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.     

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.     

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.     

Oxygen chemisorption on the PbS(001) surface: Quantum-chemical modeling

The structure and stability of local centers involving a different number of oxygen atoms on the surface of crystalline lead sulfide (001) were calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. The trends of the formation of such centers and changes in the core electron binding energies for the sulfur and lead atoms constituting these centers were considered.     

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).     

Evaporation of carbon atoms from the open surface of silicon carbide and through graphene cells: Semiempirical quantum-chemical modeling

The evaporation of silicon atoms during the epitaxial growth of graphene on the singular carbon and silicon faces of silicon carbide SiC was modeled by the semiempirical AM1 and PM3 methods. The analysis was performed for evaporation of atoms both from the open surface of SiC and through the surface of the formed graphene monolayers. The total activation barrier of the evaporation of the silicon atoms, their passage from the graphene cell, and further evaporation from graphene was shown to be lower than the barrier to evaporation of the silicon atom on a free surface of SiC. Passage through graphene is thus not the limiting stage of the process, but contributes significantly to the effective evaporation time.     

Oxygen reduction on a platinum cluster

The reaction Pt₅O₂(ads)+H⁺(aq)+e⁻→Pt₅–OOH is analyzed on the basis of density functional theory calculations. It is found that the electron transfer process takes place gradually as the hydronium ion gets close to the adsorbed oxygen. At a certain small O_ads–H⋯O_water distance, the barrier for proton and electron transfer becomes negligible. The effect of an electric field on the reaction is studied by charging the metal/adsorbate complex, and that of a solvation shell on the proton transfer process is explored by using the H₃O⁺(H₂O)₂ ion cluster to model the hydrated proton.     

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.     

Cubyl cations: A quantum-chemical study

By the quantum-chemical method PBE0/6-311G (3d5f7,p) equilibrium structural parameters of cubyl cation C₈H ₇ ⁺ , adduct C₈H ₇ ⁺ ·MeOH and methoxycubane molecule C₈H₇OMe were determined. In the free cation the bond lengths are as follows: C¹C^α 1.489, C^αC^β 1,569, C^βC^γ 1.556, and CH 1.085–1.087 Å; angles at C¹ are 98.6°; charge on the C¹ atom is +0.39 au (NPA). In the methoxycubane the cubyl-O bond is shorter, while in the cationic adduct is longer than the O-Me bond: 1.368 < 1.405 Å and 1.494 > 1.469 Å. A relatively low electrophilicity of the cubyl cation compared with CH ₃ ⁺ is concluded and the possibility of its obtaining by radiochemical methods is assumed.     

Coordination of nitroxyl radical probes to the Lewis acid sites on the surface of gallium oxide: A quantum-chemical analysis

The results of ESR investigations and quantum-chemical studies on the coordination of 2,2,6,6-tetramethylpiperidine-N-oxyl to the Lewis acid sites (LAS) on the surface of gallium oxide are reported and analyzed. The structural, spin, electrostatic, and energy characteristics of the coordination of the simplest nitroxyl radical, H₂NO^., to the Ga(OH)₃ model cluster-type LAS were calculated by the unrestricted Hartree—Fock method in the 6–31G basis set and using the semiempirical PM3 approximation. The properties of gallium-containing nitroxyl surface complexes and their aluminium-containing analogs are compared. According to the results of cluster quantum-chemical calculations, the LAS on the surface of gallium oxide are stronger than those on the alumina surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 996–1001, June, 2000.     

Activation of molecular hydrogen on platinum nanoparticles: Quantum-chemical modeling

The interaction of molecular hydrogen with platinum clusters of different size has been modeled by the density functional theory method within the generalized gradient approximation (GGA). The cluster size turns out to have little effect on the interaction energy, whereas the effect of the cluster structure is rather significant. The most efficient interaction with hydrogen is observed for clusters with a structure resembling the crystal structure of platinum metal. In such clusters, the hydrogen molecule is attached to its surface without a barrier. Configurations with the bidentate hydrogen coordination are the most stable ones. The H atoms can migrate over the cluster surface, overcoming moderate potential barriers of ∼0.3–0.4 eV.     

Potential-dependent chemisorption of carbon monoxide on platinum electrodes: new insight from quantum-chemical calculations combined with vibrational spectroscopy

Density functional theory (DFT) using the finite cluster approach is utilized to compute binding energies, bond geometries, and vibrational properties of carbon monoxide adsorbed on Pt(111) as a function of the external interfacial field, focusing attention on the metal–CO bond itself. Comparison with electrode potential-dependent frequencies for the metal–CO (ν_M–CO) as well as the much-studied intramolecular C---O (ν_CO) vibration, as measured by in-situ Raman and infrared spectroscopy, facilitate their interpretation in terms of metal-chemisorbate bonding for this archetypal electrochemical system. Decomposing the calculated metal–CO binding energy and vibrational frequencies into individual orbital and steric repulsion components enables the role of such quantum-chemical interactions to the field- (and hence potential-) dependent bonding to be assessed. No simple relationship between the field(F)-dependent binding energies and the ν_M–CO frequencies is evident. While the DFT ν_M–CO–F slopes are negative at positive and small–moderate negative fields, reflecting the prevailing influence of back-donation, a ν_M–CO–F maximum is obtained at larger negative fields for atop CO, and a plateau for hollow-site CO. This Stark-tuning behavior reflects largely offsetting field-dependent contributions from π and σ surface bonding, and can also be rationalized on the basis of changes in the electrostatic component of ν_M–CO from increasing M–CO charge polarization. A rough correlation between the field-dependent ν_M–CO frequencies and the corresponding bond distances, r_M–CO, is observed for hollow and atop CO in that r_M–CO shortens towards less positive fields, but becomes near-constant at moderate–large negative fields. A more quantitative correlation between the field-dependent C---O frequencies and bond lengths is also evident. In harmony with earlier findings (and unlike the ν_M–CO–F behavior), the ν_CO–F dependence is due chiefly to changes in the back-donation bonding component. The overall vibrational frequency-field behavior predicted by DFT is also in semi-quantitative concordance with experimental potential-dependent spectra.     

A quantum-chemical study on the Mannich reaction with polynitromethanes

According to the results of SCF-CNDO/2 computation, we propose employing the “ladder weighting summation” to define a G_e index which is used to measure the relative magnitude of the nucleophilicity of the relevant amine and pseudo acid components in a Mannich reaction. We also put forward an A_e index to measure the relative magnitude of the electrophilicity of methyleneamine cationic species in the same reaction. A combined use of both G_e and A_e indices is suggested to evaluate the feasibility of a Mannich reaction for the polynitromethanes and the stability of the products. These results are consistent with those conclusions drawn from “soft-hard acid-base theory”.     

Influence of the reaction medium on platinum surface formation

The effect of an organic reaction medium on the Pt-black surface formation has been studied. It has been shown that under the action of the reaction medium an irreversible change of the catalyst surface occurs.

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Stereoselective hydrogenation of acetylene on copper catalysts: A quantum-chemical study

The structure of the alkyne adsorption complex on the copper surface is favorable for the stereoselective formation of cis isomers in the hydrogenation reaction. The pathway of dissociative hydrogen adsorption on copper is reported. It is assumed that the electron-donating promoter acts to reduce the electron work function of the copper catalyst by partially covering or “decorating” the surface of the copper particles.     

Simple molecules on a zeolite acceptor center: A quantum-chemical approach

The effectiveness and reliability of the cluster approach to modeling the interaction of several simple molecules (CO, water, ammonia, ethylene, and methanol) with zeolite acceptor centers was shown on the basis of various quantum-chemical methods and a comparison with the data of direct measurements. The Al···CO bonds were found to form in the complex of CO with three-coordinate aluminum. This allows the CO molecule to be used as a test for the acceptor properties of zeolites. A comparison of the calculation results and experimental data on the interaction of one to three RH-type molecules with a zeolite fragment led us to conclude that water, ammonia, ethylene, and methanol acquired proton-donor properties in the field of zeolite acceptor centers. These properties were similar to those of bridge OH groups in hydrogen zeolite forms. Probable schemes of catalysis on zeolites should be constructed taking into account the possible participation of acceptor centers in the conversions of molecules of the type specified.     

Thermal behavior of platinum fluorides: I. Saturated vapor pressure of platinum tetrafluoride

Published data on the chemical and phase transformations of crystalline platinum tetrafluoride on heating are considered, and contradictions of the obtained results are explained. A method for the calculation of the activity of platinum tetrafluoride in the presence of platinum penta-and hexafluorides is proposed. The enthalpy of sublimation of platinum tetrafluoride is calculated from experimental data: Δ_s H ₂₉₈ ^o = (161.3 ± 1.4 kJ/mol.     

叠氮根电负性的量子化学研究

用ab initio, MINDO/3, MNDO和DV-Xa等量子化学方法计算研究了一些分子型和离子型(碱金属)叠氮化物及相应氯化物的平衡构型和电子结构。结构表明: 在分子型叠氮化物中叠氮根的电负性较氯小、与氮相当; 而在离子型金属叠氮化物中,叠氮根的电负性和氯相当或较氯稍大。将计算所得正则离域分子轨道进行定域化处理, 发现产生这种电负性差异的主要原因是上述两类叠氮化物中N2的成键状况不同, 本文对此进行了较为细致的分析。     

Cysteine adsorption on the Au(111) surface and the electron transfer in configuration of a scanning tunneling microscope: A quantum-chemical approach

Adsorption of two forms, molecule and radical, of amino acid L-cysteine (Cys) on the Au₁₂ cluster that simulates the (111) face of single-crystal gold is studied in the framework of the density functional theory. Effects of solvation of adsorbed Cys particles and lateral interaction in a monolayer are analyzed. The simulation predicts a commensurate adsorption energetics of the molecule and radical, with a difference between the “on-top,” “hollow,” and “bridge” positions. An analysis of lateral electrostatic interactions points to the stability of a cluster comprising six Cys particles, which conforms to the size of a fragment observed experimentally. Adsorption calculations are used to build three-dimensional isosurfaces (STM images), where the tungsten needle of the scanning tunneling microscope is simulated by a tungsten atom or by small clusters. The calculated images are sensitive to both the Cys shape and the orientation of adsorbed Cys particles. Calculation results are compared with fresh in situ submolecular-resolution STM data. Simulated images (with commensurate contributions made by sulfur atom and amino group) built for Cys radical adsorbed in the “on-top” position give best conformance to experiment.