A. S. Zyubin

Ab initio study of H photodetachment from the ethyl radical

Abstract Minimal energy pathways for hydrogen motion in excited electronic states of C 2 H 5 have been studied at the CASSCF and MRCI levels. The calculations reveal that after photoexcitation to the 3s and 3p Rydberg states a proton from the CH 3 fragment slides to the bridging position over the center of the CC bond. In 3s ( 2 2 A ′ ) the resulting configuration ( 1 2 A 1 , C 2v ) is unstable and dissociates to H and C 2 H 4 in the ground electronic states without barrier. All three 3p Rydberg states of C 2 H 5 are stable relative to the H splitting, and a rapid proton exchange can occur before the dissociation.

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 Greens 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.

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.

Modeling of processes in fuel cells based on sulfonic acid membranes and platinum clusters

The density functional theory with account for gradient correction (DFT/PBE) and periodical boundary conditions was used to model the main stages of processes occurring in hydrogen low-temperature fuel cells. Modeling was carried out at the example of calculation of catalytic anodic and cathodic processes occurring on the surface of the Pt19 catalyst supported on a SnO2 and water adsorption processes on the surface of a membrane represented by a crystal of metisylene sulfonic acid dihydrate [(CH3)3C6H2SO3− · H5O2+]. It was shown that the most energy-efficient process in the membrane is formation of crystals, in which two stoichiometric water molecules correspond to a single SO3H group. Superstoichiometric water is adsorbed on the crystal surface with the adsorption energy of 0.3–0.6 eV; its transition inside the crystal is energy-consuming (2 eV). Barriers of surface proton conductivity are 0.2–0.3 eV.

Matrix reorganization with intramolecular tunneling of H atom: Formic acid in Ar matrix

The intramolecular tunneling of hydrogen atom in formic acid HCOOH at low temperatures is considered based on literature experimental data on C-O internal rotation. The energetic and geometric parameters as well as vibrational frequencies for formic acid in cis and trans configurations surrounded by 12 Ar atoms are calculated in the frame of the MP2 approach with extended basis sets. The temperature and pressure dependence of the rate constant is analyzed taking into consideration the matrix reorganization for the Debye model of lattice motion. It has been shown that the available experimental data can be explained by the suggested matrix reorganization mechanism. Theoretical expressions for the temperature dependence of the rate constant agree well with the experimental data on the cis to trans tunneling reactions in formic acid with fitting parameters attaining reasonable values. A mechanism describing pressure dependence of the rate constant for H-atom intramolecular tunneling reactions is also proposed.

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.

Quantum chemical modeling of the structure formation and proton transfer in 2-hydroxybenzenesulfonic, 4-hydroxy-1,3-benzenedisulfonic, and 1,3-benzenedisulfonic acids

Hydrate clusters of 2-hydroxybenzenesulfonic and 1,3-benzenedisulfonic acids were calculated in terms of the density functional theory (DFT) by the B3LYP/6-31G** method. The process of water adsorption on the crystal surface of 4-hydroxy-1,3-benzenedisulfonic acid dihydrate was simulated using the generalized gradient approximation (DFT/PBE) and periodic boundary conditions. For the model system (OHC6H4SO3−)·H5O2+, the activation barriers for the proton transfer were calculated depending on the distance between the O atoms and the deviation of the proton from the O...O bond line. The presence of one H2O molecule per SO3H group is energetically most favorable for the formation of clusters of 1,3-benzenedisulfonic acid containing a stoichiometric amount of water. The simulation of the hydration of 4-hydroxy-1,3-benzenedisulfonic acid dihydrate (OHC6H3(SO3H)2·2 H2O + n H2O, n = 1–3) showed that the superstoichiometric H2O molecule is adsorbed on the crystal surface of this dihydrate with energy release of 0.75–0.95 eV. The position of this water molecule is less favorable in the bulk than 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.

Platinum nanoparticles on different types of titanium dioxide surface: A quantum-chemical modeling

The interaction of a Pt29 nanoparticle with pristine and reduced TiO2 (110), (100), (101), and (100) surfaces in the rutile and anatase modifications has been modeled by the density functional theory method within the generalized gradient approximation (GGA). It has been demonstrated that the interaction energy of platinum particles with stoichiometric surfaces of titanium dioxide crystals is noticeably lower than for tin dioxide crystals. Like for SnO2, the reduction of the surface leads in some cases to a significant increase in the energy of interaction with platinum. The reoxidation of such structures should result in platinum fixation on the surface.

Interaction of oxygen with the platinum surface: A quantum-chemical modeling

The interaction of oxygen with the (111), (110), and (100) platinum crystal surfaces has been modeled by the density functional theory method within the generalized gradient approximation (GGA). It has been demonstrated that the dissociative adsorption of a dioxygen molecule to all three types of surfaces is energetically favorable. The peroxide species are less stable than the dissociated ones, but they are also energetically favorable. There have been considered the relative stability of different structures involving one and several oxygen atoms, the mutual influence of the atoms on the surface, the adsorption energy as a function of the surface coverage, and adsorption onto the intrinsic surface defects.