Suchergebnisse

This research was partly funded by the Russian Science Foundation (under the project 14-43-0005) and ERA-NET HarvEnPiez project, with the computer resources provided by Stuttgart Supercomputing Centre (Project DEFTD 12939). A. C. also acknowledges financial support from the University of Latvia Foundation (Arnis Riekstins's ''MikroTik'' donation). Authors thank R. Merkle, A. Popov for fruitful discussions. ; Tb-doped CeO2 (ceria) is a promising mixed conductor for oxygen permeation membranes and reversible oxygen sorbents. To predict solubility of Tb ions in ceria for a wide range of concentrations, density functional theory (DFT+U) calculations with two different values of Hubbard U-parameter on Tb and Ce ions were combined with alloy thermodynamics and the Concentration Wave approach. It is shown that, to predict properties of disordered solid solutions at finite temperatures, the energy parameters in the mixing energies can be extracted from the DFT+U calculations performed at T = 0 K for two ordered configurations of the dopant in the supercells. The unlimited solubility of Tb4+ in CeO2 in the quasi-binary cross-section CeO2-TbO2 is predicted in the temperature range where both stoichiometric TbO2 and CeO2 reveal fluorite structures (above 700 °C). ; Russian Science Foundation (under the project 14-43-0005); ERA-NET HarvEnPiez project; Stuttgart Supercomputing Centre Project DEFTD 12939; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The electronic properties (such as energy gap HOMO levels. LUMO levels, density of state and density of bonds in addition to spectroscopic properties like IR spectra, Raman spectra, force constant and reduced masses as a function of frequency) of coronene C24 and reduced graphene oxide C24OX , where x=1-5, were studied.. The methodology employed was Density Functional Theory (DFT) with Hybrid function B3LYP and 6-311G** basis sets. The energy gap was calculated for C24 to be 3.5 eV and for C24Ox was from 0.89 to 1.6862 eV for x=1-5 ,respectively. These energy gaps values are comparable to the measured gap of Graphene (1-2.2 eV). The spectroscopic properties were compared with experimental measurements, specifically the longitudinal optical modes which agreed well.

PUBLISHED ; Export Date: 5 January 2017 ; The energy gap of a molecular crystal is one of the most important properties since it determines the crystal charge transport when the material is utilized in electronic devices. This is, however, a quantity difficult to calculate and standard theoretical approaches based on density functional theory (DFT) have proven unable to provide accurate estimates. In fact, besides the well-known band-gap problem, DFT completely fails in capturing the fundamental gap reduction occurring when molecules are packed in a crystal structures. The failure has to be associated with the inability of describing the electronic polarization and the real space localization of the charged states. Here we describe a scheme based on constrained DFT, which can improve upon the shortcomings of standard DFT. The method is applied to the benzene crystal, where we show that accurate results can be achieved for both the band gap and also the energy level alignment. ; The authors are grateful to S. Refaely-Abramson for useful discussions. A.D. and I.R. were sponsored by the European Union through Project No. 618082 ACMOL. A.D. received additional support from the ?Ministerio de Economia y Com- petitividad? (Mineco) of Spain (Grant No. FPDI-2013-16641). S.S. acknowledges the Quest project funded by European Research Council. Computational resources were provided by the Trinity Centre for High Performance Computing (TCHPC) and the Irish Centre for High-End Computing (ICHEC)

The energy gap of a molecular crystal is one of the most important properties since it determines the crystal charge transport when the material is utilized in electronic devices. This is, however, a quantity difficult to calculate and standard theoretical approaches based on density functional theory (DFT) have proven unable to provide accurate estimates. In fact, besides the well-known band-gap problem, DFT completely fails in capturing the fundamental gap reduction occurring when molecules are packed in a crystal structures. The failure has to be associated with the inability of describing the electronic polarization and the real space localization of the charged states. Here we describe a scheme based on constrained DFT, which can improve upon the shortcomings of standard DFT. The method is applied to the benzene crystal, where we show that accurate results can be achieved for both the band gap and also the energy level alignment. ; A.D. and I.R. were sponsored by the European Union through Project No. 618082 ACMOL. A.D. received additional support from the "Ministerio de Economia y Competitividad" (Mineco) of Spain (Grant No. FPDI-2013-16641). S.S. acknowledges the Quest project funded by European Research Council. Computational resources were provided by the Trinity Centre for High Performance Computing (TCHPC) and the Irish Centre for High-End Computing (ICHEC). ; Peer reviewed

In silico-based optimization of Ir/P,S-catalysts for the asymmetric hydrogenation of unfunctionalized olefins using (E)-1-(but-2-en-2-yl)-4-methoxybenzene as a benchmark olefin has been carried out. DFT calculations revealed that the thioether group has a major role in directing the olefin coordination. This, together with the configuration of the biphenyl phosphite group, has an impact in maximizing the energy gap between the most stable transition states leading to opposite enantiomers. As a result, the optimized catalyst proved to be efficient in the hydrogenation of a range of alkenes with the same substitution pattern and olefin geometry as the benchmark olefin, regardless of the presence of functional groups with different coordination abilities (ee values up to 97%). Appealingly, further modifications at the thioether groups and at the biaryl phosphite moiety allowed the highly enantioselective hydrogenation of olefins with different substitution patterns (e.g., α,β-unsaturated lactones and lactams, 1,1′-disubstituted enol phosphinates, and cyclic β-enamides; ee values up to >99%). ; We gratefully acknowledge financial support from the Spanish Ministry of Science and Innovation (PID2019-104904GB-I00, PGC2018-100780-B-I00, and PGC2018-096616-B-I00), European Regional Development Fund (AEI/FEDER, UE), the Catalan Government (2017SGR1472), and the University of Alicante (VOGROB-316FI). M.B. also thanks the URV for generous support.

For the relationship between thermodynamic parameters, global hardness, global electronegativity, anomeric effect, structural parameters, and structural properties of dodecahedro-spiro [isochromene-3,2'-pyran] (A), dodecahedro-spiro [isothiocromene-3,2'- thiopyran) (B), dodecahedro-spiro [isochroman-3,2'-thiopyran] (C) and dodecahedro-spiro [isothiochrome-3,2'-pyran] (D) computational methods (B3LYP / 6-311 ++ G **) were used. Results show that the calculated Gibbs free energy and enthalpy differences between the axial- and equatorial- stereoisomers [i.e. ΔG = Geq – Gax, ΔH = Heq- Hax] decrease from compound A to compound B and from compound C to compound D. NBO results show that the anomeric effect associated with the electron delocalization is in benefit of the axial-stereoisomers and with the decrease of the anomeric effect, the values of global hardness (η), and global electronegativity (χ), from compound A to compound B and from compound C to Composition D is also decrease. The correlations between the global hardness, global electronegativity, anomeric effect, structural parameters, and thermodynamic parameters [ΔH, ΔG, and ΔS] of compounds A-D have been investigated. Also, it can be concluded that the stability energy obtained from the numerical values of the anomeric effect and the values calculated theoretically (B3LYP / 6-311 ++ G **) confirmed each other.

First principle calculations are performed to theoretically predict the physical properties of hexagonal aluminium arsenide planar and buckled monolayers. The structural characteristics showed that the buckled parameter is about 0.32 A°. Cohesive energies have favourable values and it indicates the fabrication possibility. Phonon dispersion properties indicated that the planar aluminium arsenic monolayers are dynamically unstable, while the buckled is less dynamically unstable. The elastic constant parameters achieved the required characteristics of stable hexagonal monolayer structures. The study of electronic band structure prefers to indirect semiconductor band gaps, and the density of states showed strong orbital hybridization in the conduction band. Planar structure has isotropic light electron effective mass and anisotropic heavy hole effective mass. The buckled structure has isotropic light electron effective mass and isotropic heavy hole effective mass. The absorption spectra have high absorption coefficient in various visible and ultraviolet wavelength. The absorption coefficient levels off at about direct and indirect band gaps.