Photoluminescence of oxygen-containing surface defects in germanium oxides: a theoretical study

Photoabsorption and photoluminescence properties of nonbridging oxygen -O-Ge[triple bond](NBO), -OO-Ge[triple bond] (peroxy radical), O=Ge=, and (O2)Ge= defects in germanium oxides have been investigated by high-level ab initio calculations. Geometry optimization for excited electronic states of model clusters simulating these defects was carried out at the complete-active-space self-consistent-field level, and relative energies were calculated by various methods including time-dependent density-functional theory, outer-valence Green's functions, equation-of-motion coupled cluster theory with single and double excitations, symmetry-adapted cluster configuration interaction, multireference second-order perturbation theory, and multireference configuration interaction. The results demonstrate that the considered excited states of the aforementioned defects normally exhibit large Stokes shifts and that, with few exceptions, UV photoabsorption is accompanied by red or IR photoluminescence.     

Quantum chemical modeling of nanostructured silicon Si<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2—308). The snowball-type structures

To search for advanced anode materials for Li-ion rechargeable batteries, the structures, stabilities, and electronic properties of crystalline silicon and those of the snowball-type (SB) and core-shell (CS) silicon clusters Si_{zyubin@icp.ac.ru} (n = 2—308) were quantum chemically modeled within the framework of the density functional theory with inclusion of gradient correction and periodic boundary conditions. The formation of SB agglomerates from smaller Si_{zyubin@icp.ac.ru} clusters (n ≤ 7) is energetically preferable. At n ≥ 105 and circumscribed sphere diameters (D) ≥ 17—20 Å, CS isomers comprised of quasi-crystalline cores surrounded by small clusters are energetically more favorable than the SB isomers.     

Crystalline hydrates of calix[4]arene-<Emphasis Type="Italic">para</Emphasis>-sulfonic acid with <Emphasis Type="Italic">n</Emphasis> (<Emphasis Type="Italic">n</Emphasis> = 6–16) water molecules: a structure modeling

The structures of crystalline hydrates of calix[4]arene-para-sulfonic acid with n (n = 6–16) water molecules and the activation barriers to surface proton migration were calculated within the framework of the density functional theory (DFT) using the PBE gradient-corrected functional, the "hard" basis set of projector-augmented waves (PAW), a corresponding pseudopotential, periodic boundary conditions, and the VASP program package. The energies of formation of crystalline hydrates from calix[4]arene-para-sulfonic acid and n water molecules calculated per water molecule are in the range of 0.4–0.9 eV and depend on n. The adsorption energy of water on the surface is in the range of 0.5–0.7 eV. The activation barriers to proton transfer across the surface calculated for the most stable crystal (n = 8) are close to experimental data and depend on the number of superstoihiometric water molecules, being equal to ~0.2 eV provided three superstoihiometric water molecules per surface SO₃H group.     

Interaction of platinum nanoparticles with different types of tin dioxide surface: Quantum-chemical modeling

The interaction of Pt₂₉ nanoparticles with pristine and reduced (110), (100), (011), and (001) SnO₂ surfaces has been modeled using the density functional theory method within the generalized gradient approximation (GGA). It has been demonstrated that, in some cases, the reduction of the surface leads to a considerable increase in the energy of interaction with platinum. The second oxidation of such structures should lead to the platinum fixation on the surface.     

Synthesis of new polyporphines by replacing central ion in magnesium polyporphine

It is shown that the derivatives of the source electroactive polymer, magnesium polyporphine (pMgP), can be synthesized by successive treatment of electropolymerized pMgP film on the electrode surface with trifluoroacetic acid and zinc acetate solutions in the organic solvents. Based on the electrochemical and spectral characteristics of the modifying layers, it is concluded that the central magnesium ion in the porphine monomeric blocks is replaced with the formation of polyporphine in the form of free base (pH₂P) and zinc polyporphine, respectively. The oxidative transformation of thus obtained new polyporphines pH₂P and pZnP (which are the polymers of type I) is realized. It leads to a change in the molecular structure of polymer films (the transition to the type II); as a result, the potential range of their electroactivity is extended significantly.     

Dihydrogen Elimination from Hydrated Magnesium Borohydride: Quantum-Chemical Modeling

The successive elimination of H₂ from Mg(BH₄)₂ · nH₂O (n = 1, 4, 6), (Mg(BH₄)₂)₂ · 8H₂O, and (Mg(BH₄)₂)₄ · 4H₂O complexes has been modeled in the framework of the cluster approach with the use of the 6-31G* basis set and hybrid density functional (B3LYP). It has been shown that the structures in which the proton-donating water molecule is located between two metal cations have the lowest potential barriers. The barrier to elimination of the first H₂ molecule is low, but it increases as OH groups are substituted for the H atoms in the BH ₄ ^? anion. The elimination of H₂ occurs with a considerable energy release.     

Conjugately dense subgroups of free products of groups with amalgamation

A subgroup having non-empty intersection with each class of conjugate elements of the group is said to be conjugately dense. It is shown that, under certain conditions, the number of conjugately dense subgroups in a free product with amalgamation is not less than some cardinal. As a consequence, P. Neumann’s conjecture in the Kourovka notebook (Question 6.38) is refuted. It is also stated that a modular group and a non-Abelian group of countable or finite rank possess continuum many pairwise non-conjugate conjugately dense subgroups. Supported by RFBR grant No. 03-01-00905. __________ Translated from Algebra i Logika, Vol. 45, No. 5, pp. 520–537, September–October, 2006.