Quantum chemical modelling of the structure and properties of hypervalent defects in vitreous SiO2 and GeO2

A cluster approximation using a semiempirical MNDO-PM3 scheme is used to study the structure and properties of the defect structures which develop in vitreous SiO₂ and GeO₂ during the interaction of the previously discovered most probable defects (two-member cycles, fragments with double bonds O=A<(A=Si, Ge) and open chains) with valence-saturated segments of a fracture surface. During the interaction of open chains with such a surface, defects with a large dipole moment (up to 15–20 D) are formed, which may create anisotropic highly polarized regions in the glass. The bonds around hypervalent centers are weakened, and the characteristics of the newly formed and already existing bonds approach one another; that is, in a grouping of this sort, other decay paths may exist that change the direction of fracture. In structures formed by the interaction of O=A< defects and two-member cycles with the surface, hypervalent bonds are easily broken; that is, the hypervalent configuration is transformed into an ordinary one. In a number of cases, the potential surface contains two or three minima with similar energies, separated by low or moderate potential barriers. Fiz. Tverd. Tela (St. Petersburg) 41, 1419–1423 (August 1999)     

The role of intermolecular vibrations and reorganization of a reaction system in tunneling reactions with H atom transfer. A Debye model for the medium

Mechanisms of temperature dependence of the rate constants for two types of solid-state tunneling chemical reactions, namely, transfer of an H atom between two molecules and intramolecular transfer, are analyzed. To this end, an analytical expression for the rate constant for tunneling atom transfer in solids is derived in the framework of a modified theory of nonradiative transitions. The mechanisms of the temperature dependence of the rate constant considered in this work include oscillations of the potential barrier to chemical reaction in intermolecular fluctuations and reorganization of the medium. The effect of pressure on the distance between reactants and on the frequency of intermolecular vibrations is taken into account. The theory developed is used to interpret experimental data on tunneling transfer of an H atom in two reactions: a) intramolecular hydrogen transfer in a matrix-isolated formic acid molecule entrapped in an argon crystal and b) H atom transfer from a fluorene molecule to an excited acridine molecule in a fluorene crystal. Dedicated to the 90th anniversary of the L. Ya. Karpov Institute of Physical Chemistry. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1073–1085, June, 2008.     

Photoluminescence properties of silica-based mesoporous materials similar to those of nanoscale silicon

Photoluminescence (PL) from composites of 7- and 15-nm sized silica nanoparticles (SNs) and mesoporous silicas (MSs) induced by 266- (4.66-) and 532-nm (2.33-eV) laser light has been studied at room temperature. The multiband PL from MSs in the range of 1.0-2.1 eV is evidenced to originate from isolated bulk and surface non-bridging oxygens (NBOs) and from NBOs combined with variously placed 1-nm sized pore wall oxygen vacancies (OVs). The nature and diversity of NBO light-emitters are confirmed by ab initio calculations. The PL from SNs exhibits only a short wavelength part of the bands (1.5-2.1 eV) originated from isolated bulk and surface NBOs. This fact indicates that the highly OV-bearing structures occur only in extremely thin (∼ 1 nm) silica layers. The similarity of spectroscopic properties of silica-based nanoscale materials to those of surface-oxidized silicon nanocrystals and porous silicon, containing silica-passivating layers of the same width, is discussed. Received 20 November 2000     

Quantum-chemical modeling of polypyrrole structure in neutral complexes with electron density acceptors

Quantum chemical approach has been applied for modeling the change in the conformation of the polypyrrole (PPy) chain both in its neutral and positively charged (doped) states due to its interaction with incorporated counterions. Polymer has been modeled by oligopyrrole molecule of 9–15 monomer units. It is shown that it is energetically favorable to reorient the pyrrole rings closest to the anion from trans to cis position. This reorientation leads to the formation of meanders which include 3–5 pyrrole rings per loop. Reduction in the loop size decreases the energy of interaction with the ion while an increase in the loop size reduces the number of attached ionic species accompanied by a slight change in their interaction energy with the PPy chain. The observed effect of polymer chain structuring in the presence of anions is proposed as a possible reason explaining the experimentally recorded broadening of the electroactivity potential region and the conductive state of the polypyrrole film on the electrode surface as a result of multiple repetitions of charge/discharge cycles of the polymer chain. Vibrational spectra of oligopyrrole complexes have been calculated, and prospects for experimental detection of predicted conformational states by IR spectroscopy are assessed.     

Silicon- and carbon-based anode materials: Quantum-chemical modeling

With the aim of searching for promising anode materials for lithium-ion batteries, we performed quantum-chemical modeling of the structure, stability, and electronic properties of silicon-coated carbon nanotubes, silicon rods, and silicon carbide fibers by the density functional theory method including gradient correction and periodic boundary conditions. It has been demonstrated that nanotubes poorly hold silicon, whereas silicon firmly adheres to the SiC surface. Silicon rods are more favorable than clusters and have the stability close to that of the crystal. The band gap in the rods is close to zero. Silicon carbide can be transformed into a conductor by doping with nitrogen.     

Optical properties of oxygen vacancies in germanium oxides: quantum chemical modeling of photoexcitation and photoluminescence

Photoabsorption and photoluminescence properties of single and double oxygen vacancy (OV and DOV) defects in quartz-like germanium oxide have been investigated by high-level ab initio calculations. It has been found that photoabsorption for these systems occurs at lower energies as compared to the analogous defects in SiO(2). For OV, the lowest electronic excitations with high oscillator strengths have energies of 6.7-7.0 eV, whereas for DOV, the lowest-energy photoabsorption band is calculated to be in the range of 5.5-5.9 eV. Significant geometry relaxation and large Stokes shift are inherent for these excited states and, as a result, their photoluminescence bands are predicted to peak at 3.1-3.3 eV for OV and at 2.6 eV for DOV. The double oxygen vacancy is suggested to be the most suitable candidate for generating bright blue photoluminescence observed experimentally for substoichiometric quartz-like GeO(2) nanowires, as the calculated optical properties of DOV are in close agreement with the features found in experiment.     

Nonaqueous LiNafion-based polymeric electrolyte: quantum-chemical modeling

In the framework of the search for promising electrolytes for lithium-ion batteries, quantumchemical modeling of the structure, stability, and electronic properties of membranes based on LiNafion. nDMSO, n = 0–16) has been performed by the density functional theory method with inclusion of gradient correction and periodic conditions (PBE/PAW). Inasmuch as the key factor that determines the ionic conductivity is the degree of swelling of LiNafion in DMSO (according to calculations, for n = 8, the degree of swelling is close to 500%, and for n = 16, it is close to 700% by volume), similar membranes can be good candidates from the viewpoint of lithium ion transport (with barriers of 0.2–0.3 eV, which depend on n).     

Quantum-chemical modeling of the dissociative adsorption of molecular hydrogen onto the tin dioxide surface

The hydrogen adsorption, dissociation, and migration on the tin dioxide surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that dissociative chemisorption onto the tin dioxide surface depends on the adsorption site and the surface structure and that there are places on the surface where dissociation can occur with a low barrier of 0.1–0.2 eV to yield a primary isomer in which one of the hydrogen atoms is bound to the tin atom and the other, to an oxygen atom; the second dissociation even at the same place is possible only if the primary isomer overcomes a barrier of ∼1 eV and transforms to the secondary isomer with two O-H bonds.     

Behavior of molecular hydrogen on the platinum crystal surface: Quantum-chemical modeling

The interaction of molecular hydrogen with the (111), (110), and (100) surfaces of the platinum crystal has been modeled by the density functional theory method within the generalized gradient approximation (GGA). The (100) surface is the least energetically favorable one, while the (111) and (110) surfaces are close in energy. The hydrogen molecule is attached to all three types of surfaces without a barrier. The largest decrease in energy is realized for the (100) surface. The bidentate coordination of hydrogen atoms is typical of the (100) and (110) surfaces, and the tridentate coordination is characteristic of the (111) surface. The H atoms can migrate over the crystal surface, overcoming moderate potential barriers of ??0.1?C0.2 eV; however, over the (110) surface, migration is possible only along the ridges. The maximal number of attached atoms per surface atom is close to unity for the (111) or (110) surface and to 1.67 for the (100) surface.