Alexander A. Bagaturyants

First-principle investigation of the hydroxylation of zirconia and hafnia surfaces

First-principle calculations demonstrate that the adsorption energies of water on the (001) and (101) surfaces of tetragonal zirconia (t-ZrO2) and on the (001) surface of monoclinic zirconia and hafnia (m-ZrO2 and m-HfO2) strongly depend on the surface hydroxylation degree. It is found that the fully hydroxylated t-ZrO2(001) surface undergoes a 2×2 reconstruction. The influence of surface dipole-dipole interactions on the calculated adsorption energy is discussed.

DFT calculations on the electronic and geometrical structure of 18-crown-6 complexes with Ag+, Hg2+, Ag0, Hg+, Hg0, AgNO3, and HgX2 (X=Cl, Br, and I)

Abstract Density functional theory is used for molecular simulation of the electronic and geometrical structure of 18-crown-6, its complexes with Ag + , Hg 2+ , Ag 0 , Hg + , Hg 0 , AgNO 3 , and HgX 2 (X=Cl, Br, and I). Ab initio MP2/6-31G ∗ calculations are performed for the two main conformations of the free crown ether and for anion-free complexes. The complex formation energies are analysed in terms of various contributions including Pauli and electrostatic repulsion and orbital interactions according to the Morokuma scheme. It is found that the Hg 2+ ion is most strongly bound to the crown ether; silver and mercury ions in the 18-crown-6 cavity can capture an electron, and neutral 18-crown-6 complex of silver can be bound through van der Waals-type interactions. The stability of metal complexes with supporting anionic ligands is determined not only by the cation charge, but also by the type of the ligand.

Molecular simulation of the complexation effects on conformations and electronic absorption spectra of crown ether styryl dyes

Abstract The structure and spectra of new styryl dyes containing a crown ether moiety and a sulfoalkyl group along with their complexes with metal cations are investigated using spectroscopic methods and computer simulation. Large differences between the absorption spectra of the cis form and its complexes with alkaline earth metals are found. The absorption maximum of the high intensity longwave band is at 429 nm for the cis form and at 321–328 nm for the complexes. We show that this difference results from a large distortion of the chromophore geometry, which arises from the formation of an intramolecular coordination bond between the sulfo group and the metal cation captured in the crown cavity. For the distorted geometry, the first electronic transition is symmetry forbidden. The disappearance of the longwave band results in a large hypsochromic shift, observed experimentally.

Ab initio study of phosphorescent emitters based on rare-earth complexes with organic ligands for organic electroluminescent devices.

An ab initio approach is developed for calculation of low-lying excited states in Ln(3+) complexes with organic ligands. The energies of the ground and excited states are calculated using the XMCQDPT2/CASSCF approximation; the 4f electrons of the Ln(3+) ion are included in the core, and the effects of the core electrons are described by scalar quasirelativistic 4f-in-core pseudopotentials. The geometries of the complexes in the ground and triplet excited states are fully optimized at the CASSCF level, and the resulting excited states have been found to be localized on one of the ligands. The efficiency of ligand-to-lanthanide energy transfer is assessed based on the relative energies of the triplet excited states localized on the organic ligands with respect to the receiving and emitting levels of the Ln(3+) ion. It is shown that ligand relaxation in the excited state should be properly taken into account in order to adequately describe energy transfer in the complexes. It is demonstrated that the efficiency of antenna ligands for lanthanide complexes used as phosphorescent emitters in organic light-emitting devices can be reasonably predicted using the procedure suggested in this work. Hence, the best antenna ligands can be selected in silico based on theoretical calculations of ligand-localized excited energy levels.

Symmetry-Breaking in Cationic Polymethine Dyes: Part 2. Shape of Electronic Absorption Bands Explained by the Thermal Fluctuations of the Solvent Reaction Field

The electronic absorption spectra of the symmetric cyanines exhibit dramatic dependence on the conjugated chain length: whereas short-chain homologues are characterized by the narrow and sharp absorption bands of high intensity, the long-chain homologues demonstrate very broad, structureless bands of low intensity. Spectra of the intermediate homologues combine both features. These broad bands are often explained using spontaneous symmetry-breaking and charge localization at one of the termini, and the combination of broad and sharp features was interpreted as coexistence of symmetric and asymmetric species in solution. These explanations were not supported by the first principle simulations until now. Here, we employ a combination of time-dependent density functional theory, a polarizable continuum model, and Franck-Condon (FC) approximation to predict the absorption line shapes for the series of 2-azaazulene and 1-methylpyridine-4-substituted polymethine dyes. To simulate inhomogeneous broadening by the solvent, the molecular structures are optimized in the presence of a finite electric field of various strengths. The calculated FC line shapes, averaged with the Boltzmann weights of different field strengths, reproduce the experimentally observed spectra closely. Although the polarizable continuum model accounts for the equilibrium solvent reaction field at absolute zero, the finite field accounts for the thermal fluctuations in the solvent, which break the symmetry of the solute molecule. This model of inhomogeneous broadening opens the possibility for computational studies of thermochromism. The choice of the global hybrid exchange-correlation functional SOGGA11-X, including 40% of the exact exchange, plays the critical role in the success of our model.

Oxygen vacancies in tetragonal ZrO 2 : ab initio embedded cluster calculations

Formation of oxygen vacancies in bulk tetragonal ZrO2 and at its (101) and (001) surfaces was studied using ab initio embedded cluster calculations. A new technique was proposed for generating a set of point charges that represents the Coulomb field of the crystal environment both for bulk and surface cluster structures. This technique provides the rapid convergence of the calculated Madelung potential to the unique true value. The estimated bulk vacancy formation energy in tetragonal ZrO2 is 8.8 eV, while surface vacancy energy formation depends on the type of the surface and position of removed oxygen atom and varies from 8.3 to 9.3 eV. The calculated activation energy of oxygen vacancy migration in bulk ZrO2 is 1.95 eV.

NUMERICAL CALCULATIONS OF OPTICAL LINESHAPES FOR DISORDERED MOLECULAR AGGREGATES

Abstract Numerical calculations of optical lineshapes are performed for molecular aggregates with Gaussian diagonal disorder. A method is proposed that significantly increases the efficiency of numerical calculations by choosing the fluctuating average value of transition energies of all molecules in the aggregate as an origin of the energy scale. Using the proposed method, the absorption line half-width and the exciton coherence length are examined as functions of the number of molecules in the aggregate, the degree of disorder and the disorder correlation over a wide range of parameters.

Impact of strain on the surface properties of transition metal carbide films: First-principles study

The effect of in-plane lattice strain on the atomic and electronic properties of low-index transition metal (M=Ti, Nb, and Ta) carbide surfaces is studied by first-principles molecular dynamics calculations using a pseudopotential plane-wave technique. The most stable cubic rock-salt phase is considered for carbides. The first-principle study of various [(001), (110), and metal-terminated (111)] carbide surfaces reveals that both compressive and tensile strains strongly affect surface relaxation and electronic properties (work function values and band structures). The most stable (001) carbide surfaces exhibit rumpling between transition metal and carbon atoms in the topmost surface layers, which depends on the applied strain. The work function (WF) for the metal-terminated (111) surfaces varies monotonically, rather strongly depending on the applied strain (the range of variation reaches about 1 eV), while the WF for the (001) surface varies nonmonotonically with a much smaller resulting variation over t...

Macrocyclic Complexes of Palladium(II) with Benzothiacrown Ethers: Synthesis, Characterization, and Structure of cis and trans Isomers

A series of palladium(II) complexes with nitro- and formylbenzothiacrown-ether derivatives was synthesized. The spatial structure of the complexes was studied by NMR, X-ray diffraction analysis, and quantum chemical calculations (density functional theory). The cavity size and the ligand denticity were found to be crucial factors determining the geometric configuration of the thiacrown-ether complexes. Palladium(II) complexes with benzodithia-12(18)-crown-4(6) ethers were demonstrated to have a cis-configured S(2)PdY(2) fragment (Y = Cl, OAc). In the case of Pd(II) and benzodithia-21-crown-7 ethers, only complexes with a trans configuration of the S(2)PdY(2) fragment form. In the case of Pd(II) and nitrobenzomonothia-15-crown-5 ether, only 2(ligand):1(Pd) complex with trans configuration of the core fragment forms.

MO calculations on the absorption spectra of organic dimers. The interaction energy between dipole moments of electronic transitions in monomers and the shape of absorption bands

Abstract The interaction energy between dipole moments of electronic transitions in the monomers and the shape of absorption bands of organic dimers are calculated by the CNDO/S-CI method. It is found that the through-space resonance interaction between π orbitals of the monomers has a strong effect on the shifts and widths of absorption bands of dimers with the structure of H -aggregates. A large intermediate region of structures is found between structures corresponding to H - and J -aggregates. In the intermediate region, the interaction energy between the dipole moments of electronic transition in the monomers is small, and the dimer has a broad unshifted spectral band similar to that of the monomers. The large size of the intermediate region is due to a violation of the point-dipole approximation.