- electron density redistribution, the value of excited state dipole moment was found to be greater than that of the ground state. The ground state dipole moment value was determined by quantum chemical method which was used to estimate excited state dipole moment using solvatochromic correlations. Both non-specific solute-solvent interactions and hydrogen bonding interactions were investigated by using Kamlet-Abboud-Taft and Catalan method. TD-DFT (B3LYP/6-311G (d,p)) was used for the determination of HOMO-LUMO energies.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides.

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

A curious interplay in the films of N-heterocyclic carbene Pt(II) complexes upon deposition of alkali metals.

The recently synthesized series of PtII complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned PtII complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination PtII centre to Pt0 and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system “Pt complex-alkali metal ion”; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the PtII complexes for a variety of biomedical, sensing, chemical, and electronic applications.

Amination of Five Families of Room-Temperature Ionic Liquids: Computational Thermodynamics and Vibrational Spectroscopy

The amino group is important to enhance carbon dioxide capture capabilities of substances, including room-temperature ionic liquids (RTILs). We report thermodynamics of amination for five organic cations representing different families of RTILs (imidazolium, pyridinium, morpholinium, pyrrolidinium, pyrazolium) and compare all prospective amination sites. We conclude that amination of the cation ring is more favorable than amination of the side chains. Redistribution of electron density due to amination partially correlates with variation of enthalpy and Gibbs free energy, whereas entropic part is similar for all cations and all amination sites. Vibrational frequencies allow for easy distinguishing amination products from reactants and simplify identification of the reaction site (ring or side chain). Formation of strong hydrogen bonds between the amino group and the anions greatly favors amination of RTILs, compared to amination of benzene.

Synthesis, characterization and photophysical studies of a novel schiff base bearing 1, 2, 4-Triazole scaffold

A novel Schiff base derivative containing 1, 2, 4-triazole nucleus (TMPIMP) was synthesized from 4- [1,2,4] triazol-1-ylmethyl-phenylamine and salicylaldehyde in the presence of glacial acetic acid in an ethanolic medium. The synthesized compound was characterized by 1H-NMR, IR and UV spectral analysis. The excitation and emission spectra of triazolyl methyl phenyl imino methyl phenol (abbreviated as TMPIMP) were recorded in various solvents to investigate their solvatochromic behaviour. Dipole moments of the two electronic states of TMPIMP were calculated from solvatochromic spectral shifts. These were correlated with refractive index (η) and dielectric constant (ε) of various solvents. Theoretical calculations were performed to estimate the excited state dipole moment on the basis of different solvent correlation methods, like the Bilot–Kawski, Bakhshiev, Lippert–Mataga, Kawski–Chamma-Viallet and Reichardt methods. The dipole moment in the excited state was found to be higher than that in the ground state due to a substantial redistribution of electron densities and charges. Using a multiple regression analysis, the solvent–solute interactions were determined by means of Kamlet Taft parameters (α, β, π*). Computational studies were performed by Gaussian 09 W software using a time-dependent density functional theory (TD-DFT) in order to calculate the atomic charges and frontier molecular orbital energies in the solvent phase. The calculations indicated that the dipole moment of the molecule in an excited state is much higher than that in a ground state. The chemical stability of TMPIMP was determined by means of chemical hardness (η) using HOMO-LUMO energies. The reactive centers in the molecule were also identified by molecular electrostatic potential (MESP) 3D plots as a result of TD-DFT computational analysis.

Ammonia-modified Co(II) sites in zeolites: spin and electron density redistribution through the Co(II)-NO bond.

Electronic factors essential for the bonding of a non-innocent NO ligand to ammonia-modified Co(2+) sites in cobalt-exchanged zeolites are examined for small cluster models using DFT and advanced correlated wave function calculations. The analysis of charge transfer processes between the NO ligand and the cobalt center involves two protocols: valence-bond expansion of the multiconfiguration CASSCF wave function (in terms of fragment-localized active orbitals) and spin-resolved natural orbitals for chemical valence (SR-NOCV). Applicability of SR-NOCV analysis to transition metal complexes involving non-innocent fragments is critically assessed and the approach based on the CASSCF wave function turns out to be much more robust and systematic for all studied models. It is shown that the character and direction of electron density redistribution through the Co-N-O bond, quantified by relative share of the Co(II)-NO(0), Co(III)-NO(-), and Co(I)-NO(+) resonance structures in the total wave function, fully rationalize the activation of the N-O bond upon NH3 co-ligation (evidenced by calculated and measured red-shift of the NO stretching frequency and commonly ascribed to enhanced backdonation). The huge red-shift of νN-O is attributed to an effective electron transfer between the ammonia-modified Co(ii) centers and the NO antibonding π*-orbitals (related to the increased share of the Co(III)-NO(-) form). Unexpectedly, the effect is stronger for the singlet complex with three NH3 ligands than for that with five NH3 ligands bound to the cobalt center. Our results also indicate that high-efficiency electron transfers between the Co(ii) center and the NO ligand may be enabled for the selected spin state and disabled for the other spin state of the adduct. This illustrates how the cobalt center may serve to fine-tune the electronic communication between the NO ligand and its binding site.

Estimation of ground state and excited state dipole moments of a novel Schiff base derivative containing 1, 2, 4-triazole nucleus by solvatochromic method

A novel schiff base derivative containing 1, 2, 4-triazole moiety (NBTMPA) has been synthesized from 4- [1, 2, 4] triazol-1-ylmethyl-phenylamine and 4-nitrobenzaldehyde in the presence of glacial acetic acid in an ethanolic medium. The absorbance and fluorescence spectra of (4-nitro-benzylidene)-(4- [1, 2, 4] triazol-1-ylmethyl-phenyl)-amine (NBTMPA) were recorded in various solvents to investigate their solvatochromic behaviour. Dipole moments of the two electronic states of NBTMPA were calculated from solvatochromic spectral shifts. These were correlated with the refractive index (n) and dielectric constant (ε) of various solvents. Theoretical calculations were performed to estimate the excited state dipole moment on the basis of different solvent correlation methods, like the Bilot–Kawski, Bakhshiev, Lippert–Mataga, Kawski–Chamma–Viallet and Reichardt methods. The dipole moment in the excited state was found to be higher than that in the ground state due to a substantial redistribution of electron densities and charges. Using a multiple regression analysis, the solvent–solute interactions were determined by means of Kamlet Taft parameters (α, β, π*). Computational studies were performed by Gaussian 09W software using a time-dependent density functional theory (TDDFT) in order to calculate the atomic charges and frontier molecular orbital energies in the solvent phase. The calculations indicated that the dipole moment of the molecule in an excited state is much higher than that in a ground state. The chemical stability of NBTMPA was determined by means of chemical hardness (η) using HOMO–LUMO energies. The reactive centres in the molecule were also identified by molecular electrostatic potential (MESP) 3D plots as a result of a TDDFT computational analysis.

Covalency-Dependent Vibrational Dynamics in Two-Dimensional Titanium Carbides.

Structure and vibrational dynamics of T-terminated titanium carbide monosheets Ti2CT2 (T = O, F, OH) are studied by means of first-principles calculations to understand their inherent relation. Terminations modulate the crystal structures through the redistribution of valence electron density among the atoms in the monosheets, particularly Ti atoms. Phonon partial density of states analysis shows a clear feature of collaborative vibration, which reflects the covalent nature of bonds in the monosheets. Two metrics of covalency and cophonicity proposed very recently are adopted to quantitatively correlate the vibrational properties with the electrostructural characteristics of the system. A remarkable positive correlation between the covalency and vibrational dynamics specified as Raman shifts and IR wavenumbers is found. The bond-specific covalency metrics depend on not only the identity of terminations but also the thickness of the two-dimensional titanium carbides. For example, in the case of Ti3C2T2 with increased thickness, red shift in Raman shifts and IR wavenumbers occurs as a result of the decrease in covalency.

Ambience-related adsorbates on CuPc surface—Photoemission and thermal desorption spectroscopy studies for control of organic electronics degradation processes

In present work, using the angle-resolved X-ray photoelectron spectroscopy, the chemical structure of the 32nm-thick copper phthalocyanine (CuPc) films surface was revealed upon long-term ambient air exposure. The existence of ambience-related components was detected in the C1s, N1s and O1s spectra and their depth distributions within the CuPc layer were analyzed. Combined ultraviolet photoelectron and photoemission yield spectroscopies provided information concerning redistribution of electron density in surface region which was reflected in surface band bending increase by 0.91eV, shift in electron affinity of 0.26eV and work function variation of 1.17eV. The kind and amount of desorbed molecules or atoms from phthalocyanine’s surface were investigated by the thermal desorption spectroscopy. Quantified activation energies for desorption of C, N2, O2 and H2O species with values in the range of 120–166kJ/mol indicated chemical adsorption process (of rather covalent than ionic nature) of the ambience related adsorbates on organic surface.

Monoanionic Dipyrrin–Pyridine Ligands: Synthesis, Structure and Photophysical Properties

AbstractA novel monoanionic tetradentate N4 ligand (F5DPPy) based on a dipyrromethene skeleton as a molecular platform and decorated with pyridine rings at the 1‐ and 9‐positions of the dipyrrin motif has been prepared and characterized. Interestingly, although this ligand is weakly fluorescent, it presents a chelation‐enhanced fluorescence effect of around 150 times upon coordination to ZnII. Time‐dependent (TD) DFT calculations reproduce nicely the spectroscopic features of both the ligand and the complex, and analysis of the electron density redistribution in the excited state suggests that a better orbital overlap of the HOMO and LUMO in F5DPPyZnCl compared with F5DPPy is responsible for the more intense transitions observed with the former system. As such, this ligand opens interesting perspectives in the design of ratiometric sensors.

Modeling and simulation of helium behavior in tungsten: A first-principles investigation

Tungsten (W) is regarded as the most promising candidates for plasma-facing materials (PFMs) in future fusion reactors. However, W will be exposed to high fluxes of helium (He) plasma in a nuclear fusion environment, which will degrade the thermal and mechanical properties of W. We have investigated the behavior of He in W using a first-principles method based on density functional theory. It has demonstrated that the quantitative stress indicator and the qualitative hard-sphere model can be used to characterize the effective volume and the stability of interstitial He in W. The hydrostatic tensile (compressive) strain will enhance (suppress) the dissolution and diffusion of He in W. The existing of He will have negative effect on the ideal strength of W. Vacancies and grain boundaries defects in W can serve as trapping centers for He, because the defects can provide large space for He. He will spontaneously form He clusters at the near surface by self-trapping. The pre-existing impurity/alloying element can significantly reduce the solution energy of He in W due to the electron density redistribution induced by them, such as C and Nb. The results will provide good reference for developing W materials as PFMs. This paper reviews our recent findings regarding the behavior of He in W.

Density dependence of electron mobility in the accumulation mode for fully depleted SOI films

The electron mobility µeff in the accumulation mode is investigated for undepleted and fully depleted double-gate n+–n–n+ silicon-on-insulator (SOI) metal–oxide–semiconductor field-effect transistors (MOSFET). To determine the range of possible values of the mobility and the dominant scattering mechanisms in thin-film structures, it is proposed that the field dependence of the mobility µeff be replaced with the dependence on the density Ne of induced charge carriers. It is shown that the dependences µeff(Ne) can be approximated by the power functions µeff(Ne) ∝ Ne-n, where the exponent n is determined by the chargecarrier scattering mechanism as in the mobility field dependence. The values of the exponent n in the dependences µeff(Ne) are determined when the SOI-film mode near one of its surfaces varies from inversion to accumulation. The obtained results are explained from the viewpoint of the electron-density redistribution over the SOI-film thickness and changes in the scattering mechanisms.

Energetics and electronic structure of UAl4 with point defects

UAl4 ideal and defect structures were studied within the framework of the Density Functional Theory. The structural and magnetic ordering of UAl4 in paramagnetic, ferromagnetic, and antiferromagnetic states have been investigated, within the collinear and non-collinear spin approximation, using the GGA model, as embedded in the program package VASP. An antiferromagnetic (01¯1) layer structure with spins aligned to the [010] direction was found to be energetically preferred. The analysis of density of states and bonding charge density point out that the bonding mechanism consists primarily of band mixing between the U 5f and Al 3p states. Supercells were built from UAl4 unit cells with the established magnetic structure. For those supercells we calculated the energy of formation of vacancies and antisite defects taking into account the existence of three distinct aluminum sites. Point defect formation energies, local lattice relaxations, as well as the defect induced magnetic ordering and electronic density redistribution, are discussed. It is shown that antiferromagnetism is locally broken. Al antisites and U antisites in Al 4e Wyckoff positions are the constitutional point defects in Al-rich and U-rich oI20 UAl4, respectively. In this way we have presented here the first set of data which makes it possible to discuss and quantify the point defects concentrations in the experimental composition range for existence of this uranium aluminide.